Download Forest Patterns and Processes

Forest Patterns and Processes
Harry White
Introduction
An ecosystem consists of a biological community, an assemblage of species populations
co-occuring within spatial and temporal boundaries, and the chemical and physical
environment in which they live (Begon, et.al., 1990). A woodland ecosystem
encompasses the living organisms of the forest, and extends vertically from the top of the
canopy to the lowest soil layers affected by biotic processes (Waring and Schlesinger,
1985). Characteristics of the forest ecosystem, such as species richness and abundance,
the availability and movement of water, and the chemical composition and physical
nature of the soil are dependent on the climate, terrain, and the biologic and geologic
history of the forest (Daubenmire, 1968). Water, temperature, and nitrogen are the most
common limiting factors in plant growth (Kozlowski, et.al., 1991).
Succession
In stable climatic conditions, a forest community is composed of a mosaic of various tree
species and ages, and organisms, along a progression known as ecological succession. In
succession, pioneering species first colonize previously uninhabited landforms (primary
succession), facilitating the establishment of later successional forms by developing soil
and other conditions favorable to the succeeding organisms (secondary succession)
(Reichle, 1970) leading to the presence of a final self-maintaining community (Collier,
et.al., 1973). Secondary succession may also occur following the abandonment of
cultivated land or after major or minor perturbations such as fire or tree-falls which leave
some trace of previous organic activity (Bormann and Likens, 1979).
Forest communities can be characterized by soil assay, by surveying soil and leaf litter
organisms, and by determining relative importance values of tree species and their
dispersion patterns in the woodland (Wilson, 1994).
Patterns and Processes on the Forest Floor
Soil provides water, minerals, and anchorage for plant growth. In the Central Valley of
Connecticut, most soils are derived from glacial till overlaying sandstone and basalt, are
composed of sand, silt, clay, stone, and decaying organic matter and retain the general
chemical characteristics of their parent bedrock (Rodgers, 1990). In addition, air and
water (containing dissolved minerals and gases) occupy soil interstitial spaces along with
a biological community that includes bacteria, algae, fungi, invertebrates, and roots. Root
development is dependent on soil physical properties such as water content, density,
resistance to root penetration, aeration, pH, and the concentrations of essential mineral
nutrients. (Kozlowski, et.al., 1991).
Water
Water serves to transport nutrients throughout trees, to maintain structural turgor, and as
a medium for chemical reactions and is thus essential for plant survival. The availability
of water often determines the kinds of trees present in a forest (Lange, et.al., 1982) and
the structure of soil determines the availability of water in the forest. Three types of soilborne water are important to plants: gravitational, capillary, and hygroscopic.
Gravitational water occupies the larger pore spaces in the soil and drains into the water
table shortly after rainfall. Capillary water is contained by and moves under the influence
of surface tension between soil particles, and is the primary water supply for plants. Soils
suitable for root growth contain about equal volumes of capillary and non-capillary pore
space (Kozlowski, et.al., 1991). Hygroscopic water is held in a thin film on the surface of
soil particles and only moves as a vapor and is thus unavailable to plants (Oosting, 1953).
Soils
Soils serve as a mineral resource pool for forest plants, and minerals are continuously
recycled through the ecosystem by the retranslocation of minerals within plants and by
the decomposition of fallen litter, dead soil organisms, and other organic matter. Because
many of the pathways of mineral nutrient movement involve water, some minerals are
lost to leaching or runoff; these losses are countered by the contribution of essential
nutrient elements from the atmosphere, from rock weathering, and from biological
processes (Bormann and Likens, 1979). However, in temperate forests, recycling within
trees and through decomposition is so efficient that after canopy closure the demand for
minerals from the soil is greatly reduced (Kozlowski, et.al., 1991).
Nutrients
Mineral nutrients serve many functions in plants, such as constituents of plant tissues,
osmotic and membrane permeability regulators, and enzyme activators. Essential
nutrients of trees include nitrogen and nitrogen compounds, potassium, and phosphorus.
A plant's ability to extract soil nutrients is dependent on the pH of the soil (Waring and
Schlesinger, 1985). Tree species differ significantly in their ability to absorb various
nutrients and in their tolerance of limited supplies of essential elements (Kozlowski,
et.al., 1991).
pH
What is soil pH?
Soil pH is the measure of the acidity (sourness) or alkalinity (sweetness) of a soil. A
simple numerical scale is used to express pH. The scale goes from 0.0 To 14.0, with 0.0
being most acid, and 14.0 being most alkaline. The value, 7.0 is neutral--i.e., neither acid
or alkaline.
Why is pH important?
Soil pH is important because it influences several soil factors affecting plant growth, such
as (1) soil bacteria, (2) nutrient leaching, (3) nutrient availability, (4) toxic elements, and
(5) soil structure. Bacterial activity that releases nitrogen from organic matter and certain
fertilizers is particularly affected by soil pH, because bacteria operate best in the pH
range of 5.5 to 7.0. Plant nutrients leach out of soils with a pH below 5.0 much more
rapidly than from soils with values between 5.0 and 7.5. Plant nutrients are generally
most available to plants in the pH range 5.5 to 6.5. Aluminum may become toxic to plant
growth in certain soils with a pH below 5.0. The structure of the soil, especially of clay,
is affected by pH. In the optimum pH range (5.5 to 7.0) clay soils are granular and are
easily worked, whereas if the soil pH is either extremely acid or extremely alkaline, clays
tend to become sticky and hard to cultivate.
A pH determination (soil test) will tell whether your soil will produce good plant growth
or whether it will need to be treated to adjust the pH level. For most plants, the optimum
pH range is from 5.5 to 7.0, but some plants will grow in more acid soil or may require a
more alkaline level.
The pH is not an indication of fertility, but it does affect the availability of fertilizer
nutrients. A soil may contain adequate nutrients yet growth may be limited by a very
unfavorable pH. Likewise, builder's sand, which is virtually devoid of nutrients, may
have an optimum pH for plant growth.
Soils tend to become acidic because soluble basic salts leach out of the soil as
gravitational water percolates through the forest floor and because of acids produced by
biological and chemical processes in the upper layers of the forest floor (Oosting, 1953).
Acidic soils chemically bind nutrients and inhibit the bacterial production of nitrates,
although the level of this effect is dependent on soil structure. Clay soil particles, in
contrast to sand particles, possess surface negative charges that attract and hold soluble
nutrient cations, preventing their loss through runoff, and complex anion-exchange and
cation-exchange processes in the root structure of plants facilitate the withdrawal of
nutrients in soils with a pH of 5 and higher (Waring and Schlesinger, 1985).
Essential Elements: N
Nitrogen availability is a critical factor in the development of leaf area or photosynthetic
surface, and thus dictates the amount of photosynthate available for plant growth and
reproduction (Kozlowski, et.al., 1991). The effects of nitrogen on leaf area are most
pronounced on young trees growing in the understory where sunlight availability is
reduced (Packham and Harding, 1982). Soil microorganisms fix nitrogen in the soil as
nitrates; it is the nitrate form of nitrogen that is absorbed by plants. Intermediate forms
and byproducts of nitrogen fixation include nitites and ammonia, which are toxic to
plants in higher concentrations; well- drained and aerated soils prevent the accumulation
of these problematic forms of nitrogen (Mueller-Dombois and Ellenberg, 1974).
Essential Elements: P and K
Phosphorus is a component of ATP, nucleic acids, and cell membranes, and low
concentrations may limit plant growth, especially since phosphate tends to be tightly
adsorbed to soil particles (Waring and Schlesinger, 1985) and is not present in the
atmospheric storage reservoir as is nitrogen. Potassium is a key component in the gas
exchange mechanism of plants by facilitating the operations of the stomata, and plays a
role in the synthesis of starch and the translocation of carbohydrates in plants. Potassium
can thus be a limiting resource if it is unavailable to plants at levels sufficient to maintain
maintenance and growth.
Sidebar: A Better Approach.....
Soil essential element analysis does not adequately characterize the elemental abundance
conditions of plants because there is much variation in concentration around the base of
trees and from site to site in the forest; foliar analysis is replacing soil analysis as the
preferred method of determining the effects of essential element levels on maintenance,
reproduction, and growth because foliar analysis reveals the actual elemental
concentrations in plants (Kozlowski, et.al., 1991).
The Role of Microorganisms on the Forest Floor
Tree litter is comprised of leaves, twigs, and dead organisms. It is an important
constituent of woodland ecosystems, as it affects soil structure, mineral nutrient status,
and water flow through the forest. Litter contributes to the recycling of nutrients in the
forest by supporting bacterial and invertebrate populations involved in decomposition and
soil aeration processes (Packham and Harding, 1982). Soil organisms transform plant and
animal tissues within the litter, forming complex aggregates of organic matter with the
soil mineral component. The rate of litter decomposition is dependent on temperature and
moisture levels, on the tree types contributing to the litter, and on the number, species,
and activity of soil flora and fauna (Reichle, 1970).
Bacteria and fungi dominate in the soils and are concentrated in the surface layer
underneath the forest litter. Soil microorganisms metabolize cellulose, lignin, amino
acids, and pigments, making the largest contribution to the nutrient pool that is available
for uptake by plant roots (Daubenmire, 1968).
The composition of the forest is thus determined by the physical environment and the
soil, and in turn the plants of the forest exert an influence on the soil and the physical
environment (Colinvaux, 1973).
Plant Communities in Southern New England
Each species of tree, shrub, and herbaceous plant has a certain range of tolerance to
conditions of light, temperature, moisture, and essential nutrients. In southern New
England a generally homogenous broadleaf forest, dominated by oak and maple trees, has
established itself since the last ice age (Shelford, 1965). Variation in the distribution and
abundance of trees in the area is dependent on terrain and orientation to the sun; hilltop
and southerly-exposed slopes are typically deficient in moisture as compared to low slope
communities which show the greatest abundance and diversity of forest systems in
general (Vankat, 1979). The low slope community extends to within ten inches of the
water table, and low slope trees, such as sugar maple (Acer saccarinum) are adapted to
high water tables and thick saturated soils (Jorgenson, 1978). The well-developed canopy
of this forest type demands shade- tolerant understory species such as the hop hornbeam
(Ostrya virginiana).
The dispersion of a species is the arrangement of the members within a habitat (Brower
and Zar, 1984). The dispersion of members of a species is seldom uniform or regular but
is generally aggregated due to the patchy distribution of conditions favorable to the
species (Begon, et.al., 1986). A random dispersion pattern may be found when conditions
are uniform throughout a habitat.
Flux
No forest is stable in the sense of being unchanging; all are in a continuous state of flux
(Newman, 1982). Disturbance is a natural component of forest life. A study of a forest
without looking at changes over time can only characterize the state of the forest at that
moment in time.
Analysis & Conclusions Questions:
1. How does Mr. White describe an ecosystem?
2. How does Mr. White describe primary and secondary succession?
3. What is the importance of water to plants in an ecosystem?
4. Why is the demand for minerals from the soil greatly reduced in a temperate
forest ecosystem?
5. Why is soil pH important to plants?
6. Why is nitrogen important to plants?
7. Why is phosphorus important to plants?
8. Why is potassium important to plants?
9. What purpose do bacteria and fungi play in soils?