Impacts of an Extreme Precipitation Event on Dipterocarp Mortality and Habitat Filtering in a Bornean Tropical Rain Forest

The frequency of extreme precipitation events is predicted to increase in some tropical regions in response to global climate change, but the impacts of this form of disturbance on the structure and dynamics of tropical tree communities across heterogeneous landscapes remain understudied. We determined the effects of an extreme precipitation event (EPE) in July 2006 on mortality of dipterocarps on a 68ha permanent inventory plot in Sepilok Forest Reserve, Sabah. For stems 30cm dbh, 12 of the 15 species of Dipterocarpaceae on this plot have significant positive and/or negative associations to habitats defined by topography and soil type. Short-term mortality induced by the EPE was much greater for individuals growing on the alluvial floodplain (13.7%) than in the mudstone (1.4%) or sandstone (0.0%) habitats, but mortality of dipterocarps did not differ among these habitats in the subsequent 5-yr interval. The likelihood of mortality in response to the EPE was highest for a small group of fast growing dipterocarps that possess low wood density and a strong association to the alluvial forest habitat. This group of species represents a high percentage of dipterocarp individuals but a low proportion of dipterocarp diversity in this habitat. We conclude that disturbance induced by high rainfall events contributes to the episodic nature of tropical forest dynamics, and that increases in the frequency of these events would disproportionately impact low-lying alluvial forest environments and some of the species growing in them. Malay Kekerapan kejadian hujan di kawasan tropika telah diramal akan semakin bertambah, disebab oleh perubahan iklim dunia. Pada masa ini, kesan kejadian hujan yang bertambah kerap terhadap struktur serta dinamik hutan tropika di pelbagai landskap masih kurang diketahui. Penyelidikan telah dibuat untuk menyiasat kesan kejadian hujan kerap pada 2006 terhadap kadar pertumbuhan Dipterokap yang mempunyai dbh melebihi 30cm terletak dalam plot 68ha di dalam Hutan Simpan Sepilok Kabili di Sabah. Penyelidikan menunjukkan 12 daripada 15 spesis yang dikaji mempunyai kaitan positif dan/atau negative yang ketara terhadap topografi serta jenis tanah yang didapati di habitat tersebut. Didapati kadar mortality adalah tinggi untuk individu yang bertumbuh di tanah alluvial (13.7%), jika dibanding dengan individu yang bertumbuh di tanah lumpur (1.4%) dan tanah pasir (0.0%). Walaubagaimanapun, kadar mortality adalah tidak banyak berbeza antara individu yang bertumbih di jenis tanah berlainan apabila kajian diulangi 5 tahun kemudian. Adalah didapati juga, tindak balas terhadap kejadian hujan yang kerap adalah terhad kepada beberapa jenis Dipterokap yang bertumbuh cepat, mempunyai kepadatan kayu yang rendah dan yang terletak di tanah alluvial sahaja. Kesimpulannya, kekerapan kejadian hujan sebegini akan mempengaruhi kadar pertumbuhan spesis pokok yang bertumbuh di hutan bertanah alluvial.