Climatic Influence on Asian Tropical Rain Forest Dynamics

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In the Asian tropics, an El Nino event brings a draught, while a La Nina induces wet climate. This study investigates the effect of two contrasting climatic events, El Nino and La Nina, on the forest community dynamics of tropical rain forest in Southeast Asia. Field work was conducted in a lowland tropical rain forest in Malaysia and a lower montane tropical rain forest in Thailand. In the Malaysian site, all the living trees>1.0 cm dbh within a 52-hecatre plot were labeled, mapped, identified to species, and measured by dbh in 1992. The second enumeration was conducted in 1997. The dead trees were observed again in 1999. Out of the 359965 living trees in 1997, 15798 trees died after two years, showing a mortality of 0.022/yr, which is greater than 0.013/yr observed in 1992-1997. The increase of mortality suggests the impact of El Nino(1997/1998)and La Nina(1998/1999)on forest dynamics. The mortality rate of 0.013/yr gives an estimate of 9359 dead trees in 1998 and 1999, when El Nino … More and La Nina could not hypothetically occurred. Therefore 6439 trees are estimated to be killed by El Nino and Lanina. The number of dead trees within the new landslide areas of 0.32 ha in 1999 is 2235. Hence the trees killed by El Nino are equivalent to 4204. These estimates suggest the mortality rate of 0.012 due to El Nino and that of 0.0064 due to La Nina. The dependency of mortality rates on topography is clear, since mortality rates are higher on ridges than in valleys. A multiple regression analysis on illipe nut exports from Sarawak showed a strong correlation between exported illipe nut quantities and draft probability, which is defined by the ratio of the monthly number of weather stations with rainfall less than 100 mm/month to the monthly total number of available weather stations in Sarawak. In the montane forest of Thailand, population dynamics of Fagaceae and Lauraceae speices were monitored in a 15 ha plot. However, no clear sympotoms were observed, concerning the relationships between forest dynamics and El Nino/La Nina. Although the unusual cold temperature of minus 3.2 C was observed on 25 December 1999, flowering did not occurred in the spring 2000, suggesting another flowering trigger different from dipterocarps in Malaysia. Less