Structural dynamics of a natural mixed deciduous forest in western Thailand

Structural dynamics of a natural tropical seasonal mixed deciduous - forest were studied over a 4-yr period at Mae Klong Watershed Research Station, Kanchanaburi Province, western Thailand, with particular reference to the role of forest fires and undergrowth bamboos. All trees greater than 5 cm DBH in a permanent plot of 200 m x 200 m were censused every two years from 1992 to 1996. The forest was characterized by a low stem density and basal area and relatively high species diversity. Both the bamboo undergrowth and frequent forest fires could be dominant factors that prevent continuous regeneration. Recruitment, mortality, gain (growth of survival tree plus ingrowth) and loss in basal area (by tree death) during the four years were 6.70%/yr, 2.91%/yr, 1.22%/yr and 1.34%/yr, respectively. Mortality was size dependent; middle size trees (30-50 cm) had the lowest mortality, while the smallest (5-10 cm) had the highest mortality. Tree recruitment was observed particularly in the first two years, mostly in the area where die-back of undergrowth bamboo occurred. The bias of the spatial distribution of recruitment to the area of bamboo die-back was significant and stronger than that to the forest canopy gaps. Successful regeneration of trees which survive competition with other herbs and trees after die-back of bamboo could occur when repeated forest fires did not occur in subsequent years. It is suggested that both the fire disturbance regime and bamboo life-cycle greatly influence the structure and dynamics of this seasonal tropical forest.