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Environmental factors impacting plant community structure

I. Factors impacting plant community structure

A. Climate
- temperature ranges and water availability are the major determinants
- is the plant frost tolerant, drought tolerant, swamp tolerant

B. Soil

1. Soil horizons
   O - primarily dead organic material, most organisms live here
   A1 - rich humus layer, mixing of organic material and mineral soil
   A2 - minerals moved into solution here, many plant roots found here
   B - little organic material, resembles underlying rock in composition
   C - weakly altered material similar to parent rock2. Soil types

2. Soil types - influence nutrient availability & soil pH
   a. True spodsol - gray soil with a this layer of black decomposing humus at top, beneath is brown or red B layer, thick layer of needles on top
   often found in coniferous forests
   fairly acidic
   b. Gray-brown spodsol - brownish in color, leaf litter layer thinner
   found in deciduous forests
   not as acidic as true spodsol
   c. Yellow soil - coastal plain, NC to Tx., sandy
   d. Red soil - southeastern states, deep layers are red
   e. Prairie soil - plain states and north central US, deep brown at top, light brown beneath

3. Soil moisture - most trees live comfortably in only one of the following types of soils, some generalists (red maple) do well in all three.
   a. Mesic - well drained soil, loam (mixture of sand, silt & clay), rich habitat, good species diversity
   b. Xeric - sandy or other poor soils, usually overly dry, coastal forests, south-facing mountain slopes (exposed to sun for longer during a given day, tend to be drier).
   c. Hydric - low lying area that retain water, soil is saturated, swamps or bogs

4. Soil nutrients
   a. Nitrates (NO3), Phosphates (PO3), many micronutrients
   b. Mycelles - charged particles that consist of clay (inorganic) and humus (organic) - capture and hold important nutrients with their charge
   c. Bog soils are nitrogen poor because they are acidic, the H+ bumps the nutrients off the mycelles, taking their place and allowing them to be washed out in the groundwater
   i. carnivorous plants evolved to capture and digest animals in order to utilize the nitrogen form the proteins in them.

C. Wind - effects temp, availability of top soil, shape of plant

D. Salt & salt spray - plant must be salt tolerant, able to excrete salts (magrove), store water (succulents), or have tap roots to reach fresh water.

E. Fire - many species may be drought resistant, but in regions with seasonal fires, only grasses flourish, because they are capable of growing back from their roots (or rhizomes), while trees and shrubs are not.

II. Changes in physical environmental conditions

A. Allogenic - changes are not a direct result of organisms, but is a feature of the physical environment.
e.g. decrease in temp as you gain altitude (adiabatic cooling), decrease in temp as you increase in depth in a body of water.

B. Autogenic - changes to the physical environment that are a direct result of the organisms in the community.
- indirect interaction between species (not direct interaction predation , etc), through modification of the physical environment

1. Facilitation - each stage paves the way for the next. Early successional species can modify the environment in such a way as to allow later stages to grow, & prevents current stages from continuing.
   E.g. addition of soil & nutrients (allow better competitors to enter), ruderals initially grow where others cannot, improving the soil.

2. Inhibition - of growth of one plant by another, either by shading or direct competition for sunlight, nutrients, water, or by chemical warfare Here the competitors begin to replace the ruderals. Integral part of the succession of plant species (seres).

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