| On a continental scale, there are variations
in the composition and structure of forests, even in places
where climate and soil are very comparable. These differences
can be attributed to the different bio geographic and human
histories of these continents. The differences are important
- to some extent they explain differences in patterns of logging
throughout the world. In general, Neotropical and Asian forest
are richer in species than African forests, while the latter
are richest in lianas. Africa has very large trees, while trees
in the Neotropics are often smaller. Wood characteristics of
trees in Asian forests are so similar that large numbers of
species can be sold under a single commercial name (e.g. red
meranti) and high logging intensities can be achieved. Neotropical
timbers differ widely in their wood characteristics, so no commercial
lumping of species under one name occurs and only few trees
per hectare can be harvested.
Within continents, even within a single forest formation
(such as "tropical rain forest"), large variation
exists from place to place, leading to the definition of forest
types. Usually forest types are linked to the occurrence of
differences in soil type and water availability. Differences
in disturbance regimes also give rise to differences in forest
type.
Whatever the cause, forest types can always be described
using a limited number of variables. These describe some quality
of a forests' structure, composition and functioning. Parameters
frequently encountered in the literature are:
| Structure: |
Canopy height, number of stems per hectare
(stem density), the distribution of stems over different
size classes (population structure), basal area (BA -
the cross-sectional area of wood per hectare, expressed
as m2 per ha, measured at 1.3 m height along the stem),
biomass (usually derived from basal area, tree height
and a conversion factor) |
| Composition: |
Species number per ha, contribution of deciduous
vs evergreen tree species, composition of the fauna etc. |
| Function: |
Function refers to the processes that make
a forest "function": the cycling of nutrients
and water, pollination, seed dispersal and predation,
etc. It is often quantified in terms of fluxes: amount
of water intercepted and evaporated; amount of nutrients
cycling in leaf litter; etc. |
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