I arrived back from the last trip out to find that the team had discovered a huge, leaning dead tree—a ‘widow-maker’—behind our camp, and no one was comfortable staying at the site any longer. These trees can come crashing down with no warning. Often I have jolted from deep sleep into an adrenaline-flushed wakefulness (How close is it? Should I run? Which way?) by the gunshot-like popping of fibers signaling an old tree starting to topple. On a quiet night, sound travels far, and the tree is usually a long way away, but the panic is real. So, “When a tree falls in the forest…?”
The next day we moved back to the campsite by the river, accepting the small risk that there may be a huge flood again. If flooding returns, we built several racks to protect the equipment—and we’d need to wait with our feet in the water until the water subsided. Pak Manto, our colleague who has spent the most time in this part of the forest, said this river experiences a significant flooding event only about once a year… so we should be fine now. Indeed, the weather this year has been unusually dry. November and December are very wet (total rainfall here averages over 12 feet a year), followed by a few dry weeks in January before the early-year rains resume. But this year it has hardly rained since January.
If this persists until June, when the typically drier season begins, the forest will have a very tough time. The fires from land clearing for hill rice and oil palm have already started, and by September the smoke can be so bad that planes cease flying into local airports, literally stranding us on Borneo! I remember once being unable to see across a room through the smoke. Climate change models predict little temperature change for the tropics, but anticipate significant shifts in rainfall pattern and totals, although these are difficult to predict with models (but see this paper on Indonesia). It is interesting that every villager I’ve talked to has both heard about climate change and believes that he or she has witnessed its effects playing out over the last few years. Rice planting times are changing, winds are wrong, rivers are too low. Nobody seems to be denying climate change in Borneo!
It is true, despite these observed oddities, that we are prone to forget the saw-toothed nature of ‘natural’ weather events. Though rare, extreme events can shape the world in intervening years. Borneo’s forests are a prime example. Wind and animals scatter seeds throughout the forest year by year. Usually the seedlings of most species can survive anywhere in a lowland forest. However, every 50 years or so, there might be a very extreme drought. The hilltops will dry out more than the valleys, and as the drought persists, seedling of species that need higher levels of moisture will die out on the ridges. As a result, when the rains finally return, the species of seedlings will no longer be evenly distributed. As surviving seedlings mature (ever so slowly…1 to 2 centimeters per year in the shade), tree communities begin reflecting this uneven distribution of drought-tolerant species—despite the fact that mature trees seldom die from a bad drought due to their deep tap roots. In this very forest, David Peart and I found evidence for this change in species composition. Exploring the evolution of drought tolerance is one of the goals of our current project, and we’ve sited plots in both wet valleys and drier ridges to observe how species are differentially distributed.