The effects of drought and fire on seed and seedling dynamics in a tropical seasonal forest in Thailand

Abstract

Seed production and seedling dynamics were studied for 6 years (1992-1997) with particular emphasis on the effects of the drought and fire in a natural tropical seasonal forest at Mae Klong Watershed Research Station, Kantchanaburi, western Thailand. The number of seed produced and emerged seedlings varied greatly among years and species. Most tree species produced very few seeds in 1994 and many in 1995 and 1996, particularly, Shorea siamensis Miq. which displayed the most seeding habit. Some seedlings, Shorea, Dipterocarpus alatus Roxb., and Dipterocarpus turbinatus Gaertn. f., emerged at the end of the dry season (late April, after the first rain but when soils still have a low soil moisture content), while the majority of seedlings emerged at the beginning of the rainy season. After a fire disturbed the plot in April 1996, many species increased their seedling emergences, especially Berrya ammonilla. Seedlings of most species less than a year old showed a relatively low mortality in the rainy season, but those of Pterocarpus macrocarpus Kurz had high mortality. The mean survival rate for all seedling species in this forest was quite low (24.1% y-1), and different values existed for the rainy and the dry season (11.5, and 6.1% month-1, respectively). This suggests that different species have adaptations related to the season of seedling emergence and resistance to drought in this tropical seasonal forest community. The traits of tree species are classified in terms of their traits in the early stages of regeneration; seed and/or seedling bank, resprouting, and drought resistance. Most species have adapted to fire and/or drought by resprouting, seed bank, and/or seedling bank, although the few species which occur mainly in mesic evergreen forests have less adapted to these environments. The demographic variations in seed and seedling stages may contribute to the coexistence of these species in this species-rich dry tropical forest.