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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).
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