A tropical moist forest is a relatively stable environment, offering food, water and shelter to its inhabitants year-round. Yet even in southern Thailand, the least seasonal part of the country, with the shortest dry season, the months of February through to April are a time of water stress, when many of the streams and gullies, even those inside the forest, run dry. Previously, there was abundant water year-round. However, since the early to mid-1980s, our watershed of Phaen Din Samur, a raised plateau roughly 300 m above sea level, has been deforested by immigrant coffee-growers from the adjacent provinces of Nakhon Si Thammarat and Surat Thani, who moved into the area along logging roads created by the local province timber company. Now, with the loss of their watershed, many of the villages in surrounding areas are scarcely less dry and dusty then those in Esarn, and drinking water has to be trucked in from outside. What effect is this unaccustomed water stress having on the forest? Will these new, human-induced fluctuations cause the extinction of some frogs and toads, and perhaps even some of bird species most dependent upon year-round supplies of abundant freshwater? It is perfectly possible that such changes in the hydrology could, over a period of years, bring about profound changes in the plant species composition of the forest too.

When you live in a forest for a period of year, you become aware of how much each year differs from the preceding one. Besides annual cycles, there are also so-called “supra-annualEcycles, and no two years are ever quite the same. Many trees don't flower or fruit every year, but at seemingly irregular intervals of a few years. The year 1990, and again last year, 1994, were good fruiting years for the plum tamarind, a tall forest tree, Dialium platysepalum , known also as “kerinchiEin Malaysia. In each of these years, sack after sack of the velvety black fruits (luuk yii) of this species, which cracks to yield sweetish sour, tamarind like pulp, were brought off the mountains, and sold for a good price in local market, fetching Bht.30-40 per kilo for the villagers and Bth.80 per kilo for the traders.

The villagers themselves told us that in the past, the entire tree would be cut down just to harvest the fruits, These days, though, there is much less forest and many fewer “yii”trees, and the few villagers who still collect “luuk yiiEare careful to preserve the trees, lopping off only the fruit-bearing branches. In addition, the collectors don't reveal the location of “theirEtrees to rivals. If 1994 was a good year for “luuk yiiE then it was a terrible one for “satawEwith scarcely any to be found. My experience of the preceding three years had led me to think that “satawEin the forest fruited abundantly every year.

Not so , however. But if there was a shortage in 1994, this year there is a veritable glut, and every tree in the forest bears abundant, green, strap-like pods.

It's not only the trees that go in cycles. Sometimes the birds follow them. Acharn Pilai Poonswad told me that, this year, there are only half as many hornbill nests as usual in her Khao Yai and Huai Kha Khaeng study sites. Now hornbills are generally dependent upon figs, those all-important keystone species of the forest ecosystem, and this year, inexplicably, fruiting is much reduced, apparently causing many pairs to miss out on nesting altogether. In Malaysia, there is some evidence to suggest that widespread deforestation of the region may be affecting rainfall patterns, causing unusually dry years to recur with increasing frequency. This is apparently inducing the widespread failure of nesting in even many insectivorous forest birds during same years.

In 1990 , we had at least three nesting pairs of Chestnut-capped Thrushes in or Khao Nor Chuchi study area and there was a time, in May or June, when it was hardly possible to go into the forest without seeing or hearing at least one. Yet in succeeding years, we have scarcely met with any sign of Chestnut-capped Thrush although there has been no ostensible change in the habitat.

What is going on? As far as is known, this is a resident species.

Philip D. Round
September 1995