4. Fire Climax Pine Forests in South Asia’s Tropical Submontane and Montane Altitudes: In Competition with Dipterocarps
Approximately 105 species of the genus Pinus are recognized. From the main center of speciation in Central America and Southeast Asia, some species extend into the tropics. In mainland South Asia and Insular SE Asia the pines are largely confined to the zone of lower montane rain forest. They are usually found on dry sites and prefer a slight to distinct seasonal climate. Most tropical pines are pioneers and tend to occupy disturbed sites, such as landslides, abandoned cultivation lands and burned sites.
Fig.7. In continental South Asia tree species in the seasonally dry forests show remarkable adaptations to fire. Pure stands of fire tolerant Shorea robusta in northern India and southern Nepal are the result of the selective force of annual fires.
Besides the pioneer characteristics, most tropical pines show distinct adaptations to a fire environment [bark thickness, rooting depth, occasionally sprouting, high flammability of litter] (GOLDAMMER and PEÑAFIEL, 1990). The tropical pure pine forests of South Asia, e.g., Pinus khesyia, Pinus merkusii, Pinus roxburghii, most often are the result of a long history of regular burning. As in the tropical deciduous forests, fires are mainly set by graziers, but also spread from escaping shifting cultivation fires and the general careless use of fire in rural lands. Fire return intervals have become shorter during the last decades, often not exceeding one to five years. These regularly occurring fires favor the fire-adapted pines which replace fire-sensitive broadleaved species [Fig.8]. The increased frequency of human-caused fires has led to an overall increase of pines and pure pine stands outside the potential natural area of occurrence in a non-fire environment. In the mountainous zones of the tropics, fire also leads to an increase of the altidudinal distribution of pines, e.g., by expanding the mid-elevation pine forest belt downslope into the lowland Dipterocarp forest biome and upslope into the montane broadleaved forest associations, e.g., the mixed oak-chestnut forests (KOWAL, 1966; GOLDAMMER, 1993a). These tropical fire climax pine forests occur throughout submontane elevations in Burma, Thailand, Laos, Kampuchea, Viet Nam, the Philippines [Luzón] and Indonesia [Sumatra].
All over the tropical and subtropical world, fire climax pine forests provide a high degree of habitability and carrying capacity for humans. If used properly in time and space, fire creates a highly productive coniferous forest, which grants landscape stability and sustained supply of timber, fuelwood, resin, and grazing land. However, together with the effects of overgrazing [including trampling effects] and extensive illegal [fuel]wood cutting, the increasing occurrence of wildfires tend to destabilize the submontane pine forests and result in forest depletion, erosion and subsequent flooding of lowlands.