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In Tropical Asia, two major types of closed-canopy forest have been distinguished (Collins et al., 1991): rainforest and monsoon forest. Rainforests grow in ever-wet conditions where rainfall is heavy and spread throughout the year. These forests are evergreen or semi-evergreen and include lowland, montane, and swamp forests. Monsoon forests grow where rainfall is high but unevenly spread throughout the year. Most monsoon forest trees are deciduous; they shed their leaves in the dry season. These two types of forest cover about 2 million km2 in the region, although many forests are degraded by logging, farming, and fire. In Malaysia, Myanmar, and Sumatra, forests are being modified by timber extraction. In Laos, Kalimantan, and Papua New Guinea, shifting cultivation is encroaching into the forests; in Bangladesh, India, the Philippines, Sri Lanka, Thailand, and Viet Nam, only scattered relicts of relatively undamaged forests remain (Collins et al., 1991).
Studies of the potential regional impacts of climate change on the forests and forestry of Tropical Asia are limited, although a number of local studies have been carried out. Results of research from Thailand suggest that climate change would have a profound effect on the future distribution, productivity, and health of Thailand's forests. Using climate change scenarios generated by the UKMO and GISS GCMs, Boonpragob and Santisirisomboon (1996) estimated that the area of subtropical forests could decline from the current 50% to either 20% or 12% of Thailand's total forest cover (depending on the model used), whereas the area of tropical forests could increase from 45% to 80% of total forest cover. Somaratne and Dhanapala (1996) used the same model for Sri Lanka and estimated a decrease in tropical rainforest of 2-11% and an increase in tropical dry forest of 7-8%. A northward shift of tropical wet forests into areas currently occupied by tropical dry forests also is projected.
Teak is an important wood product throughout the region; in Java, it has been shown to be sensitive to variations in climate (Whetton and Rutherford, 1994). Using climate scenarios generated by ECHAm3, Achanta and Kanetkar (1996) have linked the precipitation effectiveness index (PEI) to net primary productivity of teak plantations in Kerala State, India. They estimate that a projected depletion of soil moisture would likely cause teak productivity to decline from 5.40 m3/ha to 5.07 m3/ha. The productivity of moist deciduous forests also could decline, from 1.8 m3/ha to 1.5 m3/ha.
In recent decades, deforestation has increased in Tropical Asia. The impacts have been investigated in a series of GCM simulations of the effects of deforestation on the local and regional climate (Zhang, 1994). In general, Zhang found that deforestation can modify land-surface characteristics, such as surface albedo and surface roughness, and thus redistribute the local surface energy budget. At the same time, modification of land-surface processes by deforestation may produce some disturbances of monsoon circulation over the southeast Asian region. Deforestation, along with the potential impacts of climate change, also may have a negative impact on sustainable-nutrition security in south Asia (Sinha and Swaminathan, 1991).
In semi-arid regions of Tropical Asia, tropical forests generally are sensitive to changes in temperature and rainfall, as well as changes in their seasonality. Fire-which also is influenced by these changes-significantly affects the structure, composition, and age diversity of forests in the region. Fire frequency is affected by intentional (e.g., slash and burn agriculture) and nonintentional changes in human land use, as well as by variations in climate (e.g., longer or shorter dry seasons).
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