Rehabilitation and Restoration Of Degraded Forests (PDF) - IUCN

REHABILITATION AND RESTORATION OF DEGRADED FORESTS
In Figure 8 the productivity of a plantation with just one species is
shown as 100 per cent. A mixed-species plantation may have either an
increased or decreased level of timber production compared with this
monoculture. A decrease might occur (line A) if incompatible species
are mixed. An increase might occur (line B exceeds 100 per cent) if
complementary species are combined; this allows production gains
through the mechanisms described in Table 5. However, this increase
in per-hectare commercial productivity is unlikely to persist with
increasing numbers of tree species; at some point the market price of
each additional species declines once the most profitable are included.
Increased numbers of species then simply dilute the density of the
most profitable species in the stand. (This is the reason for the difference
in the shape of these diversity-function curves from the hypothetical
curves shown in Figure 7). Of course the attractiveness to the
grower of using large numbers of tree species will necessarily depend
on whether there are markets for more than a small number of tree
species and whether commercial timber production is the primary
objective of reforestation.
The right axis of Figure 8 shows wildlife habitat quality. The best
habitat quality has a large number of plant species. Depending on the
wildlife and plant species involved, a small decline in plant species
richness may cause a rapid decline in habitat quality (line X). In other
circumstances the habitat quality might be unaffected until such time
as only a few plant species remain (line Y). Depending on the plant
and animal species and, consequently, on these trends, a manager may
be able to adopt a compromise that offers both timber production and
habitat quality not far from the optimum (intersection of lines B and
Y) or be faced with a choice that is suboptimal for both (intersection of
lines A and X). Of course, the identity as well as the proportions of
particular plant species can make a difference to these relationships.
Thus the presence or absence of particular food plants or plant life
forms may have a greater effect on habitat quality for certain wildlife
species than biodiversity alone. Additional work on exploring such
relationships might lead to the design of plantation forests where tradeoffs
between production and wildlife habitat values are much reduced.
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