Download Chapter 10 - Populations, Communities, and Ecosystems CHAPTER

Chapter 10 - Populations, Communities, and Ecosystems
CHAPTER 10
- POPULATIONS, COMMUNITIES, AND ECOSYSTEMS .................................. 1
10.1 INTRODUCTION ................................................................................................................................ 2
10.2 NICHE AND SPECIES ABUNDANCE .................................................................................................... 3
10.3 ENVIRONMENTAL GRADIENTS AND COMMUNITIES........................................................................... 6
10.4 ECOSYSTEMS ................................................................................................................................... 8
10.4.1 What is an ecosystem? ........................................................................................................... 8
10.4.2 Ecosystem structure and function......................................................................................... 9
10.4.3 Environmental controls of net primary production............................................................ 11
10.4.4 Biogeochemical cycles ......................................................................................................... 14
10.4.5 Forest production and nutrient cycling .............................................................................. 17
10.4.6 Net ecosystem production .................................................................................................... 19
10.5 LANDSCAPES ................................................................................................................................. 22
10.6 GLOBAL VEGETATION ................................................................................................................... 24
10.6.1 Biogeography ....................................................................................................................... 24
10.6.2 Net primary production ....................................................................................................... 28
10.6.3 Litterfall and soil carbon ..................................................................................................... 30
10.6.4 Global carbon cycle ............................................................................................................. 31
10.6.5 Global terrestrial ecosystem models .................................................................................... 32
10.7 TABLES ......................................................................................................................................... 34
10.8 FIGURE LEGENDS .......................................................................................................................... 38
Ecological Climatology
10.1
Introduction
This chapter focuses on the arrangement of individuals into populations, communities, and
ecosystems. The progression of study from individuals to populations to communities to ecosystems
represents a change in scientific scope. Vegetation data collected as part of the Hubbard Brook ecosystem
study illustrate the different population, community, and ecosystem depictions of the same forest
vegetation (Figure 10.1). The Hubbard Brook study has been a leading innovator in the study of forest
ecosystems (Likens et al. 1977; Bormann and Likens 1979; Likens and Bormann 1995). Other such studies
include the Coweeta watershed in the Great Smoky Mountains of North Carolina (Swank and Crossley
1988) and Walker Branch in Tennessee (Johnson and Van Hook 1989). The study site, located in the
White Mountain National Forest in central New Hampshire, is part of the extensive northern hardwood
forest ecosystem. The climate is cool temperate, humid continental, with long, cold winters and short, cool
summers. Prior to logging between about 1909 and 1917, the vegetation is thought to have been a mature,
old-age forest. The watershed depicted in Figure 10.1 covers 13.23 hectares (ha) (1 ha = 10 000 m2), with
elevations ranging from 546 m to 791 m. The mountains generally face towards the southeast with a slope
of 21-23%. The soil is predominantly sandy loam.
The forest can be described in terms of its overall community composition (Figure 10.1, top left).
The principal species are sugar maple, comprising 35% of large trees, American beech (27%), and yellow
birch (23%). Other less abundant species include paper birch, red spruce, balsam fir, pin cherry, and
striped maple. The population size structure indicates an all-aged forest with relatively few large, old trees
and an abundance of seedlings (Figure 10.1, top right). The largest trees are remnants from before logging
while the moderate size trees (11-20 cm diameter) regenerated during and following logging. Continued
reproduction provides large seedling and sapling cohorts to replace older trees as they die. The floristic
composition of the forest changes with elevation (Figure 10.1, middle). Sugar maple, beech, and yellow
birch are the most abundant trees at all elevations. Paper birch, red spruce, and balsam fir are a minor
component of low and middle elevation slopes, but are more abundant on upper slopes.
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Chapter 10 – Populations, Communities, and Ecosystems
The Hubbard Brook forest ecosystem contains 16 108 g of living biomass per square meter, of
which about 45% is carbon (Figure 10.1, bottom). (Up to 80-90% of fresh biomass is water. Biomass is
reported in terms of dry weight. Some studies report biomass in terms of carbon. The carbon content of
biomass is typically 45% of dry weight. Throughout this chapter units of g m-2 refer to dry biomass while
units of g C m-2 refer to the carbon in biomass.) Most of this biomass (82%) is contained in aboveground
plant material (leaves, branches, stems); only 18% is belowground in roots. Annually, plants gain 1002 g
m-2 of biomass in net primary production, representing an uptake of 451 g C m-2 from the atmosphere. Not
all of this material remains in the forest. Each year leaves, twigs, and woody debris fall to the ground as
litter. This litter slowly decomposes over time, releasing carbon back to the atmosphere. Annually, 680 g
m-2 of biomass decomposes, returning 306 g C m-2 to the atmosphere. The net biomass gained by the
ecosystem, known as net ecosystem production, is 322 g m-2 so that 145 g C m-2 is stored annually in the
ecosystem.
10.2
Niche and species abundance
The environment is spatially heterogeneous, varying in light, temperature, soil water, nutrients,
and other conditions. Just as physiological processes vary depending on the specific environmental
conditions encountered by a plant, so too do plant species thrive over a specific range of environmental
conditions. Biological performance for a particular species is typically optimal at some level of an
environmental condition (e.g., soil water) and decreases with conditions less than or greater than optimal
(Figure 10.2). In two dimensions (e.g., soil water and temperature), this space is represented by an area
formed by the intersection of soil water and temperature tolerances (Figure 10.2). For example, the
abundance of pollen in eastern North America varies with temperature and precipitation (Figure 10.3). Oak
pollen is most abundant with annual precipitation of 1000 mm and July temperature in excess of 24 °C.
Northern pine pollen is most abundant in cooler, drier climates while spruce pollen is greatest in cold,
moist climates.
The niche concept invokes attributes of species, specifically their tolerance of environmental
conditions, and the way species interact with others to understand the spatial distribution of species in a
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Ecological Climatology
landscape (Bazzaz 1996; Shugart 1998). A niche represents all the components of the environment, both
abiotic and biotic, to which a species is adapted. It includes both the environment where a species is found
– its habitat – and the role of the species in relation to other species – what it does and how it lives. The
functional role of species is perhaps most obvious in animals, where different species have preferences for
certain size prey. Indeed, the concept of niche originally was formulated for animals. Among plants,
functional roles include how resources are utilized, the timing of biological activity, and the partitioning of
light into overstory and understory environments and soil into deep- and shallow-rooted plants. Life form,
phenology, and regeneration are additional functional aspects of plant niche (Grubb 1977).
The concept of niche can be seen in the distribution of a species across the landscape. Light,
water, nutrients, and temperature are important to plants, and different species have evolved different
preferences. In this respect, niche is a multidimensional volume within which the species is likely to be
found (Figure 10.2). It is closely tied to the notion of ecological limits and physiological tolerances. For
example, the occurrence of Eucalyptus pauciflora, a common alpine eucalypt in southeastern Australia,
varies in relation to type of rock (granite or sedimentary), temperature, and precipitation (Austin et al.
1990). On granite rocks, this tree species is most common where annual rainfall is greater than 1400 mm
and mean annual temperature ranges from 6 °C to 8 °C (Figure 10.4).
A niche represents more than just physiological tolerances and adaptations to the physical
environment. A niche is shaped by the functional roles of species in communities and how these roles alter
the competitive balance among species. The fundamental niche of a species grown in isolation may be
quite different from the realized niche when grown in the presence of other species. Competition with other
species may exclude a species from parts of its fundamental niche.
Figure 10.5 illustrates the way in which physiological tolerances, functional roles, and
competition influence species distributions. In this example, the physiology and growth of trees is a
tradeoff between shade and drought tolerance. Shade tolerant trees tend to be drought intolerant while
shade intolerant trees are tolerant of low soil water. Furthermore, growth rate decreases as tolerance to low
light or low soil water increases. Simulations with a model of forest dynamics show the consequences of
this tradeoff. When five hypothetical plant species are grown in monocultures (i.e., in the absence of
competition with other species), their abundance along a soil water gradient reflects their physiological
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Chapter 10 – Populations, Communities, and Ecosystems
tolerances. All species grow best when soil is wet and the range of soil water over which they grow
increases with drought tolerance. When grown in mixtures of all 15 species, the same five species are
distributed along the soil water gradient with bell-shaped curves. Tradeoffs in life history and physiology
preclude a single species from always being successful. Moreover, competition displaces each species
towards the environmental conditions that it is able to tolerate but which cannot be tolerated by species that
are better competitors under optimal conditions. This ecological optimum is closer to the physiological
limit than to the physiological optimum.
The introduction of non-native species provides an example of how physiological tolerances and
life history patterns
affect the competitive balance among species and thereby alter vegetation
composition. The dominant plant of pristine grasslands in the northern intermountain region of the United
States., in the semi-arid region between the Rocky Mountains and Cascade Mountains, used to be the
native bluebunch wheatgrass (Harris 1967). Human activities introduced cheatgrass, which has since
expanded to dominate much of the region. Both species are winter plants. Bluebunch wheatgrass, a
perennial, breaks dormancy in autumn and begins leaf growth in September or October when temperatures
cool and soil moisture increases. It grows slowly during winter, with rapid growth in spring after snow
melt. Flowers and seeds form in June and July, with summer dormancy by mid-July. Cheatgrass is a winter
annual. Seeds germinate in autumn, and plants maintain slow growth throughout winter. Spring brings
strong growth, followed by rapid flowering and seed production and then death by late May.
In a study of the ecology of these two species, it was found that the presence of cheatgrass
reduces the growth and survival of bluebunch wheatgrass (Figure 10.6). In one experiment, wheatgrass
seeds were sown so that plants grew in different densities of cheatgrass: sparse (0-4 per m2); moderate (1520 per m2); dense (90-100 per m2). After October sowing, survival of wheatgrass seedlings during the
following summer was inversely related to cheatgrass density. In June, there was little effect of cheatgrass
density on survival. By October, however, only 39% of the plants survived in the dense cheatgrass while
86% survived in the sparse cheatgrass. The average height of wheatgrass decreased from 38 cm with sparse
competition to 18 cm with dense competition. The detrimental effect of cheatgrass on wheatgrass growth
was apparent when both species were grown at the same densities but in different proportions of the two
species. Wheatgrass root growth declined with greater proportion of cheatgrass while cheatgrass root
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Ecological Climatology
growth was unaffected. The cause of reduced wheatgrass growth is apparently the longer roots of
cheatgrass, which provides access to soil water at the expense of wheatgrass. Cheatgrass roots grow
throughout winter following germination in October. In contrast, wheatgrass roots grow little during
winter. Cheatgrass grows and thrives at the expense of wheatgrass because of its winter root growth, which
gives it an early growing season advantage over wheatgrass, and its earlier maturation, which depletes soil
water.
10.3
Environmental gradients and communities
Evolutionary pressures in which success is determined by the number of offspring that survive to
maturity have lead to niche differentiation so as to reduce competition among species. Each species has its
own niche, with a central location that differs from other species so as to minimize the detrimental effects
of competition for resources. This can be seen in the dispersion of plant species along environmental
gradients. For example, the vegetation of the Great Smoky Mountains along the Tennessee-North Carolina
state border illustrates the typical distribution of vegetation along environmental gradients (Whittaker
1956). In this region, elevation ranges from 460 m along the bottomlands to 2000 m at the summits of the
highest peaks. Annual precipitation increases from less than 1500 mm in the lower valleys to more than
2000 mm at high elevations. The abundance of tree species changes markedly with respect to elevation and
moisture (Figure 10.7). Species distributions in relation to elevation show rounded or bell-shaped curves
and overlap broadly, but with distinct population centers distributed along the elevation gradient. Yellow
poplar dominates low elevations on mesic sites. With higher elevation, yellow birch, mountain silverbell,
sugar maple, white basswood, and yellow buckeye become progressively more dominant. High elevations
above 1400 m are almost exclusively beech forests. The floristic composition of xeric sites is distinctly
different, but again has broad, overlapping population dispersions with respect to elevation.
Vegetation can be classified into distinct communities or associations of species based on similar
physiognomy and floristic composition. Physiognomy considers the form and structure of communities in
terms of the type of plant (e.g., woody, herbaceous) and vertical structure (e.g., overstory, understory).
Floristic composition considers the dominant species (e.g., oak-hickory forest). Numerous such
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Chapter 10 – Populations, Communities, and Ecosystems
communities occur in the Smoky Mountains, segregated along moisture and elevation gradients (Figure
10.8). Vegetation at lower and middle elevations varies from mixed deciduous cove forest and hemlock
forest on mesic sites through a variety of oak forests and heaths on drier sites to pine forests and heaths on
xeric slopes. Red spruce and Fraser fir forests prevail at high elevations, with beech and oak forests on
favorable sites. Patches of grasses form grassy balds on the summits of mountains. Shrub communities
dominated by evergreen ericaceous shrubs form heath balds along dry ridges.
Although one or two dominant species define certain communities, several or more species
comprise most communities. For example, beech forests on sheltered south slopes above 1372 m are a
mixture of beech and lesser amounts of other species (Figure 10.9). Beech, comprising 81% of the trees
with a diameter greater than 2.54 cm, is clearly the most dominant. The next most abundant species is
mountain silverbell (8% of trees). Yellow birch and yellow buckeye together comprise 5% of trees, and
various other species occur in minor amounts. Cove forests between 762 m and 1067 m have a more
diverse community structure (Figure 10.9). Four species (mountain silverbell, white basswood, sugar
maple, yellow birch) share dominance in these forests, accounting for 69% of trees. Six other species, with
abundances of 2-6%, comprise 21% of the forest.
Most communities intergrade continuously and exist within a continuum of populations (Figure
10.10). Some populations are restricted to particular communities. For example, white basswood grows
almost exclusively in cove forests. However, chestnut oak and hemlock, while having greatest abundance
in chestnut oak and hemlock communities, occur in numerous other communities. Chestnut oak can be
found in abundances of 5% or more in oak and pine forests. Hemlock can be found in abundances of 10%
or more in cove forests and oak-hickory forests. Other species such as red maple never form a distinct
community type, but rather intermingle with other species throughout many communities.
Much of the history of ecology has been dominated by a debate about the nature of plant
communities (McIntosh 1981, 1985; Golley 1993). Are they emergent units representing a distinct level of
ecological organization (Clements 1916, 1928; Odum 1953, 1969, 1971) or are they merely temporally and
spatially co-occurring species (Gleason 1917, 1926, 1939)? Studies of vegetation in the Smoky Mountains
and other areas reveal species are not grouped along environmental gradients in distinct natural
associations. They are distributed individualistically according to their own physiology and life history
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Ecological Climatology
patterns. The floristic composition changes continuously along environmental gradients rather than
discretely, resulting in a continuum of populations not a series of distinct plant associations (Austin and
Smith 1989). Communities with distinct floristic composition and physiognomy are merely formed from
the overlap of species distributions, and vegetation units such as communities are arbitrary products of
classification rather than natural units clearly defined in the field. They are not emergent units, but merely
comprise plant species that coexist at a given point in space and time.
10.4
Ecosystems
10.4.1 What is an ecosystem?
A terrestrial ecosystem is the sum of individual organisms interacting with their neighbors to
acquire the resources essential for growth and development and interacting with the physical environment
to alter resource availability and the characteristics of the environment (Aber and Melillo 1991; Waring
and Running 1998). It combines all living organisms – plants, animals, and microbes – and their physical
environment into a functional system linked through a variety of biological, chemical, and physical
processes. It includes climate, living and decomposing material, soil, and the circulation of energy and
materials that link them. Ecosystem studies explicitly recognize that the biological and physical
components interact so that it is not meaningful to separate the two.
The term ecosystem was first coined in 1935 (Tansley 1935), but it took many years to articulate
what an ecosystem is and how to study it (McIntosh 1985; Golley 1993). Over time, ecosystem studies
became organized around the cycling of carbon and nutrients (Lindeman 1942; Odum 1953, 1971;
Bormann and Likens 1967; Likens et al. 1977). In this view, the structure of an ecosystem is measured by
the amount of materials such as carbon and nitrogen and their distribution among living, decaying, and
inorganic components. The functioning of an ecosystem is measured by processes such as photosynthesis,
respiration, evapotranspiration, and elemental cycling. These functions link the biotic and abiotic
environment into a dynamic system. Like communities, much of the history of ecology has been a debate
about whether ecosystems are superorganisms with emergent properties (Clements 1916, 1928; Odum
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Chapter 10 – Populations, Communities, and Ecosystems
1953, 1969, 1971) or merely the sum of its individual organisms interacting with each other and the
environment (Gleason 1917, 1926, 1939).
The spatial extent of an ecosystem depends on the particular research question posed. The largest
ecosystem is the planet, where the biosphere (all living organisms), hydrosphere (water), atmosphere,
pedosphere (soil), and lithosphere (rock) interact to regulate temperature, precipitation, atmospheric CO2,
biodiversity, water quality and other aspects of Earth’s environment. In contrast, a decomposing log on the
forest floor is a small ecosystem characterized by microbial organisms and other living creatures that
inhabit the log and by flows of materials into and out of the log. Field studies of terrestrial ecosystems are
often conducted at two scales: watersheds and stands. Watershed studies such as Hubbard Brook allow for
accurate measurements of material flows out of the system in streamflow. A less precise scale of study is
that of a stand. A stand is a relatively homogenous area of similar floristic composition, vegetation
structure, soils, and microclimate such that it can be treated as a single unit of study. A typical spatial scale
is several hectares.
10.4.2 Ecosystem structure and function
The emphasis in ecosystem studies on the standing stock and cyling of materials provides a means
to compare diverse ecosystems such as deserts, grasslands, and forests in terms of common processes, their
rates, and factors controlling these rates. For example, Figure 10.11 depicts the flow of energy in the form
of carbon among green plants (producers), animals (consumers), and soil micoorganisms (decomposers).
This representation of an ecosystem is independent of the type of ecosystem. Only the amount of carbon,
the rates of transfer, and the factors controlling these rates vary among ecosystems.
The net carbon stored within a terrestrial ecosystem is, ignoring herbivores, the difference
between carbon uptake by plants during photosynthesis, carbon loss during plant growth and maintenance
respiration, and carbon loss during decomposition of soil organic matter. Overall, the net carbon storage,
termed net ecosystem production (NEP), is given by
NEP = ( GPP − Ra ) − Rh = NPP − Rh = GPP − R
The first term in this equation, GPP, is gross primary production – the carbon uptake during
photosynthesis. The second term, Ra, is autrotrophic respiration – the carbon loss during growth and
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Ecological Climatology
maintenance respiration by plants. The difference (i.e., GPP-Ra) is net primary production (NPP). This is
the net carbon uptake by plants that can be used to grow new biomass. Measurements in grassland and
forest ecosystems suggest total plant respiration is about 50% of gross primary production (Ryan 1991).
Heterotrophic respiration (Rh) is the loss of carbon by soil microorganisms as they slowly decompose
organic debris on the forest floor and in the mineral soil. Together, autotrophic and heterotrophic
respiration comprise total ecosystem respiration (R).
Figure 10.12 shows the structure of three broadleaf deciduous and needleleaf evergreen boreal
forest ecosystems in the Prince Albert National Park in central Saskatchewan, Canada. All are even aged,
with the black spruce forest older than the other two. The trees are tallest and have the largest average
diameter in the aspen forest. Here, hazel forms a continuous shrub understory. The soil is moderately
drained loam. The jack pine forest is shorter, and the trees are smaller in diameter. Reindeer lichen covers
the ground in this forest. The soil is well-drained coarse sand. The black spruce forest has the smallest
trees, but their density is five to six times greater than the other forests. Feathermoss is the dominant
ground cover, and there is a shrub understory. The soil is a 20-30 cm thick layer of peat over poorly
drained mineral soil.
The three forests differ greatly in total carbon (excluding roots) and in their distribution of carbon.
The black spruce forest contains 2.8 times more carbon than the aspen forest and 6.5 times more than the
jack pine forest. Woody material (stems and branches) is the largest living carbon pool in each forest,
comprising 87% of living carbon in the black spruce and jack pine forests and 98% of living carbon in the
aspen forest. The aspen forest, with the tallest and largest trees, has the most wood. The greatest difference
among these forests, though, is in organic carbon on the forest floor and in mineral soil. The black spruce
forest, with its thick peat layer, contains 7 and 14 times more soil organic carbon than the aspen and jack
pine forests, respectively. Soil organic carbon accounts for 88% of total ecosystem carbon in the black
spruce forest, but only 35% in the aspen forest and 42% in the jack pine forest. Clearly, these three forests
differ in carbon storage and the ecological processes controlling carbon storage.
The carbon cycle depends on parallel flows of essential nutrients that are absorbed by plants
during growth, released to the soil in litterfall and decomposition, and washed out of the system to streams
in runoff (Figure 10.13). A productive forest, for example, may gain 500 g C m-2 yr-1 as the balance
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Chapter 10 – Populations, Communities, and Ecosystems
between carbon uptake during photosynthesis and carbon loss during respiration. This net primary
production may be allocated as 200 g C m-2 yr-1 to foliage, 175 g C m-2 yr-1 to stem sapwood, and 125 g C
m-2 yr-1 to roots. Foliage, sapwood, and roots require nitrogen in certain ratios of carbon-to-nitrogen (C:N).
Foliage has the highest nitrogen requirement. Roots and woody biomass require less nitrogen. A total of
9.3 g N m-2 yr-1 is required to grow this biomass, with 86% used in foliage growth. Leaves, twigs,
branches, and bark fall to the ground as a result of mechanical abrasion, wind, and ice storms and, in the
case of foliage, physiological processes that trigger the end of a growing season. A typical litterfall may
deposit 220 g C m-2 yr-1 on the ground, with the vast majority coming from the annual shedding of leaves.
The nitrogen held in these leaves is also returned to the ground, but not as much as when the leaves were
fresh. Typically, one-half the nitrogen in deciduous leaves is moved from foliage into the tree prior to leaf
abscission in a process known as translocation. The death of fine roots also contributes organic debris and
nutrients into the soil. This debris slowly decays over time from the activities of bacteria, fungi,
earthworms, and other microorganisms living in the soil. Each year fresh new litter pools are formed while
existing pools are slowly transformed to humus. The decomposition of organic material releases carbon to
the atmosphere and mineralizes nutrients bound in the organic material to be used again in plant growth.
10.4.3 Environmental controls of net primary production
Net primary production is influenced by stand age, species composition, and site conditions such
as temperature, soil moisture, and nutrient availability. Locally, changes in elevation and slope, by altering
temperature and moisture, have a strong effect on production. In general, annual tree production decreases
with higher elevation due to colder temperatures (Figure 10.14). The importance of nutrients and water for
net primary production has been routinely demonstrated by experiments that artificially manipulate these
conditions. In one such study, portions of a 50-year-old Douglas fir forest in the Rocky Mountains of New
Mexico were for two years either irrigated weekly throughout the growing season so that precipitation was
effectively doubled or fertilized once in spring with nitrogen and other nutrients (Figure 10.15). Production
measurements prior to treatment and in untreated control plots provided baseline comparisons in the
absence of water and nutrient addition. Over the two years of study, aboveground net primary production
increased by 13% of the pre-treatment value in the control untreated plots. Application of water and
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Ecological Climatology
nutrients greatly increased production compared with this baseline increase. Aboveground net primary
production increased by 57% in the irrigated plots and 70% in the fertilized plots compared with their pretreatment values. This increase was realized in more new foliage biomass for similar sized trees that were
irrigated or fertilized compared with untreated trees. In addition, the proportion of total net primary
production that was belowground in roots decreased from 46% in the untreated control to 31% in the
irrigated plots and 23% in the fertilized plots.
Leaf area index also influences net primary production. Production efficiency models simulate net
primary production proportional to absorbed photosynthetically active radiation times a light use
efficiency, which relates carbon gain to light absorbed (e.g., 1.5 g C per MJ) (Ruimy et al. 1994, 1996;
Maisongrande et al. 1995; Prince and Goward 1995; Prince et al. 1995; Waring et al. 1995; Goetz et al.
1999, 2000). Leaf area index influences the amount of solar radiation absorbed by the canopy. With a leaf
area index of 1 m2 m-2, only about 50% of incoming photosynthetically active radiation is absorbed by
vegetation (Figure 8.17). Absorption increases to almost 75% with a leaf area index of 2 m2 m-2 and to 95%
with a leaf area index of 4 m2 m-2. Hence, carbon uptake during gross primary production increases with
greater leaf area index. However, light absorption saturates at high leaf area index, as does the fraction of
the canopy that is sunlit (Figure 9.16). As a result, gross primary production attains a maximum value at
some leaf area index beyond which further increase in leaf area does not increase carbon uptake. In fact,
net primary production can decline because respiration loss increases as woody biomass accumulates.
Site conditions, species composition, and stand structure can create large differences among
forests in gross primary production, respiration, and the allocation of carbon into above- and belowground
production. Figure 10.16 shows net primary production for the three boreal forests described in Figure
10.12. Total net primary production in these forests ranges from 222 to 392 g C m-2 yr-1, with greater
production in the aspen stand than for black spruce or jack pine. The aspen stand allocates less net primary
production to roots (10%) than black spruce (42%) or jack pine (47%). This, combined with its greater
total production, gives the aspen forest an aboveground tree production (352 g C m-2 yr-1) that is two to
three times that of the other forests. Greater allocation of carbon to roots by needleleaf evergreen trees than
broadleaf deciduous trees is a general pattern found throughout the boreal forest.
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Chapter 10 – Populations, Communities, and Ecosystems
A detailed study of carbon allocation in six needleleaf evergreen forests along a west-to-east
transect in Oregon beginning at the Pacific coast and extending 225 km inland illustrates the effect of site
conditions on net primary production (Figure 10.17). The Coast Range and Cascade Mountains heavily
influence climate in this region. The westernmost site near the coast has a maritime climate, with cool
temperatures and 2510 mm annual precipitation. The easternmost site is located in the desert interior region
created by the rain shadow of the Cascades. At this site, annual precipitation is only 220 mm, and the
climate is continental, with hot, dry summers and cold winters. Elevation across the transect ranges from
170 m to 1460 m, with mean annual temperature of 6.0-11.2 °C. Snow is rare in the coast and valley sites
(Sitka spruce, Douglas fir), but deep in the subalpine mountain hemlock forest. Drought is common in the
eastern ponderosa pine and juniper woodlands. Across the transect, leaf area index ranges from 0.4 to 8.6
m2 m-2; gross primary production ranges from 302 to 2404 g C m-2 yr-1. The tallest forests with the densest
canopy and highest production are found to the west of the Cascade Mountains. Gross primary production
decreases and a greater portion of this is allocated to roots in the harsher alpine forest and arid woodlands
east of the Cascades.
Not all of net primary production results in biomass increment. Foliage, twigs, branches, and bark
fall to the ground as litterfall. Fine roots continually die, providing a large source of organic carbon to the
soil. Trees die and create coarse woody debris as stems topple over. Figure 10.16 gives an indication of
how much of net primary production is litterfall in boreal forests. Total detritus production ranges from
32% to 43% of aboveground net primary production. Foliage litterfall is high in all three forests. In
deciduous forests, leaf biomass turns over annually with leaf abscission in autumn. However, even
evergreen trees lose their needles, not all at once but slowly over several years. Needle longevity is
typically a few years in pines and several years or more in spruces.
Coarse woody debris in the form of standing dead trees, downed boles, and large branches is an
important component of forest ecosystems (Harmon et al. 1986). This debris is produced when wind
uproots and snaps trees, branches break, and when fire, insects, disease, and slow growth kill trees. Typical
rates of debris production vary from between 20 to 200 g m-2 yr-1, increasing to 300-3000 g m-2 yr-1 in oldgrowth coniferous forests of the Pacific Northwest region of North America. In the quaking aspen, black
spruce, and jack pine forests of Saskatchewan (Figure 10.16), coarse woody debris production is 7-11% of
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Ecological Climatology
aboveground net primary production or about 12-25 g C m-2 yr-1 (27-56 g m-2 yr-1). Logs and large
branches decay slowly over time so that the accumulated mass of coarse woody debris on the ground can
be quite large, ranging from 1100-3800 g m-2 in deciduous forests to 1000-51 100 g m-2 in coniferous
forests (Harmon et al. 1986). This typically covers 2% of the forest floor, but logs can cover 14-25% of
forest floor in old-growth coniferous forests of the Pacific Northwest.
10.4.4 Biogeochemical cycles
Elements such as nitrogen, phosphorus, potassium, calcium, magnesium, and other essential plant
nutrients reside within ecosystems and are continually recycled between soil and living organisms (Table
10.1). These elements are released for use by plants during the decay of organic material or during the
weathering of rocks (Chapter 6). Decomposition of litter is an important source of nutrients in terrestrial
ecosystems. Slow rates of decomposition result in accumulation of nutrients bound in organic debris that
are unavailable for plant growth. Many of these elements are imported into an ecosystem in small
quantities by rainfall. Others, especially nitrogen, are also obtained from gases in the atmosphere. Elements
are washed out of the soil solution into streams during storm runoff.
Figure 10.18 shows the calcium cycle for a 55-year-old forest at Hubbard Brook, which is typical
of elements without a prominent gas phase and where organic cycling, weathering, and cation exchange in
the soil solution are the important processes. Calcium is stored in living plant material, in decomposing
material on the forest floor, and in the soil, where it is either available in the soil solution or in mineral soil
and rock. Most of the calcium (99%) is contained in the soil complex. Relatively little is stored in living
plant material. The amount of calcium available annually in the soil solution for plant use is a balance of
several processes. Precipitation imports 0.22 g m-2 annually. Calcium leaches from leaves and bark as
water drips off leaves (throughfall) or flows down branches and stems (stemflow). A total of 0.67 g m-2
enters the soil solution annually in this process. Chemical exudates from roots provide 0.35 g m-2 yr-1 to the
soil solution. The two largest sources of calcium are mineralization of litter on the forest floor (4.24 g m-2
yr-1) and weathering (2.11 g m-2 yr-1). This annually available calcium is taken up by vegetation, held on
exchange sites in the soil, or lost during runoff. Eighteen percent (1.39 g m-2) of the annually available
calcium is exported from the system in runoff. The remainder (6.22 g m-2) is taken up by vegetation
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Chapter 10 – Populations, Communities, and Ecosystems
annually during growth. However, only 0.81 g m-2 yr-1 (13% of annual uptake) accumulates in plant
biomass. Sixty-five percent of the uptake (4.07 g m-2 yr-1) returns to the ground annually in aboveground
litterfall. Another
0.32 g m-2 yr-1 is lost through root mortality. Leaching from leaves and bark in
throughfall and stemflow and from roots results in a loss of 1.02 g m-2 yr-1.
The nitrogen cycle is more complex because nitrogen has a gaseous phase, occurring as diatomic
nitrogen (N2), ammonia (NH3), nitrous oxide (N2O), nitric oxide (NO), and nitrogen dioxide (NO2), and
because the nitrogen used in plant growth is the inorganic ions of nitrate (NO3-) and ammonium (NH4+).
Nitrogen is not a significant component of primary or secondary minerals so that there is little release in the
weathering of rocks. Instead, the source of nitrogen over geologic timescales is the atmosphere, and the
major stores of nitrogen in an ecosystem are bound in plants and soil organic matter. The nitrogen cycle
consists of four processes: nitrogen fixation – the conversion of atmospheric nitrogen (N2) to a usable
form; mineralization – the release of organically bound nitrogen during decomposition; nitrification – the
oxidation of ammonium to nitrite (NO2-) and nitrate; and denitrification – the reduction of nitrate to N2.
Although 78% of the atmosphere is comprised of N2, this gas is unusable by plants and must be
converted to a usable form in a process known as nitrogen fixation. Some species of plants, especially
legumes such as alfalfa, clover, peas, and beans, have symbiotic relationships with bacteria and algae that
allow for fixation of atmospheric nitrogen into organic ammonium for use by plants. Nitrogen fixation is
most prevalent in agricultural land due to the cultivation of nitrogen fixing crops, but also occurs in natural
ecosystems by some 285 species of woody plants (Barbour et al. 1999). Biological nitrogen fixation is
thought to be about 195 × 1012 g N per year, with a range of 100-290 × 1012 g N per year (Cleveland et al.
1999). Lightning strikes also fix nitrogen. Nitrogen in the form of nitrate and ammonium is also deposited
in precipitation. Annually, these atmospheric inputs are typically much less than the nitrogen released in
the mineralization of soil organic matter.
Most of the nitrogen used in plant growth comes from internal recycling and decomposition of
organic debris, which releases inorganic nitrogen in the form of nitrate and ammonium. This internal cycle
is dominated by uptake of nitrate and ammonium from the soil solution, incorporation into plant tissues,
return of nitrogen to the soil in litter, and mineralization of organically bound nitrogen, which is then
released into the soil solution. Ammonium is the first inorganic product of mineralization. This is absorbed
15
Ecological Climatology
by plants from the soil solution. Some of the ammonium is oxidized to nitrite and nitrate by bacteria in a
process known as nitrification. Energy is released in the oxidation of ammonium to nitrite and the
oxidation of nitrite to nitrate. Nitrate is absorbed by plants from the soil solution during growth. Nitrate is
more readily leached from the soil solution than ammonium because soils have a greater capacity to
exchange cations than anions. A variety of gases such as ammonia, nitric oxide, nitrous oxide, and N2 are
produced through microbial activity and released to the atmosphere. One such process is denitrification,
which occurs when bacteria convert nitrate to nitric oxide, nitrous oxide, or N2. Denitrification is a major
process that returns nitrogen to the atmosphere and has an important role in regulating the concentration of
nitrous oxide, a major greenhouse gas, in the atmosphere.
A significant portion of the nitrogen required in plant growth is internally recycled within plants
in a process known as translocation. Translocation is the withdrawal of nutrients from senescing leaves and
subsequent storage within the plant. It is common in trees, where the nitrogen is stored in woody tissue and
used for growth in the following year. Grasses and other herbaceous perennial plants also store nutrients in
underground plant parts. Table 10.2 shows the magnitude of this translocation of nutrients in the Hubbard
Brook study, where the deciduous sugar maple, beech, and yellow birch trees are dominant. One-half of the
nitrogen and 60% of the phosphorus in leaves prior to senescence are withdrawn before leaf fall. The
nutrients withdrawn from leaves and stored within the trees are available for reuse in subsequent growth. A
large portion of the annual elemental requirement can be met in this fashion. For example, 34% of the
annual nitrogen and 30% of the annual phosphorus requirements during growth are supplied from
translocated stores. This contributes to the tightness of these biogeochemical cycles. Nutrients withdrawn
prior to leaf fall are not subject to losses.
Figure 10.19 shows the nitrogen cycle for a 55-year-old forest at Hubbard Brook. In contrast to
calcium, which is almost exclusively stored in soil and rock, nitrogen is stored either in vegetation or in
debris on the forest floor. A total of 0.65 g m-2 of nitrogen, in the form of both nitrate and ammonium, is
imported annually by precipitation. Nitrogen fixation imports another 1.42 g m-2 yr-1. Little nitrogen is
added by rock weathering. Of the 2.07 g m-2 entering the ecosystem annually, 81% is retained within the
ecosystem. Only 0.40 g m-2 wash out each year into streams. This illustrates the overall tightness of the
nitrogen cycle. Leaching from foliage, bark, and roots returns both organically bound nitrogen and
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Chapter 10 – Populations, Communities, and Ecosystems
inorganic nitrogen to the soil. However, the largest source of nitrogen is aboveground litterfall, which
returns 5.42 g m-2 yr-1 to the soil. Root mortality adds a smaller amount of nitrogen. This litter decomposes
over time, first immobilizing nitrogen as microbes utilize more nitrogen than is available in the debris and
then mineralizing nitrogen for use by plants. The available inorganic nitrogen in the soil solution that is not
washed out in runoff is taken up by plants during growth (7.96 g m-2 yr-1). Only 11% (0.9 g m-2 yr-1) of this
nitrogen is retained by plants. The rest is returned to the soil in litterfall or leaching. More nitrogen
accumulates in the forest floor from nitrogen fixation, litter, and leaching than is lost during mineralization
for a net gain of 0.77 g m-2 yr-1.
10.4.5 Forest production and nutrient cycling
Ecosystem production and nutrient cycling are linked (Figure 10.20). High nutrient availability
leads to high nutrient uptake during plant growth. High concentrations of nutrients, especially nitrogen, in
foliage allow for high photosynthetic rates. This increases net primary production so that more nutrients are
returned to the soil in litterfall. The good quality of the litter allows for rapid decomposition and
mineralization, which reinforces the high nutrient availability. Low nutrient availability has the opposite
effect, reducing net primary production and nutrient return in litterfall. Litter quality is poor, having high
C:N or high lignin:N ratios, and litter decomposes slowly. Relatively few nutrients are mineralized,
reinforcing the low nutrient availability.
A study of production and nutrient cycling in the northern hardwood forests of Wisconsin
illustrates these linkages (Pastor et al. 1984). In this study area, forest composition is related to soil texture.
Sugar maple forms productive stands on silty clay loam. Less productive oak stands grow on sandy clay
loam. Pines and hemlock grow on sandy soils with low productivity. In these stands, aboveground net
primary production, litterfall, nutrient return in litterfall, nitrogen mineralization, and species composition
are all highly interrelated (Figure 10.21). Annual aboveground net primary production increases with
greater nitrogen mineralization. Needleleaf evergreen trees (red pine, hemlock, white pine) have the lowest
production and mineralization. Sugar maple has the highest production and nitrogen mineralization. Oaks
are intermediate. Nitrogen mineralization is negatively related to the C:N ratio of litter. Needleleaf
evergreen forests have the poorest quality litter and least nitrogen mineralization. Sugar maple has the
17
Ecological Climatology
lowest C:N ratio and highest nitrogen mineralization. Soil nitrogen mineralization is also positively
correlated with litter production and nitrogen and phosphorus return in litterfall. Needleleaf evergreen
forests have the lowest return of nitrogen and phosphorus. Broadleaf deciduous forests have higher
elemental return. In this region, soil texture and species composition create a nitrogen mineralization
gradient. The needleleaf evergreen trees that dominate xeric sites produce low quality litter and return little
nutrients in litterfall, leading to low mineralization and low production. Sugar maple, which grows on
mesic soils, has better quality litter, returns more nutrients in litterfall, and has high rates of nitrogen
mineralization and production.
Studies of production and nutrient cycling in the boreal forests of interior Alaska further illustrate
interactions between the carbon and nitrogen cycles (Viereck et al. 1983; Van Cleve et al. 1983a,b, 1986,
1991; Bonan 1989, 1990a,b, 1991a,b, 1993a; Bonan and Van Cleve 1992). The forest landscape near
Fairbanks, Alaska, is a mosaic of black spruce, white spruce, quaking aspen, paper birch, and balsam
poplar forests arising from variation in slope, elevation, soil parent material, and recurring disturbances
such as fires and floods (Figure 10.22). White spruce and balsam poplar form a mosaic of productive
forests along river floodplains, where erosion and deposition of sediments create warm, nutrient-rich
alluvial deposits. Black spruce forms unproductive stands on cold, wet, nutrient-poor soils underlain with
permafrost on terraces further back from the river. In the uplands, recurring fires create a mosaic of
productive white spruce, paper birch, and quaking aspen forests on warm, well-drained, nutrient-rich,
south-facing slopes and black spruce forests on cold, wet, nutrient-poor soils with permafrost. White
spruce, paper birch, quaking aspen, and balsam poplar stands have large aboveground tree biomass (11
221-16 801 g m-2 on average), aboveground tree production (366-562 g m-2 yr-1 on average), and litterfall
(155-389 g m-2 yr-1 on average). Black spruce stands accumulate much less tree biomass (4490 g m-2) and
have lower aboveground tree production (110 g m-2 yr-1) and litterfall (43 g m-2 yr-1). In these forests, fine
root production is 157-439 g m-2 yr-1 and accounts for 49% of total production in spruce forests but only
32% of total production in deciduous stands (Ruess et al. 1996). Broadleaf deciduous stands accumulate
less biomass on the forest floor (2195-5797 g m-2 on average) than spruce stands (7426-7646 g m-2).
Interactions among soil temperature, the forest floor, and litter quality control productivity and
nutrient cycling in these forests (Figure 10.23). Black spruce stands have thick forest floors between 12 cm
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Chapter 10 – Populations, Communities, and Ecosystems
and 38 cm deep. White spruce, paper birch, quaking aspen, and balsam poplar have thinner forest floors (218 cm). With its low thermal conductivity, a thick organic layer on the forest floor prevents heating in
summer, and in general soil temperature decreases as the forest floor becomes thicker. Aboveground tree
production decreases with colder soils. Black spruce stands occur on the coldest soils and have the lowest
production. White spruce and deciduous stands grow on warmer soils and have higher production. Forest
floor decomposition and nitrogen mineralization are positively correlated with soil temperature. Black
spruce stands have the lowest annual decomposition rate and nitrogen mineralization. Cold soil
temperatures in black spruce forests underlain with permafrost slow decomposition and nutrient
mineralization. This restricts tree growth while promoting the accumulation of a thick forest floor that
further cools the soil. In contrast, warm soils on permafrost-free sites enhance productivity and nutrient
cycling through more rapid decomposition and nutrient mineralization. Forest floor chemistry interacts
with soil temperature to control production and nutrient cycling. Forests growing on soils of low substrate
quality recycle material that is lower in nutrients, reinforcing the low productivity. Spruce forests have
forest floors with low nitrogen and high lignin concentrations, which further slows decomposition. These
forests have the lowest nitrogen uptake and the lowest return of nitrogen in litterfall. In contrast, more
productive broadleaf deciduous forests have forest floors with higher nitrogen and lower lignin
concentrations and return more nitrogen in litterfall.
10.4.6 Net ecosystem production
Measurement of CO2 fluxes within and above forest canopies provides a record of ecosystem
metabolism. For example, over the course of a day ecosystems typically lose carbon at night from
autotrophic and heterotrophic respiration and gain carbon during the day when stomata open and when
carbon uptake during photosynthesis exceeds respiratory losses (Figure 7.17). Consequently, the
concentration of CO2 in the near-surface atmosphere can have a strong diurnal cycle, as seen in
measurements in a quaking aspen forest (Figure 10.24). In the dormant season, when trees are leafless,
there is little diurnal variation in CO2 concentration; nor is there a strong vertical gradient. In the growing
season, however, there is a strong diurnal cycle at heights of 0.8 m and 2.3 m. Nighttime concentrations
range from 400 to 500 parts per million by volume (ppm) while daytime concentrations are about 350 ppm.
19
Ecological Climatology
The high nighttime concentrations arise from respiration losses and the occurrence of a strong temperature
inversion that suppresses the upward turbulent transport of CO2. The magnitude of this diurnal cycle
diminishes with height so that at a height of 9.5 m and above, nighttime and daytime CO2 concentrations
are similar.
Long-term flux measurements at the Harvard Forest in central Massachusetts from 1992 through
1999 illustrate seasonal and interannual changes in net ecosystem production (Goulden et al. 1996a,b,).
This area of Massachusetts is a humid continental climate characterized by warm to cool summers, cold
winters, and large seasonal variation in temperature. Red oak, red maple, and scattered nearby stands of
white pine, red pine, and hemlock dominate the particular site where the fluxes were measured. The forest
is about 50-70 years old with a canopy height of 20-24 m. Carbon uptake by the forest varied seasonally,
with carbon sequestration during the spring, summer, and autumn growing season and carbon loss during
the cold dormant season (Figure 10.25). Between 1992 and 1999, the forest typically gained 3-6 g C m-2
per day during the growing season and lost 1-2 g C m-2 per day during the non-growing season. However,
large day-to-day variation in net carbon uptake occurred during the growing season in response to weather
fluctuations, with some days having minimal uptake or even net efflux.
While only net ecosystem production was measured, this flux can be partitioned into gross
primary production and ecosystem respiration (Figure 10.26). Carbon uptake during gross primary
production was restricted to the growing season, with peak uptake in excess of 10 g C m-2 day-1. Carbon
loss during respiration occurred throughout the year, with relatively low values (1 g C m-2 day-1) during the
dormant season and higher values (greater than 5 g C m-2 day-1) during the summer when plants and
microbes were active. Gross primary production varied from year to year as result of changes in length of
growing season. Relatively small differences in the time of leaf emergence or senescence (e.g., 6 to 10
days) resulted in large differences in annual gross primary production (e.g., 50 g C m-2). Prolonged cloudy
periods in July 1992, August 1992, and August 1994 each reduced gross primary production by 40 g C m-2.
Unusually warm soils in the dormant season, arising from deep snow that insulated the ground, resulted in
high respiration losses in some years. Summer drought also increased respiration and reduced gross
primary production.
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Chapter 10 – Populations, Communities, and Ecosystems
Annual net carbon uptake between 1992 and 1999 ranged from 154 to 273 g C m-2 yr-1. This net
uptake was the difference between large carbon uptake during gross primary production and comparably
large carbon loss during respiration (Figure 10.27). As a result, small changes in annual gross primary
production or annual respiration resulted in large changes in annual net ecosystem production. For
example, the highest annual net carbon uptake occurred in 1995 (273 g C m-2 yr-1). In this year, low gross
primary production (1075 g C m-2 yr-1) was offset by low respiration (802 g C m-2 yr-1). In 1997, high
carbon uptake during gross primary production (1286 g C m-2 yr-1) was offset by high respiration (1107 g
C m-2 yr-1) so that less net carbon uptake occurred compared with 1995 (179 g C m-2 yr-1).
Not all forests gain carbon. Similar carbon fluxes have been measured at a black spruce forest in
central Manitoba (Goulden et al. 1997, 1998). The climate is subarctic, with short, cool summers and long,
cold winters. Upland areas around the tower site are dominated by dense, 10 m tall, 120-year-old black
spruce trees with a feathermoss ground cover. Lowland areas are dominated by sparse, 1-6 m tall trees with
sphagnum moss ground cover. Through the measurement period, carbon uptake during gross primary
production ranged from 5 to 8 g C m-2 day-1 in the growing season while respiration loss ranged from 5 to
10 g C m-2 day-1 (Figure 10.26). A significant portion of ecosystem photosynthesis resulted from mosses
growing on the forest floor. A similar result has been found in black spruce forests of interior Alaska,
where annual moss productivity can range from 50% of aboveground tree production to in excess of tree
production on sites with slow-growing trees (Oechel and Van Cleve 1986).
There was little photosynthetic uptake by the black spruce forest in early spring when soils were
frozen despite favorable air temperatures (Figure 10.26). Instead, photosynthetic uptake began with soil
thaw. Dry soil and high evaporative demand did not limit photosynthesis. Solar radiation was the dominant
controller of photosynthetic rates during the growing season. Photosynthesis was consistently higher
during cloudy periods than during sunny periods with the same photosynthetically active radiation. This is
because diffuse radiation penetrates the forest canopy more effectively than direct beam radiation. High
cloud cover increased the proportion of incoming solar radiation that was diffuse, allowing light to
penetrate deeper into the canopy. Similar results occur in other forests, where net CO2 uptake per unit
absorbed solar radiation has been found to be 50% greater when radiation is predominantly diffuse
compared with when it is primarily direct beam (Hollinger et al. 1994).
21
Ecological Climatology
Gross primary production and respiration fluxes in the black spruce forest were comparable to
those of the Harvard Forest, but whereas the Harvard forest gained carbon over the course of
measurements, the black spruce forest lost carbon (Figure 10.25). From the end of October 1994 to the end
of October 1997, the Harvard Forest gained 674 g C m-2 while the black spruce forest lost 53 g C m-2. In
the black spruce forest, high rates of daily gross primary production were balanced by high respiration loss.
Annually, about 800 g C m-2 were gained during gross primary production and a similar amount lost in
respiration. Net ecosystem production represents a small residual between these two large fluxes.
10.5
Landscapes
Landscapes represent another level of ecological organization, merging the concepts of
populations, communities, and ecosystems (Forman and Godron 1986; Forman 1995). A landscape is a
mosaic of communities and ecosystems formed by a gradient of environmental conditions that vary in
space and time. Mountains, with their variety of hillslopes, valleys, and soils, are one such landscape. Plant
species are distributed in arrangements of distinct patches across the landscape related to environmental
conditions and disturbance history. Patches are homogenous units of land with similar topography, soil,
microclimate, and vegetation. They are similar in concept to that of a stand. These patches are the
individual elements of the landscape and are embedded in a matrix of other patches, which forms the
pattern of the landscape. Patches occur within a framework of ecosystems. Ecosystems are the metabolic
units of the landscape and are interconnected by the movement of organisms and flows of materials across
the landscape. Forest, shrub, grass, crop, lake, stream, and cities are broad classes of ecosystems that
commonly form a landscape.
Transport of carbon among forests, streams, and lakes illustrates the linkages among ecosystems
in a landscape (Figure 10.28). Outputs of water, particulate matter, and dissolved substances from forest
ecosytems are inputs to stream ecosystems, where they flow downstream to lakes and eventually discharge
into oceans. At the Hubbard Brook Experimental Forest, Bear Brook is one of more than 20 streams of
varying size that are tributaries to Hubbard Brook, which drains the valley before entering the
Pemigewasset River. Stream width ranges from 2 m in the upper section to 4 m at the base of the stream.
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Chapter 10 – Populations, Communities, and Ecosystems
The slope of the stream bed averages 14%. The stream consists of a series of small pools formed by
organic debris dams connected by small waterfalls. Depth is variable, ranging from shallow to small pools
up to 60 cm deep. A carbon budget was constructed for a 1700 m section of the stream ecosystem with a
total surface area of 5877 m2.
A total of 620 g C m-2 enters the 1700 m section of Bear Brook annually. Carbon enters the stream
section in three ways. Upstream ecosystems contribute particulate and dissolved organic matter via
streamflow. These two inputs amount to 322 g C m-2 yr-1 (52% of total input). Adjacent trees, which form a
continuous canopy over the stream, contribute dissolved organic material that leaches from the canopy in
throughfall and particulate organic matter (i.e., leaves, twigs) that falls directly into the stream or is blown
into the stream. A total 297 g C m-2 yr-1 (48% of total input) enters the stream in this way. Direct leaffall
(154 g C m-2 yr-1) alone is 25% of total carbon input. Carbon uptake by mosses in the stream accounts for
less than 1% of total carbon input. Carbon is lost from the stream as respiration by plants, consumers, and
decomposers and as downstream export. These losses amount to 620 g C m-2 yr-1 and balance annual
carbon inputs. Microbial respiration, the dominant respiration loss, is 227 g C m-2 yr-1 so that about 36% of
the organic load in the stream is returned to the atmosphere annually. The remainder is exported
downstream as particulate and dissolved carbon for a total carbon export of 2304 kg C per year.
Mirror Lake is a 15 ha lake located near the mouth of the Hubbard Brook valley. Maximum depth
is 11 m and the average depth is about 6 m. Mirror Lake has its own watershed, covering 85 ha, that is
separate from upstream areas of the valley, but outflow from the lake drains into Hubbard Brook. The
forest of the Mirror Lake watershed is similar to other forests in the valley. Inflow to the lake is from three
small streams that collect and transport water, nutrients, and organic debris into the lake. Carbon is stored
in Mirror Lake in living organisms, particulate organic matter, dissolved organic and inorganic matter, and
in sediments. Only a small portion of the organic carbon in the water occurs in living organisms. Most
carbon is contained in sediments on the lake bottom. A total of 60 g C m-2 enters the lake annually through
streamflow, precipitation, net primary production of plants, and litterfall from surrounding trees. The vast
majority of this carbon (71%) is net uptake during plant production. Carbon carried from the surrounding
forest watershed into the lake via stream inflow is 19% of annual inputs. Annual litterfall contributes 354 g
C per meter of shoreline, with foliage 62% of total litterfall. Annual carbon exports are similar to inputs.
23
Ecological Climatology
Most (59%) is respired by zooplankton, benthic invertebrates, fish, and bacteria. A lesser amount (21%)
accumulates annually in lake sediments. The rest (19% of annual input or 1740 kg C per year) is exported
from the lake as particulate and dissolved organic matter. This material flows into Hubbard Brook, where it
eventually joins the Pemigewasset River.
10.6
Global vegetation
10.6.1 Biogeography
The structure and composition of vegetation, which at a local scale is shaped by environmental
factors such as temperature and moisture, is also influenced by global climate patterns. This is evident
when the complexity and diversity of terrestrial communities and ecosystems are reduced to a few biomes,
or broad classes of vegetation (e.g., forest, grassland, desert) that are similar in structure and composition
(Barbour and Billings 2000). The natural vegetation of Earth has a distinct geographic pattern that
corresponds to climate zones (Figure 10.29, color plate). The close correspondence between climate zones
and major vegetation zones is readily apparent as climate zones such as tropical savanna, tropical
rainforest, and tundra are named after vegetation (Figure 2.25).
Tropical evergreen forests (tropical rainforests) are the dominant vegetation in hot, wet equatorial
regions of South America, Africa, Southeast Asia, and Indonesia (Figure 10.29, color plate). In these
regions, monthly temperatures are warm year-round, precipitation is abundant, and there is little seasonal
variation in temperature or rainfall. Plants grow rapidly and continuously. Annual production is high. Trees
are tall, often higher than 30 m, and form a thick canopy of broadleaf evergreen leaves through which little
sunlight penetrates (Figure 10.30). The warm, wet conditions are optimal for decomposition so little litter
accumulates on the forest floor. Most nutrients are held in vegetation and are quickly recycled as litter
rapidly decomposes. Climate corresponds to the tropical rainforest zone (Figure 2.26, bottom). In tropical
regions that are warm year-round but have a dry season, tropical deciduous forests are common (Figure
10.29, color plate). These drought-deciduous trees lose their leaves during the dry season in response to
moisture stress. They are smaller than their rainforest counterparts and have less dense canopy coverage
(Figure 10.31).
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Chapter 10 – Populations, Communities, and Ecosystems
In drier tropical regions, generally to the north and south of rainforests, forests give way to
tropical savanna with small, widely spaced trees interspersed among tall (one to three meters) grasses
(Figure 10.32). In this tropical savanna climate, temperatures are warm throughout the year but there is a
pronounced dry season (Figure 2.26, top). Growth form reflects hot temperatures and lack of moisture. The
short vegetation, low biomass, and deep roots limit water loss during evapotranspiration. Recurring fires
are common in this dry landscape. Tropical savannas occur most prominently to the north and south of the
Amazon Basin in South America, to the north and south of the Congo Basin in Africa, east Africa, and
northern and eastern Australia (Figure 10.29, color plate).
In temperate regions, grasslands grow in dry, seasonally hot climates with annual precipitation
less than about 1000 mm (Figure 10.32). Grasslands are most prominent in the prairie of the United States
Great Plains, the steppes of Central Asia, the pampa of Argentina, and the veld of South Africa (Figure
10.29, color plate), where the climate is semi-arid (Figure 2.27, top). In cool climates, grasses with the C3
photosynthetic pathway dominate the vegetation. In warmer climates, C4 plants are dominant. Like in
savanna, the dry grasses lead to recurring fires, which enrich the soil and promote productivity.
Arid grasslands are transitional between forest and desert vegetation. Less precipitation results in
short, widely spaced desert scrub vegetation and shrubland (Figure 10.33). The arid climate of deserts is
hot and dry (Figure 2.27, bottom). Maximum daily temperatures in excess of 40 °C are common. Annual
precipitation is typically less than 250 mm and some months may have less than 5 mm rainfall. The
vegetation is a mix of plant types that have developed different mechanisms to survive in the hot, dry
environment. Winter annuals germinate in autumn or winter and flower in late spring. Summer annuals
germinate in mid-summer and flower in late summer or autumn. Both types of annuals typically germinate
after heavy rains to ensure adequate moisture. Drought-deciduous plants lose leaves during dry spells. In
contrast, evergreen shrubs grow throughout the year, even in periods of drought. Cacti and other succulents
have specialized organs to store water and use the CAM photosynthetic pathway to minimize water loss
during transpiration. Deserts occur on the eastern flanks of the subtropical high pressures near latitudes 30o
N and 30o S – in southwestern United States, North Africa, southern South America, South Africa, and
western Australia – and in regions far removed from sources of atmospheric moisture such as central Asia,
central Australia, and the Great Basin of western United States (Figure 10.29, color plate).
25
Ecological Climatology
Chaparral, also known as ‘Mediterranean’, vegetation is found primarily in Mediterranean regions
and southern California. In these regions, summers are hot and dry and winters are mild and moist (Figure
2.28, top). As a result, this vegetation grows during winter rather than summer. The plants are
characteristically short, dense woody bushes with thick, waxy, evergreen leaves (Figure 10.34). The oily
nature of these bushes promotes wildfire.
Greater annual precipitation supports forest. Deciduous and evergreen trees form dense,
productive temperate forests in eastern United States and the Pacific Northwest, in regions of central and
southern Europe, and eastern China (Figure 10.29, color plate). In these humid subtropical, marine, and
warm summer humid continental climates, rainfall exceeds 1000 mm per year, summers are long and
warm, and winters are cool to cold (Figures 2.28, 2.29). Moderate to pronounced seasonality is a dominant
feature of climate. Trees can be as tall as in tropical forests, but are composed primarily of broadleaf
deciduous species, which drop their leaves in winter, and needleleaf evergreen species (Figure 10.35). In
the Pacific Northwest region of North America, the mild temperatures and abundant precipitation support
thick, luxurious, extremely productive forests (Figure 10.35).
Farther north, in Alaska, northern Canada, northern Europe, and northern Russia, where the
winters are bitterly cold and the summers are cool and short, the natural vegetation is a mix of evergreen
and deciduous trees that form boreal forests or taiga (Figure 10.29, color plate). The climate is cool
summer humid continental or subarctic (Figure 2.29). Annual production and decomposition are low in this
cold landscape. Trees are predominantly needleleaf evergreen and are shorter and more open than their
temperate counterparts (Figure 10.36). Similar forests occur in high mountain ranges such as the Rocky
Mountains, Cascade Mountains, and Sierra Nevada Mountains of western U.S. In the extreme cold of
Siberia, deciduous needleleaf trees that drop their needles in winter are common.
At very high latitudes, between about 65o N and 70o N, the boreal forest gives way to the treeless
tundra comprised of grass-like sedges, dwarf shrubs, lichens, and mosses (Figure 10.37). Here, winter
temperatures are extremely cold, the growing season is short (two months or so), and soils are frozen yearround (Figure 2.30). Only the upper 50 cm or so of soil thaws during summer. Frozen soil acts as an
impervious layer, impeding drainage and creating waterlogged soil. Cold, wet soil restricts root growth,
decomposition, and plant productivity.
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Chapter 10 – Populations, Communities, and Ecosystems
Temperature and precipitation are common climate variables that correlate with biogeographic
patterns (Figure 10.38). Temperature is important because sufficient warmth, but not excessive heat, is a
prerequisite for the biochemical reactions that support life. Water is important because 80-90% of the
weight of a plant is water. Two environmental gradients are evident. This first is a latitudinal gradient from
tropical forest to temperate forest to boreal forest to arctic tundra. This reflects increasingly cold climates
with northern latitudes. Precipitation forms a second axis for vegetation differentiation. In the tropics,
vegetation changes from rainforest to seasonal forest to savanna to desert as annual precipitation decreases.
In temperate regions, forests give way to grasslands, shrublands, and deserts as precipitation decreases.
Evapotranspiration is another important determinant of where plants grow (Woodward 1987a;
Stephenson 1990). Evapotranspiration causes a typical plant to lose one liter of water for every one to five
grams of biomass produced (Table 9.2). Evapotranspiration is an integrated measure of temperature and
precipitation. Temperature indicates how much energy is available to evaporate water. Precipitation
determines the amount of water that can evaporate. In his pioneering research in the mid-1900s, Charles
Thornthwaite demonstrated a relationship between evapotranspiration and plant distributions
(Thornthwaite 1948; Thornthwaite and Mather 1955, 1957). Less water is lost via evapotranspiration in
cool climates than in warm climates. As a result, less precipitation is needed to support plant growth.
Conversely, more precipitation is needed to support plant growth in hot climates with high potential
evapotranspiration loss. For example, productive forests grow in eastern United States, where annual
precipitation ranges from 1000 mm to 1500 mm. In the tropics, a similar amount of rain creates a semi-arid
climate and supports savanna.
Annual temperature, precipitation, and evapotranspiration are three variables commonly used in
ecological classifications of bioclimatic regions (Woodward 1987a; Prentice et al. 1992; Neilson 1995;
Foley et al. 1996; Haxeltine and Prentice 1996; Kucharik et al. 2000). One such scheme is the
classification into Holdridge life zones (Figure 10.39). This scheme depicts life zones as hexagons formed
by the intersection of mean annual biotemperature and annual precipitation. For example, wet tundra
occurs with temperatures between 1.5 °C and 3.0 °C and precipitation of 250-500 mm. Wet tropical forest
occurs with temperatures greater than 24 °C and precipitation between 4000 mm and 8000 mm.
Evapotranspiration, expressed as the ratio of annual potential evapotranspiration to annual precipitation,
27
Ecological Climatology
forms a third classification dimension. For example, wet tundra and wet tropical forest are perhumid
despite a more than tenfold difference in precipitation because of the lower demand for water in the cold
tundra environment. The Holdridge scheme has been widely used in ecological studies to map the global
distribution of vegetation in response to past and future climates (Emanuel et al. 1985; Prentice 1990;
Prentice and Fung 1990; T. Smith et al. 1992a,b).
The diversity of plant species is strongly related to climate. The diversity of tree species in North
America, Europe, and eastern Asia generally declines with greater distance from the tropics and increases
with warmer temperature, greater precipitation, and greater evapotranspiration (Currie and Paquin 1987;
Adams and Woodward 1989; Kleidon and Mooney 2000). Many theories exist for why species diversity is
high in the tropics (Huston 1994; Waide et al. 1999). It reflects historical events such as extinction or
survival in refugia during past climates. It reflects different rates of disturbances. Recurring disturbances of
intermediate frequency provide a variety of habitats ranging from clearings to old-growth forests that allow
species to coexist. It also reflects a favorable climate that promotes high productivity, diverse habitats, and
allows numerous species to coexist.
10.6.2 Net primary production
Terrestrial ecosystems store large quantities of carbon and play an important role in controlling
the concentration of CO2 in the atmosphere. The total carbon stored in plant biomass is estimated to be 561
× 1015 g (Table 10.3). Forests contain three-quarters of this carbon, with tropical wet forests (28%) and
boreal forests (25%) accounting for more than one-half of the world’s plant carbon. Net primary
production is estimated to be 60 × 1015 g C yr-1. Three ecosystems – cropland (20%), tropical savanna
(18%), and tropical wet forest (14%) – account for 52% of the world’s annual carbon uptake by terrestrial
vegetation. Together, forests comprise 42% of terrestrial net primary production.
Temperature, precipitation, and evapotranspiration regulate net primary production at the global
scale (Figure 10.40). Across a variety of ecosystems from tundra to tropical rainforest, annual net primary
production increases with warmer climate. Plant production is low in extremely cold climates, increasing as
mean annual air temperature increases. In very warm climates, excessive temperatures, often combined
with low water availability, limit production. In arid climates, there is a near linear increase in production
28
Chapter 10 – Populations, Communities, and Ecosystems
as precipitation increases. Productivity plateaus in more humid climates where water no longer limits
production. The effect of temperature and water on plant production is clearly evident in the relationship
with evapotranspiration. Sites with low annual evapotranspiration have low productivity because
temperatures are cold (e.g., tundra) or water is limiting (desert). As evapotranspiration increases,
temperature or water no longer limits production.
Satellite-derived measures of plant physiological activity provide a means to examine net primary
production in terrestrial ecosystems worldwide (Figure 10.41, color plate). The strong relationship between
climate and plant productivity is clear. Highest productivity occurs in the tropical rainforests of South
America, Africa, and Southeast Asia, temperate forests of eastern United States and the Pacific Northwest,
Europe, eastern China, Japan, and eastern Australia. Arctic tundra and portions of the boreal forest have
low productivity. Lowest productivity occurs in deserts and semi-arid vegetation.
Changes in net primary production and allocation in response to climate are reflected in the
structure of vegetation (Figure 10.42). As annual precipitation increases, vegetation stature changes from
short grasses and shrubs to taller trees. There is also a change from sparse canopies with low leaf area
index to denser canopies with large leaf area. Similar structural changes occur locally, such as along the
prairie-forest border in Wisconsin, where tree height decreases and the canopy opens as soils become
progressively drier (Aber et al. 1982). These changes are a manifestation of the dynamic balance among
precipitation, soil water, and leaf area (Grier and Running 1977; Nemani and Running 1989; Woodward
1987a, 1993a). There is a maximum leaf area index for which precipitation is balanced by
evapotranspiration.
Root profiles and the ratio of belowground-to-aboveground biomass (called the root:shoot ratio)
show how vegetation structure changes in different environments (Jackson et al. 1996, 2000; Canadell et
al. 1996) Tundra, boreal forest, and temperate grassland have the shallowest root profiles, with 83-93% of
root biomass in the top 30 cm of soil (Figure 10.43). Deserts and temperate coniferous forests have the
deepest root profiles, with only about 50% of roots in the top 30 cm. Shallow-rooted plants are most
prominent in the arctic, where permafrost and poorly-drained soils restrict root growth. Plants growing in
this cold environment (tundra, cold desert) have the highest ratio of root-to-shoot biomass (6.6 and 4.5,
respectively). Temperate grasslands also have high root:shoot ratio. Woody vegetation, with large tap
29
Ecological Climatology
roots, tends to have deep root profiles. For temperate and tropical trees, 26% of root biomass is in the top
10 cm, 60% is in the top 30 cm, and 78% is in the top 50 cm. In contrast, grasses have 44% of their root
biomass in the top 10 cm and 75% in the top 30 cm. Forests typically have low root:shoot ratios (0.18 to
0.34), reflecting their large aboveground woody biomass.
Although most root biomass is located in the upper 50-100 cm of soil, roots can extend much
deeper (Figure 10.43). Tundra plants are the shallowest rooted, extending only to a depth of 50 cm on
average. Desert and tropical savanna plants have roots 10-15 m deep. Deep-rooted plants are common in
woody and herbaceous species across most terrestrial biomes. On average, roots of trees extend 7.0 m
deep, shrubs 5.1 m, and herbaceous perennials 2.6 m (Canadell et al. 1996). Plants from arid and
Mediterranean climates have the deepest rooting depths. The roots of some deserts plants have been found
to depths of 53-68 m. While these deep roots are a relatively small portion of total root biomass, they are
important hydrologically. Deep roots allow plants to tap water reserves deep in the ground, providing a
supply of water to protect against intermittent droughts.
10.6.3 Litterfall and soil carbon
Carbon storage in soil is typically comparable to or larger than plant biomass (Table 10.4). Total
soil organic matter is estimated to be 1455 × 1015 g C – almost three times that in plants. Forests contain
40% of this carbon. Cold or wet ecosystems (tundra, wetlands) account for 21%. Savanna and grassland
account for another 16%. On a per unit area basis, wetlands, with low rates of decomposition in
waterlogged soil, contain on average 68.6 kg C m-2. Temperate grasslands also contain a vast amount of
soil carbon (19.2 kg C m-2). Despite their low annual production, boreal forest and tundra ecosystems
contain large amounts of soil carbon because of low decomposition rates in cold, wet soil. Soil carbon
represents the balance between litter input and decomposition.
Litter production is related to net primary production. More productive ecosystems produce more
litter each year. Across a variety of broadleaf, needleleaf, evergreen, and deciduous forests ranging from
tropical to boreal, annual litter production increases with warmer and wetter climates (Figure 10.44).
Tropical broadleaf forests have the highest litterfall – about 950 g m-2 yr-1. Boreal needleleaf evergreen
30
Chapter 10 – Populations, Communities, and Ecosystems
forests have the lowest litter production – only 243 g m-2 yr-1. Litter production in temperate forests ranges
from 314 to 648 g m-2 yr-1.
Across a wide range of climates and plant communities, decomposition depends strongly on
evapotranspiration and litter quality (Meentemeyer 1978; Pastor and Post 1986; Vogt et al. 1986;
Matthews 1997). For example, a study of the decay of pine needles placed in 26 sites from arctic to tropical
and reflecting a wide variety of natural and managed ecosystems found annual decomposition increased
with warmer and wetter climates (Figure 10.45). Annual evapotranspiration was the single best predictor of
decomposition. Litter placed in sites with high evapotranspiration (tropical forest) lost about 40% of its
mass over a year while litter in sites with low evapotranspiration (desert, tundra) lost only about 10% of its
mass. Because annual decomposition increases with warmer and wetter climates, residence time of soil
organic material (i.e., the number of years required for material to decompose) decreases with warmer
temperature and greater annual precipitation (Figure 10.46). Residence time is as short as 0.4 to 2.4 years
in tropical forests and as high as 60 years in boreal needleleaf evergreen forests.
10.6.4 Global carbon cycle
Through gross primary production, autotrophic respiration by plants, and heterotrophic respiration
by soil microorganisms, terrestrial ecosystems influence the amount of CO2 in the atmosphere. Calculations
based on measurements of CO2 in the atmosphere suggest the Northern Hemisphere temperate and boreal
forests store about 2 × 1015 g C per year (Tans et al. 1990; Ciais et al. 1995, 2000; Keeling et al. 1996;
Prentice et al. 2000a). Figure 10.47 shows how this net annual uptake is achieved. Like that shown in
Figure 4.11, this net uptake is based on gross primary production of 120 × 1015 g C yr-1, with 50% returned
to the atmosphere during plant respiration. The remaining carbon (60 × 1015 g C yr-1) is used in net
primary production to increase plant biomass. The two views of the global carbon cycle differ in the longterm fate of this carbon. In contrast to Figure 4.11, carbon released in microbial respiration does not
balance carbon gained in net primary production. The efflux is estimated to be 50 × 1015 g C yr-1, resulting
in a net ecosystem storage of 10 × 1015 g C yr-1. Some of this carbon is lost during fires, timber harvesting,
insect attacks, or other disturbances that kill trees. The remaining carbon is the long-term carbon storage.
31
Ecological Climatology
This net biome production (1-2 × 1015 g C yr-1) is only a small fraction of the initial carbon uptake during
photosynthesis.
The metabolic activity of terrestrial ecosystems is clearly evident in seasonal variations in
atmospheric CO2 concentrations. At Mauna Loa, Hawaii, atmospheric CO2 concentration varies by several
parts per million over the course of a year, with high concentration in winter and low concentration in
summer (Figure 10.48). This is in response to the seasonal growth of terrestrial ecosystems worldwide,
which absorb CO2 during the growing season and respire CO2 during the dormant season (Tucker et al.
1986; D’Arrigo et al. 1987; Randerson et al. 1997). This seasonal pulse of CO2 is seen in measurements
made throughout the world (Figure 10.49). The seasonal change in atmospheric CO2 is greatest in high
latitudes of the Northern Hemisphere and declines with latitudes closer to the equator. South of the equator,
there is virtually no seasonality to atmospheric CO2.
10.6.5 Global terrestrial ecosystem models
Numerous physiologically based ecosystem process models simulate net primary production,
decomposition, and nutrient availability in relation to stand structure, species composition, and site
conditions (Running and Coughlan 1988; Running and Gower 1991; Raich et al. 1991; McGuire et al.
1992; Melillo et al. 1993; Running and Hunt 1993; Potter et al. 1993; Pan et al. 1996; Foley et al. 1996;
Churkina and Running 1998; Kucharik et al. 2000). These models include light, temperature, water, and
nutrient limitations to net primary production, allocate carbon to grow foliage, stems, and roots, and
decompose litter (Figure 10.50). Foley et al. (1996), Kaduk and Heimann (1996), White et al. (1997),
Botta et al. (2000), and Kucharik et al. (2000) are examples of phenology schemes used in these models.
Running and Gower (1991), Foley et al. (1996), Friedlingstein et al. (1999), and Kucharik et al. (2000) are
examples of allocation schemes used in vegetation models. These models can be used to gain insight into
the physiological processes determining observed relationships of net primary production with
temperature, precipitation, and evapotranspiration (Figure 10.40). Such relationships, rather than being
parameterizations of net primary production, are important validations of the models (Bonan 1993b; Foley
1994).
32
Chapter 10 – Populations, Communities, and Ecosystems
Ecosystem models differ greatly in how they calculate environmental conditions such as soil water
and temperature, how these conditions individually affect metabolic processes, and how multiple resource
limitations affect net primary production and carbon allocation. Some models place great emphasis on
physical control (e.g., light absorption, soil water, soil temperature) of ecosystem processes. Others
emphasize biogeochemical controls such as nutrient availability. As a result, simulated carbon uptake and
storage differ greatly among models (VEMAP 1995; Schimel et al. 1997; Cramer et al. 1999; Kicklighter
et al. 1999a; Schloss et al. 1999; Churkina et al. 1999). In general, models produce similar results in
temperate regions where temperature is the main controlling variable and differ where soil moisture limits
net primary production due to different implementations of water availability and its effect on production.
These models also differ in the simulation of net primary production in boreal forests in summer due to
differences in the direct effects of temperature on metabolic processes and indirect effects of snow and
permafrost on soil temperature.
33
Ecological Climatology
10.7
Tables
Table 10.1. Standing stocks of nutrients in a 55-year-old northern hardwood forest in the Hubbard Brook
Experimental Forest
Nutrient (g m-2)
Pool
Ca
Mg
Na
K
N
S
P
Aboveground biomass
38.3
3.6
0.16
15.5
35.1
4.2
3.4
Belowground biomass
10.1
1.3
0.38
6.3
18.1
1.7
5.3
Forest floor
37.2
3.8
0.36
6.6
125.6
12.4
7.8
Source: Data from Likens et al. (1977, p. 101).
34
Chapter 10 – Populations, Communities, and Ecosystems
Table 10.2. Translocation of nutrients (g m-2 yr-1) in a 55-year-old northern hardwood forest in the
Hubbard Brook Experimental Forest
N
P
K
Ca
Mg
Foliage before senescence (B)
7.1
0.56
2.8
2.0
0.49
Foliage after senescence (A)
3.3
0.21
1.4
2.5
0.41
Foliage leaching during senescence (L)
0.2
0.01
1.1
0.2
0.06
Translocation (B-A-L)
3.6
0.34
0.3
-0.7
0.02
Source: Data from Ryan and Bormann (1982).
35
Ecological Climatology
Table 10.3. Plant biomass and net primary production for the world’s terrestrial vegetation
Vegetation type
Area
Mean plant
Total carbon
Mean net primary
Total net primary
(1012 m2)
biomass
(1015 g C)
production
production
(g C m-2 yr-1)
(1015 g C yr-1)
(g C m-2)
Forest
Tropical wet
10.4
15 000
156
800
8.3
Tropical dry
7.7
6 500
50
620
4.8
Temperate
9.2
8 000
74
650
6.0
Boreal
15.0
9 500
142
430
6.4
Tropical savanna
24.6
2 000
49
450
11.1
Temperate
15.1
3 000
45
320
4.8
Tundra
11.0
800
9
130
1.4
Wetland
2.9
2 700
8
1 300
3.8
Desert
18.2
300
6
80
1.4
Cropland
15.9
1 400
22
760
12.1
Rock and ice
15.2
0
0
0
0.0
Total
145.2
grassland
561
60.1
Source: Data from Schlesinger (1997, p. 142). The numbers in this table are similar to more recent
estimates (Field et al. 1998).
36
Chapter 10 – Populations, Communities, and Ecosystems
Table 10.4. Soil organic matter for the world’s terrestrial vegetation
Vegetation type
Area
Mean soil carbon
Total soil carbon
(1012 m2)
(g C m-2)
(1015 g C)
Tropical
24.5
10 400
255
Temperate
12.0
11 800
142
Boreal
12.0
14 900
179
Shrubland
8.5
6 900
59
Tropical savanna
15.0
3 700
56
Temperate grassland
9.0
19 200
173
Tundra
8.0
21 600
173
Wetland
2.0
68 600
137
Desert
18.0
5 600
101
Cropland
14.0
12 700
178
Barren
24.0
100
2
Total
147.0
Forest
1 455
Source: Data from Schlesinger (1997, p. 157). See also Post et al. (1982).
37
Ecological Climatology
10.8
Figure Legends
Figure 10.1. Population structure, community composition, and ecosystem structure and function in a small
watershed of the Hubbard Brook Experimental Forest circa 1956 to 1965. Top left: Community
composition in terms of the abundance of trees with a diameter at breast height (DBH) greater than 10 cm.
Top right: Size structure of sugar maple, beech, and yellow birch populations. Middle: Population density
with respect to elevation (lower, middle, and upper third of slopes). Graphs show the density of trees with
a diameter greater than 10 cm. Bottom: Biomass distribution for foliage and fruit, branches, stems, and
roots. The figure on the left shows biomass (g m-2) while the figure on the right shows net primary
production and decomposition (g m-2 yr-1). Boxes are proportional in size to pools and fluxes. Data from
Bormann et al. (1970) and Whittaker et al. (1974).
Figure 10.2. Tolerance of a species in relation to environmental conditions. Top left: Response to soil
water. Top right: Response to soil water and temperature showing the range of conditions for which growth
can occur and is optimal. Bottom: Range of conditions along three dimensions (soil water, temperature,
light) over which growth is possible.
Figure 10.3. Pollen abundance (%) in relation to July temperature and annual precipitation. Left: Oaks.
Middle: Northern pines. Right: Spruces. Adapted from Webb et al. (1993).
Figure 10.4. Probability of occurrence of Eucalyptus pauciflora (snow gum) growing on granite rock in
southeastern Australia in relation to mean annual temperature and annual precipitation. Adapted from
Austin et al. (1990).
Figure 10.5. Relationship between physiological tolerances and plant abundance along environmental
gradients. Left: Possible plant strategies for light and water use represented as a continuum (top) and as 15
discrete plant functional types (bottom). The shaded triangle shows possible strategies based on tradeoff
between light and water use. Right: Biomass of five plant functional types along a moisture gradient when
38
Chapter 10 – Populations, Communities, and Ecosystems
grown in monocultures (top) and mixed stands of all 15 plant types (bottom). Numbers denote plant
functional types. Adapted from Smith and Huston (1989).
Figure 10.6. Bluebunch wheatgrass growth and survival in the presence of cheatgrass. Top: Survival and
height of bluebunch wheatgrass under sparse, moderate, and dense amounts of cheatgrass. Middle:
Maximum root length of both grasses when grown in mixtures of 273 plants of varying amounts of
bluebunch wheatgrass and cheatgrass. Bottom: Temporal dynamics of root growth for bluebunch
wheatgrass and cheatgrass grown in the absence of competition. Data from Harris (1967).
Figure 10.7. Distribution of tree species, as a percent of the number of trees in the stand, in relation to
elevation in the Great Smoky Mountains circa 1940s and 1950s. Top: Mesic sites. Bottom: Xeric sites.
Data from Whittaker (1956).
Figure 10.8. Topographic distribution of vegetation types for a west-facing slope in the Great Smoky
Mountains circa 1940s and 1950s. Adapted from Whittaker (1956).
Figure 10.9. Dominance-diversity curves for beech forests growing on sheltered south slopes above 1372
m and cove forests between 762 and 1067 m in the Great Smoky Mountains circa 1940s and 1950s.
Graphs show the relative importance of species, in terms of percent of trees in the stand, from most
important to least important. Data from Whittaker (1956).
Figure 10.10. Population distributions for four tree species in relation to elevation and moisture in the
Great Smoky Mountains circa 1940s and 1950s. Contour lines show species abundance in terms of percent
of trees. Major vegetation types are delimitated. Adapted from Whittaker (1956).
Figure 10.11. Generalized representation of a terrestrial ecosystem into components and the carbon flows
that connect these components.
Figure 10.12. Stand structure and carbon storage for quaking aspen, black spruce, and jack pine forests in
central Saskatchewan. Living biomass is aboveground only. Standing dead, forest floor, and mineral soil
39
Ecological Climatology
carbon comprise total detritus (decaying material). Boxes are proportional in size to carbon pools and have
units of g C m-2. Data from Gower et al. (1997).
Figure 10.13. Carbon and nitrogen cycling illustrated for a typical boreal broadleaf deciduous forest. Top:
Net primary production (500 g C m-2 yr-1) as the balance between photosynthesis and respiration. Middle
left: Allocation of net primary production to foliage, stem, and root growth and corresponding nitrogen
requirements to support this growth. Middle right: Above- and belowground litterfall returns carbon and
nitrogen to the ground. Bottom: Litter decays over time releasing carbon to the atmosphere and
mineralizing nitrogen to support plant growth.
Figure 10.14. Aboveground tree production in relation to elevation. Top: Great Smoky Mountain forests
without evergreen heath vegetation circa mid-1960s. Data from Whittaker (1966). Bottom: Hubbard Brook
Experimental Forest circa 1956-65. Data from Whittaker et al. (1974).
Figure 10.15. Effect of fertilizer and irrigation on the growth of Douglas fir trees. Top: New foliage
biomass in relation to stem diameter of an individual tree. Bottom: Aboveground net primary production
of whole stand over three years. Data from Gower et al. (1992).
Figure 10.16. Annual productivity for quaking aspen, black spruce, and jack pine forests in central
Saskatchewan. Left: Net primary production and its allocation into aboveground and belowground
production. Right: Aboveground net primary production and its allocation into biomass increment (shaded)
and litterfall (dashed). Data from Gower et al. (1997) and Steele et al. (1997).
Figure 10.17. Climate, stand structure, and carbon allocation for six forests along a west-to-east transect in
Oregon between latitudes 44 °N and 45 °N beginning at the coast and extending 225 km inland. The
elevation profile is the approximate elevation based on a 5-minute dataset at latitude 44°33.25′ N. Carbon
allocation sums to gross primary production, given at the top of each bar chart. Data from Runyon et al.
(1994) and Williams et al. (1997).
40
Chapter 10 – Populations, Communities, and Ecosystems
Figure 10.18. Annual calcium budget for a 55-year-old northern hardwood forest ecosystem in the
Hubbard Brook Experimental Forest. Boxes show major stores. Arrows show annual fluxes. Data from
Likens et al. (1977, p. 96).
Figure 10.19. Annual nitrogen budget for a 55-year-old northern hardwood forest ecosystem in the
Hubbard Brook Experimental Forest. Boxes show major stores. Arrows show annual fluxes. Nitrogen is
partitioned into organic and inorganic forms as appropriate. Data from Bormann et al. (1977), Likens et al.
(1977, p. 101), and Bormann and Likens (1979, p. 76).
Figure 10.20. Feedback between nutrient availability and net primary production. Top: High nitrogen
availability reinforces high production. Bottom: Low nitrogen availability reinforces low production.
Figure 10.21. Forest production and nutrient cycling on Blackhawk Island, Wisconsin. Top left: Annual
aboveground net primary production in relation to annual nitrogen mineralization. Top right: Annual
nitrogen mineralization in relation to litter C:N ratio. Bottom left: Annual nitrogen return in litterfall in
relation to annual nitrogen mineralization. Bottom right: Annual phosphorus return in litterfall in relation to
annual nitrogen mineralization. Data from Pastor et al. (1984).
Figure 10.22. Aboveground biomass and net primary production for black spruce, white spruce, paper
birch, quaking aspen, and balsam poplar forests near Fairbanks, Alaska. Left: Distribution of forest stands
in relation to topography. Right: Average aboveground biomass and net primary production for each forest
type. Boxes are proportional in size to pools and fluxes. Data from Viereck et al. (1983) and Van Cleve et
al. (1983b).
Figure 10.23. Controls of net primary production and nutrient cycling in boreal forests near Fairbanks,
Alaska. Top left: Forest floor thickness in relation to soil degree-days above 0 °C accumulated at a 10 cm
depth from May 20 to September 10. Top right: Annual aboveground tree production in relation to soil
degree-days. Middle left: Annual forest floor decomposition in relation to soil degree-days. Middle right:
Annual aboveground tree production in relation to annual nitrogen mineralization. Bottom left: Annual
nitrogen requirement in aboveground tree production in relation to the ratio of forest floor biomass to
41
Ecological Climatology
nitrogen. Bottom right: Ratio of litterfall biomass to nitrogen in relation to forest floor biomass:nitrogen
ratio. Forest floor and soil degree-days (top left) are from Viereck et al. (1983) for 8 black spruce, 4 white
spruce, 2 paper birch, 2 quaking aspen, and 2 balsam poplar stands. All other data are averages for the five
forest types (Van Cleve et al. 1983b; Fox and Van Cleve 1983). Van Cleve et al. (1983b) show data for
individual stands. The overall patterns and conclusions are the same.
Figure 10.24. Average diurnal cycle of CO2 concentration within and above a 70-year-old quaking aspen
forest with a canopy height of 21.5 m. Top: Leafless. Bottom: Full leaf. Adapted from Yang et al. (1999).
Figure 10.25. Net ecosystem production over several years for Harvard Forest, Massachusetts (left) and a
black spruce forest in Manitoba, Canada (right). Negative values indicate carbon uptake. Positive values
show carbon loss. Top: Daily net ecosystem production. Bottom: Cumulative net ecosystem production.
Data from Goulden et al. (1996a,b, 1997, 1998).
Figure 10.26. Daily gross primary production (negative fluxes) and ecosystem respiration (positive fluxes)
over several years. Top: Harvard Forest. Bottom: Black spruce forest, Manitoba. Data from Goulden et al.
(1996a,b, 1997, 1998).
Figure 10.27. Annual gross primary production, ecosystem respiration, and net ecosystem production for
Harvard Forest. In this diagram, GPP is shown as a positive value so that net ecosystem production is the
difference between the GPP and R lines, shown as the shaded area and separately on the right axis.
Figure 10.28. Annual carbon flow among forest, stream, and lake ecosystems in the Hubbard Brook
Experimental Forest. Boxes show biomass (g C m-2). Arrows show fluxes (g C m-2 yr-1). Dotted arrows
denote carbon transfers from one ecosystem to another. POC, particulate organic carbon. DOC, dissolved
organic carbon. DIC, dissolved inorganic carbon. Adapted from Whittaker (1975, p. 286) with data for
forest (Whittaker et al. 1974), Bear Brook (Fisher and Likens 1973), and Mirror Lake (Likens 1985, pp.
40-53, pp. 292-301). For similarity with Mirror Lake data, forest and Bear Brook data were converted from
biomass to carbon using a carbon fraction of 0.45.
42
Chapter 10 – Populations, Communities, and Ecosystems
Figure 10.29. Biogeography of natural vegetation prior to human land use. Data from Ramankutty and
Foley (1999a).
Figure 10.30. Tropical rainforest, Costa Rica. Top: Canopy. Reproduced from Barbour and Billings (1988,
p. 375). Bottom: Interior. Reproduced from Daubenmire (1978, p. 262).
Figure 10.31. Tropical deciduous forest, Costa Rica. Top: Full leaf in rainy season. Bottom: Leafless
condition. Reproduced from Daubenmire (1978, p. 242).
Figure 10.32. Top: Tropical savanna, Mexico. Reproduced from Daubenmire (1978, p. 236). Bottom:
Temperate grassland, North Dakota. Reproduced from Daubenmire (1978, p. 192).
Figure 10.33. Semi-desert shrub and desert scrub vegetation. Top: Sagebrush-bluebunch wheatgrass,
Washington. Reproduced from Daubenmire (1978, p. 205). Middle: Creosote bush, Mojave Desert.
Reproduced from Daubenmire (1978, p. 224). Bottom: Sonoran Desert. Reproduced from Daubenmire
(1978, p. 226).
Figure 10.34. Chaparral vegetation, California. Reproduced from Daubenmire (1978, p. 181).
Figure 10.35. Top: Oak-hickory forest, Tennessee. Reproduced from Daubenmire (1978, p. 133). Bottom:
Douglas fir-western hemlock forest, Oregon. Reproduced from Barbour and Billings (1988, p. 108).
Figure 10.36. Top: Spruce forest, British Columbia. Reproduced from Daubenmire (1978, p. 94). Bottom:
Subalpine spruce-fir forest, Colorado. Reproduced from Barbour and Billings (1988, p. 87).
Figure 10.37. Top: Shrub tundra, Alaska. Reproduced from Barbour and Billings (1988, p. 12). Bottom:
Alpine treeline, Colorado. Reproduced from Barbour and Billings (1988, p. 92).
Figure 10.38. Generalized relationships among major plant formations, mean annual temperature, and
annual precipitation. Adapted from Whittaker (1975, p. 167).
Figure 10.39.
Holdridge vegetation classification showing relationships among mean annual
biotemperature, annual precipitation, annual potential evapotranspiration, and vegetation type. Annual
43
Ecological Climatology
average biotemperature is calculated from monthly temperature, converting temperature below 0 °C to
zero. Temperature demarcations run horizontally ranging from 1.5 °C to 24 °C. Precipitation lines are
parallel to the potential evapotranspiration ratio axis and range from 62.5 mm to 8000 mm. Potential
evapotranspiration lines are parallel to the precipitation axis and range from 0.125 to 32. Adapted from
Holdridge (1967).
Figure 10.40. Annual net primary production (above- and belowground) in relation to climate. Top: Mean
annual temperature (Lieth 1975). Middle: Annual precipitation (Lieth 1975). Bottom: Annual
evapotranspiration (Rosenzweig 1968). These relationships have been used to model global vegetation
productivity (Lieth 1975; Esser 1987; Friedlingstein et al. 1992; Dai and Fung 1993; Kaduk and Heimann
1994; Post et al. 1997).
Figure 10.41. Plant production as measured by satellite using the Normalized Difference Vegetation Index
for the period 1982 to 1993. High numbers indicate regions of high production. Low numbers indicate
regions of low production. Data are the NOAA/NASA Pathfinder data product archived at the NASA
Goddard Space Flight Center Earth Sciences Distributed Active Archive Center (GSFC DAAC).
Figure 10.42. Vegetation height and leaf area index in relation to annual precipitation. Adapted from
Woodward (1993a, p. 97).
Figure 10.43. Relative root abundance for tundra, boreal forest, temperate grassland, tropical deciduous
forest, tropical evergreen forest, temperate deciduous forest, tropical savanna, desert, and temperate
coniferous forest. Graphs show cumulative root distribution as a function of soil depth. Text boxes show
the proportion of roots in the top 30 cm, average maximum rooting depth, and the average ratio of
root:shoot biomass. Root:shoot ratios for desert are given separately for warm and cold deserts. Data from
Jackson et al. (1996) and Canadell et al. (1996).
Figure 10.44. Annual litterfall in forests in relation to climate. Data are means for 13 forest types. Top:
Mean annual temperature. Bottom: Annual precipitation. Data from Vogt et al. (1986). See also Matthews
(1997).
44
Chapter 10 – Populations, Communities, and Ecosystems
Figure 10.45. Annual decomposition of pine litter in relation to climate. Symbols identify the vegetation
type in which the litter was placed. Top: Mean annual temperature. Middle: Annual precipitation. Bottom:
Annual evapotranspiration. Data from Gholz et al. (2000). See Moore et al. (1999) for a similar study in 18
sites across Canada.
Figure 10.46. Residence time of soil organic matter in relation to climate. Data are means for 13 forest
types. Top: Mean annual temperature. Bottom: Annual precipitation. Data from Vogt et al. (1986). See also
Schlesinger (1997, p. 194).
Figure 10.47. Global terrestrial carbon cycle. Adapted from Steffen et al. (1998).
Figure 10.48. Monthly mean atmospheric CO2 concentration in parts per million by volume (ppm)
measured at Mauna Loa, Hawaii, from 1959 to 1999. The thick solid line shows annual mean
concentration. Data collected by C.D. Keeling and colleagues at the Scripps Institution of Oceanography
(La Jolla, California) and archived at the Carbon Dioxide Information Analysis Center (Oak Ridge
National Laboratory, Oak Ridge, Tennessee).
Figure 10.49. Monthly mean atmospheric CO2 concentration in parts per million by volume (ppm)
measured at 11 locations from 1986 to 1992. Data from the Climate Monitoring and Diagnostics
Laboratory (National Oceanic and Atmospheric Administration, Boulder, Colorado) air sampling network
and archived at the Carbon Dioxide Information Analysis Center (Oak Ridge National Laboratory, Oak
Ridge, Tennessee).
Figure 10.50. Processes typically included in ecosystem models. Top: Ecosystem carbon balance and
environmental controls of photosynthesis and respiration. Internal carbon and nutrient cycling are also
shown. Bottom: Many models also include vegetation dynamics in which species composition changes
over time.
45