where B= the percent change in annual biomass
increment above the mean, P= the percent deviation
in annual precipitation above the mean.
Considering the annual rainfall changes mentioned
earlier for the three regions based on the
UKMO model results reported by the IPCC,
Eq. (1) implies yield decreases of 6% in
Amazonia and 8% in Southern Brazil, and an
increase of 4% in the Northeast. Considering the
June±July±August (dry season) rainfall believed
to be most critical, yields in this period would
decrease by 12% in Amazonia, 14% in Southern
Brazil and 21% in the Northeast. These results
must be approached with caution, given the high
uncertainty of both climatic change predictions
and the magnitude of yield response to precipitation
changes. In addition, use of Eq. (1)
assumes that single-year changes in growth increment
(observed) would be the same as a change
over many years. The longer-term changes would
be in¯uenced by accumulated stress and by
changes in carbon allocation in individuals and
ecosystems.
In practice, the relation of precipitation reduction
to plantation yield will not be a straight
line decline as implied by Eq. (1). The yield of
each tree species can be expected to follow a
curve when related to precipitation, with a steep
decline at low precipitation values, tapering to a
plateau where precipitation is sucient for the
species. As climatic change progresses, ®rms can
be expected to change the species planted in
favor of more drought-resistant ones, such as E.
camaldulensis. Losses may be greater than an
ideal sequence of species changes would suggest
if ®rms fail to switch species due to misjudgment
and due to the rapidity and unpredictability of
climatic changes. The composite of individual
species curves would approximate a straight line
with a shallower slope than the one describing
the yield of any particular species (Fig. 2). The
slope would necessarily be shallower than the
average for individual species (independent of the
sharpness of the response of each species)
because of the horizontal displacement of the individual
species curves along the axis representing
annual rainfall (Fig. 2). Droughts can aect
mortality, as well as yield: in 1993 a drought in a
former cerrado (Central Brazilian dry scrub
savanna) area of Mato Grosso caused high mortality
in stands of E. urophylla, E. pellita and E.
cloeziana that had previously been highly productive,
although stands of E. camaldulensis
maintained their more modest levels of productivity
despite the drought [8]