Drought in a Bornean everwet rain forest

1 In 1998, the forests of Lambir Hills National Park experienced a severe drought. Between late January and mid-April, less than one-fifth of the normal expected rainfall was received. To investigate the effects of the drought, survival among 6993 trees (greater than or equal to1 cm d.b.h.) in the drought period (1997-98) was compared with survival among 7270 trees in the same area before the drought (1993-97). 2 Mortality rates for the pre-drought and drought periods were calculated using maximum likelihood techniques. Forest wide mortality rates during the drought were 7.63% year(-1) as compared to 2.40% year(-1) during the pre-drought period. Logistic regression was used to investigate habitat effects. During the pre-drought period, soil type was the most important predictor of tree survival, while during the drought period, slope was the most important. 3 The mortality rate of large (greater than or equal to10 cm d.b.h.) rare trees (< 2 individuals >= 1 cm d.b.h. ha(-1)) did not differ between pre-drought and drought periods, while that of large common trees (> 35 individuals greater than or equal to1 cm d.b.h. ha(-1)) increased 6.5 fold in response to the drought suggesting a possible compensatory mechanism maintaining the persistence of rare species. Mortality of large common and rare species however, did not differ significantly during the drought due to small sample sizes. 4 The results of a comparative analysis of the impacts of droughts in four tropical forest sites provide evidence in support of the Intermediate Disturbance hypothesis.