Fire regimes and their ecological effects in seasonally dry tropical ecosystems in the Western Ghats, India.

摘要

The tropics are currently witnessing significant deforestation, recent studies from the Global forest resources assessment of FAO suggests, that of the mean annual global deforestation of 16.1 million hectares for the time period 1990--2000, more than 90% of deforestation was from the tropics. In India greater than 75% of the original forest area has been deforested and in the Western Ghats only 6.8% of the original vegetation remains. A number of disturbances in these ecosystems such as logging, grazing, and forest fires are common to the Western Ghats. This dissertation investigates forest fires which are annual disturbance events in these forests, compares and contrasts fire regimes across the different vegetation types, and also examines their ecological effects.; I combined information from remote sensing imagery and a meticulous ground mapping effort of all fires in the Mudumalai Wildlife Sanctuary (MWLS) in the Western Ghats of India over the past 14 years (1989--2002). These spatial data on fire occurrence were integrated with maps of the specific vegetation types found in the MWLS to examine fire conditions in each class of vegetation. For the MWLS, all forest types were found to have an average fire-return interval (FRI) of 7 years and the sanctuary as a whole had a FRI of 2.7 years. Compared to a similar 13-year MWLS fire dataset from the 1910s, this represents an almost three-fold increase in fire frequency over the last 80 years. I estimate a FRI of roughly 5 years for both the larger Nilgiri Biosphere Reserve and the entire Western Ghats. The estimated fire frequencies for the Western Ghats forests outside of protected reserves are considered very conservative given other recent reports.; I determined the spatial relationship between park boundary and mean fire-return interval from satellite data obtained between (1996 and 2005). Different models explain significant variations in the FRI. The linear model is the most realistic and explains 30% of the variation in FRI as a function of distance from park boundary. I implemented a logistic regression model to determine other significant determinants of forest fires in the landscape. Elevation, mean annual rainfall, and forest fractional coverage emerge as significant predictors of fire in the landscape. Results from variogram models indicated the increased importance of the spatial structure in predicting fire occurrence in the landscape.; The study of fires between 1996 and 2005 shows that the mean fire sizes in tropical dry deciduous forests were four fold larger than mean fire sizes in tropical moist deciduous forests and two fold larger than tropical dry thorn forests. Maximum fire size in the tropical dry deciduous forest was 20 fold larger than moist deciduous forest while it was only 6 fold larger than the tropical dry thorn forest. Fires have resulted in significant mortality on small (0--5 cm dbh) size class in the tropical dry deciduous and tropical moist deciduous forests. The differences in fire regimes have resulted in significant effects on the regeneration, structure, composition and diversity in these vegetation types.