Drought frequency changes in Sabah and adjacent parts of northern Borneo since the late nineteenth century and possible implications for tropical rain forest dynamics

Abstract

Archival rainfall data are used to investigate changes in drought frequency and severity in Sabah and other parts of northern Borneo since the late nineteenth century. Two measures of drought severity are used: drought duration (given by the number of consecutive months with less than 100 mm rain); and drought intensity (indexed by the cumulative rainfall deficit below 100 mm per month of a drought sequence). Within northern Borneo dry periods are very short (>3 months) and infrequent in Sarawak, south-western Sabah, Brunei and central and western Kalimantan; droughts occur seasonally, but are comparatively short in north-western Sabah; droughts are less frequent) but more severe in eastern Sabah and parts of eastern Kalimantan. In coastal Sabah and Brunei, there has been a statistically significant increase in the frequency and severity of droughts since the late 1960s. At Sandakan, two drought-prone epochs in 1877–1915 and 1968–92 (each experiencing five droughts of at least 4 months duration) are identified, separated by a 52-year period that was nearly drought-free. At Sandakan also, the ecologically damaging 1982–3 drought was neither as long or severe as those of 1903 and 1915. Links with El Niño-Southern Oscillation events are found to be not as strong as previous studies have suggested. Possible implications of the spatial and temporal patterns in drought magnitude-frequency for differences in tropical rain forest character within the region are discussed.

Resistance of a lowland rain forest to increasing drought intensity in Sabah, Borneo

“…This 1998 event ranked reasonably highly among the recent Bornean droughts of ecological significance (cf. Beaman et al 1985, Brünig 1969, 1971Goldammer & Siebert 1990, Harrison 2001, Leighton & Wirawan 1996, Mori 2000: it was not as intense as that in 1982-83 judging by the Tawau and Lahad Datu records (Walsh 1996b, Walsh & Newbery 1999. During the 1998 main event many of the canopy and emergent trees became defoliated (G. Reynolds & R. P. D. Walsh, pers.…”

Fire, drought and El Niño relationships on Borneo (Southeast Asia) in the pre-MODIS era (1980–2000)

“…In general, we see that the 1982-1983 fire activity is confined to a zone extending inland a maximum of c. 200 km. Goldammer and Seibert (1990), Walsh (1995) and Goldammer (2007) report this to be the approximate area within which monthly precipitation fell below the critical 100 mm threshold, with higher rainfall generally seen within the more mountainous areas further inland (Fig. 1).…”

Interannual variation in rainfall, drought stress and seedling mortality may mediate monodominance in tropical flooded forests

“…Therefore, significant soil water shortage develops when rainfall is below this threshold. We quantified the strength of the dry season as the cumulative millimeters by which each dry month was below 100 mm rainfall, termed the rainfall deficit (Walsh 1996). At each census, starting in 1997 we assessed plant water stress as predawn and midday leaf water potentials (i/^L) in from five to ten seedlings of Prioria and Carapa located adjacent to the plots using a Scholander pressure chamber (PMS Instruments, Corvallis, Oreg.)…”