Geography Reference
In-Depth Information
dead leaves and wood), and the soil - store minerals; and the
size of each circle represents the quantity of minerals stored.
The arrows represent the cycling of minerals between the stores,
and other inputs and outputs; the thickness of each arrow
representing the proportion of the minerals that is transferred
from the source store each year. In the tropical rainforest (A) most
of the minerals are stored in the biomass (mostly trees). Only a
small proportion of this biomass falls as litter to form the litter
compartment, which is a relatively small circle also because most
litter is broken down rapidly (thick arrow) in the temperature
and moisture conditions on the forest fl oor. The environment is
optimal not only for tree growth but also for decomposition of
the litter and chemical weathering of the substrate and the soil
minerals. Although the soil store is of intermediate size, available
nutrients are taken up rapidly by the tree roots (another thick
arrow) and tend to accumulate in the biomass. By providing
an understanding of how the rainforest functions, a systems
approach also explains the reduced fertility of the rainforest
soils after a few years of cultivation. Slash-and-burn agriculture,
whereby minerals are released from the biomass by fi re and
the cultivators move on before the soil is completely exhausted,
can be seen as closely adapted to the rapid mineral cycling that
occurs in this environment. Important geographical differences
between the three types of forests are also highlighted in
Figure 7. Progressively smaller quantities of minerals are stored
in the biomass compartment of the higher-latitude forests
which, in contrast, store larger quantities of minerals in the
litter compartment. The latter is largest in the boreal forest
(C), where a relatively small proportion of the minerals is
transferred to the soil and made available to the trees because
litter decomposition is slowest there, especially under cold winter
conditions. In contrast, the soil compartment of the summer
deciduous forest (B) is largest because both decomposition of
the litter and uptake of minerals by the roots are at intermediate
levels.
