14.2.2.3. Plantation Silviculture
Plantation forestry is a major land use in Brazil and is expected to expand
substantially over coming decades (Fearnside, 1998). Climatic change can be
expected to reduce silvicultural yields to the extent that the climate becomes
drier in major plantation states such as Minas Gerais, Espírito Santo,
São Paulo, and Paraná as a result of global warming and/or reduced
water vapor transport from Amazonia (e.g., Eagleson, 1986). Dry-season changes
can be expected to have the greatest impact on silvicultural yields. Water often
limits growth during this part of the year under present conditions, yet there
may be water to spare during the rainiest part of the year. In areas outside
of Brazil's extreme south, annual rings that are evident in the wood of
plantation trees correspond to dry (as opposed to cold) seasons.
The effect of precipitation changes on plantation yields can be approximated
by using a regression equation developed by Ferraz (1993) that relates biomass
increment in Eucalyptus to precipitation at three sites in the state of São
Paulo (Fearnside, 1999). UKMO model results (Gates et al., 1992) indicate
that annual rainfall changes for regions of Brazil would cause yields to decrease
by 6% in Amazonia and 8% in southern Brazil and increase by 4% in the northeast.
During the June-July-August (JJA) rainfall period, yields would decrease by
12% in Amazonia, 14% in southern Brazil, and 21% in the northeast (Fearnside,
1999).
The foregoing discussion of precipitation decreases considers only the effect
of global warming. Brazil is likely to suffer additional losses of precipitation
as a result of reductions in evapotranspiration caused by deforestation in Amazonia
(see Section 14.5.1.1.1). Some of the water vapor originating in Amazonia is
transported to southern Brazil (Salati and Vose, 1984; Eagleson, 1986). Decreased
water vapor supply to southern Brazil, where most of the country's silviculture
is located, would aggravate precipitation declines stemming from global warming.
The direct effects of rainfall reduction on yields are likely to underestimate
the true effect of climate change. Synergistic effects with other factors could
reduce yield substantially more—for example, through attack by pests (Cammell
and Knight, 1992).
A drier climate in plantation areas also could be expected to lead to greater
fire hazard. Fire is a problem in plantation silviculture even in the absence
of climatic change, requiring a certain level of investment in fire control
and a certain level of losses when burns occur. Pine plantations in Paraná
require continuous vigilance (Soares, 1990). Eucalyptus also is fire-prone because
of the high content of volatile oils in the leaves and bark.
Temperature changes can affect plantation yields. The models reviewed in the
IPCC's Second Assessment Report (SAR) indicate a temperature increase of
2-3°C in Amazonia (Mitchell et al., 1995; Kattenberg et al.,
1996). Considering a hypothetical increase of 1.5°C by the year 2050
in Espírito Santo and Minas Gerais, Reis et al. (1994) conclude that
the present plantation area would have to be moved to a higher elevation (a
shift that is considered impractical) or the genetic material would have to
be completely replaced, following the global strategy proposed by Ledig and
Kitzmiller (1992). In addition to direct effects of temperature considered by
Reis et al. (1994), temperature increases have a synergistic effect with
drought; the impact of dryness is worse at higher temperatures (lower elevations)
as a result of higher water demands in plantations.
CO2 enrichment would be beneficial for plantations. Higher atmospheric concentrations
of CO2 increase the water-use efficiency (WUE) of Eucalyptus. Photosynthetic
rate increased in these experiments from 96% (E. urophylla) to 134% (E. grandis).
Growth of different plant parts showed similar responses. Higher levels of CO2
also stimulate nitrogen fixation, which could be expected to lower the fertilizer
demands of plantations (Hall et al., 1992).
Climatic change would require larger areas of plantations (and consequently
greater expense) to meet the same levels of demand. The percentage increase
in areas required can be greater than the percentage decline in per-hectare
yields caused by climatic change because expansion of plantation area implies
moving onto progressively poorer sites where productivity will be lower. Taking
as examples rainfall reductions of 5, 10, 25, and 50%, plantation area requirements
are calculated to increase as much as 38% over those without climatic change,
which would bring the total plantation area by 2050 to 4.5 times the 1991 area
(Fearnside, 1999).
|