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Encyclopedia of Paleoclimatology and Ancient Environments (in press) Kluwer
Academic Publishers Earth Science Series.
Quaternary Vegetation Distributions
John W. Williams
National Center for Ecological Analysis and Synthesis
University of California, Santa Barbara
Santa Barbara, CA 93101
805-892-2523 (phone)
805-892-2510 (fax)
[email protected]
1
QUATERNARY VEGETATION DISTRIBUTION
Introduction
All aspects of Quaternary vegetation distributions, from the range and abundance of individual species, to
the global position, extent, and composition of major vegetation formations, have changed extensively and
repeatedly in response to the orbitally paced oscillations in climate, atmospheric CO2, and ice extent that
are characteristic of the Quaternary. At centennial and longer timescales, the distribution and abundance
of plant species closely track shifts in climate, despite the rooted habit of individual plants, via the dispersal
of propagules to newly favorable regions, local changes in population abundances, and the dying out of
populations in unfavorable locations. Distributions of vegetation formations and biomes emerge from these
species-level responses. The history of Quaternary vegetation distributions is thus indirectly a history of
global and regional climate change, and paleovegetational records are an important source of information
about Quaternary environments.
As an active component of the earth system, shifts in the global
vegetation affect the biogeochemical and biogeophysical fluxes between the land surface and atmosphere
as well as the functioning of the global carbon cycle.
Moreover, the present genetic structure and
distribution of populations, species, and communities is in part a legacy of late-Quaternary species
migrations and colonization history.
Primary sources of information for Quaternary vegetation history are the fossil pollen records
collected from lake, wetland, and near-shore marine sediments, supplemented extensively by plant
macrofossil, phytolith, and stable carbon isotope records. The present genetic structure of populations and
species provides additional information about colonization routes and refugia. Detailed information about
Quaternary vegetation distributions is mainly limited to the past 25,000 years, due to the limited length of
most terrestrial records and because radiocarbon dating, the primary source of age estimates for lateQuaternary organic sediments, becomes increasingly uncertain after 40,000 years as radioactivity declines
to background levels. The accuracy of radiocarbon ages varies over time, due to secular variations in
atmospheric radiocarbon concentrations; all radiocarbon ages here have been converted to calendar years
unless otherwise indicated.
The past 40,000 years include the last glacial maximum, last glacial-
interglacial transition, and current interglacial and so may serve as a model for vegetation distributions
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during previous glacial-interglacial sequences. Individual records can span one million years or more,
allowing longer-term reconstructions of vegetation history for certain locales.
Vegetation Processes and Dynamics
At Quaternary time scales (101-105 years), vegetation dynamics are mainly forced by climate change, and
plant taxa have primarily accommodated late-Quaternary climate change via local changes in abundance
and regional to continental shifts in distribution (Huntley and Webb, 1989). These biogeographic shifts can
be dramatic: for example Picea (spruce), which today is abundant in boreal and alpine regions, at the last
glacial maximum was apparently most abundant in the eastern interior of the US (Figure 1), a geographic
shift of approximately 2000 km. The maps of spruce also illustrate that although migration is an important
component of Quaternary vegetation dynamics, within-range changes in abundance are equally significant.
The range of spruce (approximated by the 5% pollen contour in Figure 1) has been relatively stable during
the middle to late Holocene, but the area of highest abundance shifted from western to eastern Canada
(Figure 1). These internal shifts in abundances are the result of changing rates of reproduction, growth, and
mortality within and among populations, mediated by climate and disturbance regimes. The changes in
extent and abundance differ among plant taxa, and the associations among taxa have changed with time.
Species such as Fagus grandifolia (American beech) and Tsuga canadensis (eastern hemlock) that today
are closely associated, had differing late-glacial distributions and colonization histories (Figure 1).
Conversely, plant associations existed in the past that no longer exist today (indicated by high
dissimilarities in Figure 1), and may have grown under climatic conditions also unlike any found today
(Williams et al., 2001). Species movements along altitudinal gradients in response to climate change also
display individualistic behavior, rather than wholesale shifts in vegetation zones.
The available evidence indicates that vegetation is in dynamic equilibrium with climate at
millennial time scales (Webb, 1986). Climatic reconstructions based on pollen-response surfaces are
generally consistent with other climatic proxies and general circulation model simulations and can
successfully predict pollen abundances for other plant taxa (Prentice et al., 1991). Multiproxy sediment
records with evidence of abrupt climate change typically show that terrestrial vegetation composition
responded to such changes with response times on the order of decades to a century (Tinner and Lotter,
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2001). Holocene migration rates based upon pollen data range from 10 to 1000 m/yr. However, these
migration rates are at the edge or beyond those predicted by current dispersal theory (Clark et al., 2001),
and may be over-estimates in cases where outlier populations invisible to the pollen record are nucleating
range expansions. Disturbance agents (fire, flood, windthrow, extreme climate events) contribute to and
accelerate vegetation change by increasing plant mortality and opening up gaps for colonization.
At subcontinental to global scales, the individualistic responses of plant taxa to climate change
cause the position, extent, and species composition of plant functional types (PFT’s) and biomes to change
over time. PFT’s and biomes are useful units of vegetation at global scales, because they are based upon
structural features of the vegetation and so are not constrained by intercontinental differences in
evolutionary history and floristic lists (Prentice et al., 2000). Moreover, the number of plant morphological
and functional types is limited relative to the number of species, so biomes can persist despite
individualistic species behavior.
Vegetation History
Plio-Pleistocene Transition to the Last Glacial Maximum (ca. 2.5 million to 21,000 yr BP)
The major floristic elements of the Quaternary vegetation were in place by the end of the Tertiary, with few
new species or morphological changes apparent in the fossil record. Vegetation distributions during the
Plio-Pleistocene transition changed in response to the onset of widespread glaciation in the Northern
Hemisphere at 2.45 million yr BP, decreased global mean temperatures, and increased climate variability.
Vegetation change was most pronounced in high latitudes, where, during the relatively warm Middle
Pliocene, coniferous forests had grown to the Arctic coast, in regions today occupied by tundra, while in
the Southern Hemisphere beech grew in coastal areas of Antarctica (Thompson and Fleming, 1996).
Despite the catastrophic climate changes during the Plio-Pleistocene transition there is little evidence of
significant global plant extinctions, although local extinctions were widespread (Watts, 1988).
For the past 700,000 years, vegetation history has been paced by the pronounced 100,000-year
cycle in climate, atmospheric CO2, and global ice volume.
Vegetation distributions at previous
interglacials are broadly similar to the current interglacial, with expansions in the abundance and range of
thermophilous taxa, but just as climates differed among interglacials, so too did the distribution and
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composition of the vegetation. The Eemian Interglacial in Europe (130 to 107 thousand yr BP), which is
the last interglacial prior to the current interglacial and is particularly well documented, is characterized by
high pollen abundances of oak (Quercus), hazel (Corylus), alder (Alnus), beech (Fagus), hornbeam
(Carpinus), and other temperate woody taxa, with differences among sites likely reflecting interregional
differences in Eemian climates. After the last interglacial ended, the proportion of treeless vegetation
increased, with high pollen abundances of grass (Poaceae), sagebrush (Artemisia), chenopods
(Chenopodiaceae), pine (Pinus), and juniper (Juniperus) reported from records spanning the last glacial
period (Allen et al., 1999). On both sides of the North Atlantic, the proportion of woody vegetation
increased during Heinrich events and interstadials (Grimm et al., 1993). In western North America, high
abundances of oak, willow (Salix), and alder pollen during interglacials alternate with high abundances of
fir (Abies), pine, and Taxaceae/Cupressaceae/Taxodiaceae (key taxa include redwood, cedar, and cypress)
pollen during glacial periods (Adam, 1988; Heusser, 2000).
Last Glacial Maximum (LGM; 21,000 yr BP)
By the last glacial maximum, enough well-dated paleovegetation records are available to systematically
map global vegetation distributions, although tropical regions remain underrepresented. LGM vegetation
distributions (Figure 2a) formed in response to climates that were colder and drier than present.
Atmospheric CO2 concentrations 80 ppm lower than pre-industrial levels may have amplified the
ecological impact of decreased moisture availability. Treeless conditions predominated in high-latitude
regions not buried by ice, with tundra intergrading into steppe in central Asia and China (Figure 1a) and
steppic vegetation in North America along the margin of the Cordilleran and Laurentide Ice Sheets, in
southern Europe and Northern Africa, in southern Africa, and in Australasia. Deserts extended to the east
coast of China in now-forested regions. In contrast, lake level records from the western interior of North
America indicate that full-glacial climates were wetter than present, and conifer woodlands grew widely in
areas that today are desert or shrubland-steppe (Thompson and Anderson, 2000). Northern Hemisphere
temperate and boreal forests were mainly confined to the southeastern US, southeastern China, and Japan,
and included a large component of cold-tolerant evergreen taxa.
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The difference between LGM and modern climate and vegetation in the tropics is less pronounced
than in higher latitudes, but neither tropical climates nor vegetation were stable during the late Quaternary
(Bush et al., 2001). Tropical lowland rain forests were somewhat diminished in extent, although the extent
of the decrease and degree of fragmentation has been controversial. In Africa, tropical rain forest was
partially replaced by tropical seasonal forest, whereas in southern Amazonia and Australasia grasslands and
savanna encroached on areas today occupied by tropical rain forest, subtropical forest, and savanna
(Kershaw, 1988; Behling, 2002). Upper treeline was as much as 1000 m lower than present, as the
elevational ranges of upland types moved downslope, and upland evergreen forests were replaced by steppe
and schlerophyll shrubland.
Recent explanations for LGM tropical treeline depressions have focused upon the effects of
increased aridity and low CO2 concentrations rather than lowered temperatures (Jolly and Haxeltine, 1997).
Tropical plant communities above treeline are dominated by plants utilizing the C4 photosynthetic pathway,
which enables maintenance of high intracellular CO2 concentrations despite low ambient CO2
concentrations. C3 plants, conversely, are adversely affected by increased photorespiration and decreased
water-use efficiency under low ambient CO2 concentrations and the resulting competitive disadvantage
may have depressed tropical treelines. In some areas, however, full-glacial C3 communities were replaced
by C4 communities during deglaciation, indicating that the physiological effects of varying CO2
concentrations can be overridden by local climatic controls (Huang et al., 2001). The importance of varying
CO2 concentrations for Quaternary vegetation dynamics remains under debate.
Deglaciation (14,000 to 8,000 yr BP)
Global vegetation distributions were fundamentally transformed during deglaciation as glacial climates
warmed to interglacial conditions during a time of higher-than-present seasonal contrasts in insolation.
Forest cover increased, thermophilous taxa expanded upslope and northwards, and vegetation formations
characteristic of the Holocene (e.g. taiga) emerged from the reshuffling of species distributions.
In
northwestern Canada and northern Eurasia, northern treeline reached its northernmost position during the
early Holocene, advancing near to Arctic coastline, then retreated to its present position by 4000 to 3000 yr
BP (MacDonald et al., 2000). Montane forest developed in central Africa by 10,000 yr BP (Jolly et al.,
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1997) and podocarp forests in New Zealand by 13,000 yr BP (McGlone, 1988). In Europe and eastern
North America, thermophilous plant taxa began migrating north between 14,000 and 13,000 yr BP. In
Africa, C4 plants diminished in abundance (Street-Perrott et al., 1997).
This period is further marked by abrupt changes in atmospheric and oceanic circulation that were
superimposed upon the general transition from glacial to interglacial climates, and vegetation distributions
closely tracked these rapid climate events. During the Younger Dryas Chronozone (12,900 to 11,600 yr
BP), plant communities adjacent to the North Atlantic reverted to cold-tolerant assemblages, switching to
tundra in European Alps and to alder shrublands in Maritime Canada (Mayle et al., 1993; Ammann et al.,
2000). Vegetation changes consistent with a reversion to lower temperatures have been reported in Alaska,
the Pacific Northwest, and at individual sites in South America and the Indo-Pacific region (Peteet, 1995).
In other regions, vegetation shifts coincident with the Younger Dryas suggest variations in moisture
balance rather than temperature. In eastern North America, pine rapidly migrated westwards into the Great
Lakes region, and spruce abundances declined (Shuman et al., 2002). Tropical African pollen records
indicate that the Younger Dryas coincided with increased aridity. Vegetation distribution shifts during the
Younger Dryas thus appear to have varied in direction among regions and continents as changes in climate
varied among these regions.
Middle Holocene (7000 to 4000 yr BP)
Global vegetation distributions during the mid-Holocene were similar to present, although differences
persisted regionally (Figure 1b). The position of northern treeline was asymmetric relative to its present
position, approximately 25 km north in northwestern Canada, 200-300 km north in central Siberia, and
little or no change in Alaska and eastern Canada (Prentice et al., 2000). Temperate forests in eastern North
America, Europe, and China were poleward of their present position, with shifts on the order of 100 km and
as much as 800 km in northeastern China. In New Zealand, southern beech (Nothofagus) forests expanded
(McGlone, 1988). These patterns suggest warmer-than-present temperatures in middle and upper latitudes.
Other distributional differences between the mid-Holocene and present vegetation were likely due
to interregional variations in moisture balance. These differences are greatest in northern Africa, where
steppe, xerophytic woodlands, and even warm mixed forest grew across a broad area now occupied by
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Saharan desert (Figure 2b). Lake level highstands and modeling studies show that the African monsoon
was enhanced relative to present (COHMAP Members, 1988). In the Columbian Andes, xeric vegetation
types increased in extent during the middle Holocene although vegetation distributions in Chile suggest
wetter-than-present conditions (Marchant et al., 2002). Steppe expanded at the expense of forest in the
northern Midwest but was reduced in central Asia relative to present.
Consequences of Quaternary Vegetation Change
Earth System Effects and Feedbacks
The terrestrial biosphere is an integral component of the earth system, and changes in vegetation
distributions are likely to have affected atmospheric circulation and the functioning of global energy and
biogeochemical cycles. General circulation model sensitivity experiments for the Holocene show that
changes in surface albedo associated with shifts in northern treeline may have amplified the orbitally
induced variations in temperature (Foley et al., 1994). In Northern Africa, the mid-Holocene expansion of
steppe at the expense of desert may have enhanced monsoonal strength by increasing rates of
evapotranspiration (Kutzbach et al., 1996). At the last glacial maximum (LGM), decreased vegetation
cover in mid- and high-latitudes may have amplified cooling, due to an albedo feedback, whereas in the
tropics decreased evapotranspiration and a weakened hydrological cycle may have enhanced tropical
aridity (Levis et al., 1999). Furthermore, the LGM decrease in vegetation cover may have increased the
source area for dust emissions and increased atmospheric LGM dust loadings (Harrison et al., 2001),
thereby indirectly affecting the earth’s radiative budget and increasing rates of nutrient transport. The
effects of vegetation change upon glacial-interglacial carbon budgets are large but uncertain, with most
estimates ranging from 300 to 1000 Pg carbon lost from the terrestrial biosphere at the LGM, compared to
present-day stocks of 2000 Pg C (Peng et al., 1998). Atmospheric methane concentrations increased from
350 ppbv at the LGM to 700 ppbv at the beginning of the Holocene, primarily due to expansion in tropical
and boreal wetlands.
Evolutionary and Genetic Impacts
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The close similarities between late Tertiary and Pleistocene faunas and the corresponding lack of
widespread extinction and macroevolutionary change during the Quaternary is somewhat surprising given
that the signal feature of the Quaternary is its extensive and repeated large climatic fluctuations.
Apparently, plant taxa have accommodated Quaternary climate change primarily by ecological rather than
evolutionary processes.
Bennett (1997) has argued that individualistic species behavior effectively
decouples microevolutionary and macroevolutionary processes, as orbitally-paced reorganizations of
species associations remove coevolutionary adaptations.
Nevertheless, the repeated range shifts characteristic of the Quaternary are highly likely to have
affected species’ genetic structure, as individual populations have propagated or gone extinct (Davis and
Shaw, 2001). Whether Quaternary vegetation dynamics has tended to foster increased or decreased genetic
diversity is not yet known. The genetic structure observed in present populations and species is in part a
legacy of the biogeographic responses to the last deglaciation. For example, haplotype distributions within
European white oaks (Quercus spp., subgenus Lepidobalanus) and North American whitebark pines have
been convincingly related to likely colonization routes and glacial population distributions (Petit et al.,
2002; Richardson et al., 2002). Differences in genetic structures among modern populations thus may be
used to refine inferences about vegetation history.
Quaternary environmental variability may have helped shape the modern latitudinal gradient in
species richness and other biogeographic patterns of species diversity. Historical explanations for high
levels of tropical species diversity have focused upon two contrasting hypotheses: 1) long-term stability of
tropical climates encouraging persistence of tropical rainforests and low rates of extinction versus 2) high
levels of endemism caused by repeated fragmentation of rainforests during glacial periods (Hooghiemstra
and van der Hammen, 1998). The extent of Amazonian rain forest fragmentation remains controversial,
but lowland rainforests appear to have persisted as intact, albeit diminished, bodies of vegetation
(Colinvaux et al., 2000), weakening refugial hypotheses.
John W. Williams
9
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Cross-references
Carbon dioxide, Quaternary variations
Carbon isotopes, stable
Paleobiogeography
Paleobotany
Palynology
Pollen Analysis
Quaternary climates
Tropics, Quaternary
Vegetation proxies
Younger Dryas
12
Figure legends
Figure 1, Rows 1-3: Mapped pollen percentages for spruce (Picea), hemlock (Tsuga), and beech (Fagus)
in northern and eastern North America for selected 1000-year time periods since the last glacial maximum.
Each column header indicates the age in yr BP. Maps are based upon fossil pollen records from the North
American Pollen Database, and consist of percent pollen abundances calculated relative to a summed
pollen count and interpolated to a 50-km grid. Such maps are widely interpreted to represent past shifts of
plant distributions, and analyses of modern pollen-vegetation relationships have shown that such maps
capture present patterns of plant abundances. Row 4: Mapped dissimilarities between fossil pollen
assemblages and a dataset of modern and 500 year BP pollen samples. High dissimilarities indicate that the
fossil pollen assemblage has no close counterpart among modern pollen samples, and is interpreted to
indicate plant taxon associations unlike any today.
Figure 2: Global vegetation maps produced by the Biome 6000 working group (Prentice et al., 2000) for
the last glacial maximum (18,000 radiocarbon yr BP; ca. 21,000 calendar yr BP), mid-Holocene (6,000 yr
BP), and present (0 yr BP), based upon systematic application of a standardized classification algorithm to
pollen and macrofossil data. The present-day vegetation reconstructions closely agree with independent
maps of the vegetation.
Ice sheet extent and changes in sea level are not shown.
Additional
paleoecological datasets exist for India, the Indo-Pacific, South America, and Australia that were not
included in this generation of the Biome 6000 synthesis. Redrawn from (Prentice et al., 2000).
13
21,000
14,000
11,000
9,000
6,000
3,000
Picea (Spruce)
0 to 5%
5 to 20%
20 to 40%
40 to 100%
No Data
Tsuga (Hemlock)
0 to 1%
1 to 5%
5 to 10%
10 to 100%
No Data
Fagus (Beech)
0 to 1%
1 to 5%
5 to 10%
10 to 100%
No Data
Dissimilarity from Present
0.00 to 0.10
0.10 to 0.15
>0.15
No Data
500
-180°
-120°
-60°
-20°
20°
60°
100° 140° 180°
A. LGM
ca. 21,000 yr BP
60°
60°
40°
40°
20°
20°
0°
0°
-20°
-20°
-40°
-40°
-60°
-60°
-180°
Biomes
-120°
-60°
-20°
20°
60°
100° 140° 180°
Tundra
60°
Taiga
B. Mid-Holocene
6,000 yr BP
60°
Cold Decid. Forest
Cold Mixed Forest
40°
40°
Temperate Decid. For
Temperate Conif. For.
20°
20°
Cool Conif. Forest
Cool Mixed 0°
Forest
0°
Warm Mixed Forest
Tropical Rain
Forest
-20°
-20°
Tropical Dry For.
Tropical Seasonal
-40°For.
-40°
Conifer Woodland
Xerophytic Scrub
-60°
-60°
Savanna
Steppe
-180°
Desert
-120°
-60°
-20°
20°
60°
100° 140° 180°
C. Present
0 yr BP
60°
60°
40°
40°
20°
20°
0°
0°
-20°
-20°
-40°
-40°
-60°
-60°
-180°
-120°
-60°
-20°
20°
60°
100° 140° 180°