The occurrence of seasonal dry periods in the tropics of South Asia increases with distance from the perhumid equatorial zone. The forests gradually develop to more open, semi-deciduous and deciduous formations [e.g., moist and dry deciduous forests, monsoon forests]. The main fire-related characteristics of these formations are seasonally available flammable fuels [grass-herb layer, shed leaves] which allow the spread of surface fires. Grass species, understory plants [shrub layer] and the overstory [tree layer] are adapted to regular fire influence. The most important adaptive traits are thick bark, ability to heal fire scars, resprouting capability [coppicing, epicormic sprouts, dormant buds, lignotubers, etc.] and seed characteristics [dispersal, serotiny, fire cracking, soil seed bank and other germination requirements] (STOTT et al., 1990; GOLDAMMER, 1993a). These features are characteristic elements of a fire ecosystem.
During the dry season the deciduous trees shed their leaves and provide annually available surface fuel. In addition the desiccated and dried grass layer, together with the shrub layer, add to the available fuel which overall generally ranges between 5-10 t ha-1. The fires are mainly set by forest users [graziers, collectors of non-wood-forest products]. The forests are underburned in order to remove dead plant material, to stimulate grass growth, and to facilitate or improve the harvest of other forest products. The fires usually develop as surface fires of moderate intensity [usually less than 400 kW m-1; cf. STOTT et al., 1990], and tend to spread over large areas of forested lands. The tree layer is generally not affected by the flames, although crowning may occur earlier in the dry season when the leaves are not yet shed. In some cases fires may affect the same area two or three times per year, e.g., one early dry season fire consuming the grass layer and one subsequent fire burning in the shed leaf litter layer (GOLDAMMER, 1993a,c).
Dry deciduous forests and moist deciduous forests occur on c.250×106 ha and 530×106 ha respectively. No reliable information exists on the extent of recurring fires in these areas. It was estimated that in Burma between 3-6.5×106 ha of forests are annually affected by fire. A report from Thailand in the late 80’s estimated an annually burned area of ca. 3.1×106 ha, predominantly in dipterocarp monsoon forests. The affected areahas diminuished considerably since then: measures of fire protection have reduced the average area burned to ca. 1.5×106 ha (unpubl. fire inventory from Thailand, 1994). The analysis of historic information from British India reveals that during the last century and early this century almost all Indian deciduous forests were burned every year (GOLDAMMER, 1993a).
The ecological impact of the yearly fires on the deciduous and semi-deciduous forest formations is significant. Fire strongly promotes fire tolerant trees, which replace the species potentially growing in an undisturbed environment. Many of the monsoon forests of continental Southeast Asia would be reconverted to evergreen rain forest biomes if the human-made fires were eliminated [Fig.6]. Such phenomena have also been observed in Australia where the aboriginal fire practices and fire regimes were controlled and rain forest vegetation started to replace the fire-prone tree-grass savannas. The fire adaptations and the possible fire dependance of economically important trees such as Sal [Shorea robusta, Fig.7] and Teak [Tectona grandis] have long been the focus of a controversial discussion regarding the traditional fire control policy in British Indian Forestry (GOLDAMMER, 1993a).
Fig.6. Examples of evolution of seasonal and perhumid forest biomes in continental and insular Southeast Asia as influenced by fire and fire protection [after Blasco, 1983].
The fire climax deciduous forests are not necessarily in an ecologically stable condition. Long-term impacts of the frequent fires lead to considerable erosion processes because of the removal of the protective litter layer just before the return of the monsoon rains. The erosion rates under standing Teak forests regularly affected by fire may exceed 60 t yr-1 ha-1 (GOLDAMMER, 1991a).