Forest Sites -
Slope, Aspect, Elevation, Climate,
and Soil
The sum of all the natural factors that influence forest
growth is characterized as "the site". Foresters
often talk about site index, or good sites
versus bad sites, or hardwood sites versus pine sites.
They refer to the combination of environmental factors
such as climate, soils, slope and aspect, and elevation,
all of which affect not only which trees grow where, but
how fast they grow and their form at maturity. Slope,
aspect, elevation, and the local climate determine the
micro-climate of a site.
As examples: northeast-facing
slopes tend to be cooler and moister than
southwest-facing slopes; high-elevation sites are colder
and more exposed than sites in the lowlands; steeper
slopes shed more rain. Just by knowing the aspect,
elevation, and slope of a site, one can get a good idea
about its potential productivity and which species will
grow well there. The concept of site is one of the most
important in
forestry because the interaction between a stand and its
site determines growth potential. It is always a
combination of factors that determine site quality.
However, if all other factors are equal within a stand,
then the site quality for timber production is a
function of the soils, especially their physical,
chemical, and biological properties.
Forest Soils
The characteristics of a forest soil are defined by
varying combinations of its four main ingredients -
mineral particles, organic matter, water, and air.
Chemical properties are determined mostly by the type of
minerals fro which the soil is derived. For example,
limestone-derived soils tend to be more
fertile, while soils derived from granite tend to be
infertile.
Fertility is largely a function of a soil's
chemical properties. Soil pH, a measure of the
acid-intensity of a soil solution, is commonly related
to fertility. Forest soils tend to be acidic; but more
acid soils are generally less fertile. Physical
properties of a soil - whether it is coarse or fine -
depends not only on the type of minerals present, but
also on the particle size.
Fine soils tend to be more
productive than coarse soils; however, they usually do
not drain well and are more easily eroded. Fine soils
are also more susceptible to damage from compaction. The
presence of organic matter - rotting debris such as
leaves and twigs - influences both the physical and
chemical properties of a soil. Soils with a high
proportion of organic matter tend to have better
structure and are more fertile. Organic matter also
helps forest soils hold water. Over 50 percent of the
volume in the upper layers of an undisturbed forest soil
is made up of pore space that holds water and air. The
proportion of air and water is both affected and
determined by the physical properties of the soil. Since
roots need air to breathe and water to supply the rest
of the tree, the soil pore space is extremely important.
Activities that compact the soil, eliminating space for
air and water, will lower the productivity of a site.
This is why it is so important for timber extraction
equipment to stay on established trails; especially on
sites with fine-textured soils that tend to compact more
readily. Consider, too, that more than half of the
feeder roots in a forest are found in the top six inches
of soil. Biological properties of a soil refer to the
communities of fungi, bacteria, and insects that live on
organic matter produced by trees and shrubs.
Though mostly microscopic, these organisms are
absolutely essential to the growth and development of
forests. While some can occasionally cause disease in
trees, most feed on fallen leaves and woody debris.
Their main role is to recycle nutrients, such as
phosphorus, potassium, and calcium, tied up in
vegetation and dead animals. Without these decomposers,
the forest floor would be littered with debris
accumulated over thousands of years, and forest
nutrients would be locked up in dead material.
Some
organisms, such as a group known as mycorrhizae-forming fungi, cause beneficial infections
on tree roots. In exchange for a share of the energy
manufactured by the tree, these fungi tremendously
improve a tree's ability to take up water and nutrients.
The tree and mycorrhizae-forming fungi depend on one
another., each supplying the other with sustenance for a
small cost. Unfortunately, mycorrhizae roots are very
susceptible to changes in environmental conditions,
especially those caused by soil compaction. (Hence,
forest management decisions must take into consideration
the way soil organisms will react to a harvesting
operation.) Usually, their activity increases due to
higher temperatures on the forest floor; however, soil
compaction will inhibit them.
[This information has been excerpted from the
Introduction to Forest Ecology and Silviculture.
Thom J. McEvoy, Extension Forester, published by the
University of Vermont in 1995] |