The trailing edges of retreating tropical forests are under-studied. And they are under even greater pressure than temperate forests from the direct and synergistic effects of climate change (Opinion, Nature 464, 1271; 2010).

In rainforest remnants of equatorial Singapore, understorey ecosystems could be becoming dangerously dry. During a near-drought in February and March this year, the water table sank and some forest streams temporarily disappeared from the country's two nature reserves.

Rainfall is likely to become more seasonal under changing global climate patterns such as El Niño (S. Yeh et al. Nature 461, 511–514; 2009). This is expected to make a larger contribution than warming in the shift towards arid conditions in the perhumid tropics, which normally have plentiful rain throughout the year. How tropical rainforest plants will respond to the new conditions is unclear, because field measurements are rare. Unlike temperate forests, no dramatic diebacks have been documented and we have fewer historical clues.

There may also be insidious changes under way in the community composition of rainforests in tropical Asia. In Singapore, beach-type forest species such as Calophyllum ferrugineum and Syzygium grande (sea apple) are becoming increasingly dominant among seedlings in the inland forest understorey. Mass fruiting of dipterocarps, the dominant plant family in tropical Asian rainforests, has been triggered more frequently as El Niño events have increased. With less time to accumulate resources, parent trees may be forced to produce smaller seeds that have less chance of survival.

Synergistic interactions with other conservation threats could also be a bigger problem in tropical forests, which have many more species and more complex mutualisms than temperate forests.