The forests of South-East Asia are dominated by trees belonging to one plant family: the Dipterocarpaceae. They get their name from the heavy two-winged fruits that they produce sporadically and synchronously, a phenomenon known as mast seeding. Mast seeding is thought to have evolved as an anti-predator strategy: if vast numbers of seeds are only produced infrequently, then the seed predators have little chance of eating them all.
Dipterocarps are a highly diverse group, with many apparently similar species growing side-by-side, raising the interesting ecological question of how so many species coexist. One popular and controversial explanation for this diversity is that species specialise on different regeneration conditions: in particular, species take advantage of the mosaic of light conditions that exists in a forest1. For example, in tree-fall gaps where light levels are high, the fastest-growing seedlings might belong to one particular species, while under much more shaded conditions, a different species might grow fastest. Because each species is the fastest growing under some light level, each species wins some sites, hence diversity is maintained, as long as the mosaic of light conditions persists. The hypothesis is sometimes known as the “cross-over” hypothesis, as the growth rate rankings “cross-over” from one light regime to another.
Support for this hypothesis is generally rather poor, with many studies finding that the ranking of species with regard to growth rate does not change with light availability. To test this hypothesis with dipterocarps we grew seedlings in shade-houses with three different light levels and quantified their growth rates over two years2. We found that the rank order of species’ growth rates did indeed change between low, medium and high light, lending support to the cross-over hypothesis for this group of species. Of course, that’s not to say that it’s the only mechanism supporting diversity in these forests, but it might be one way that the forests of South-East Asia maintain their incredible diversity.
- Sack L & Grubb PJ. (2001) Why do species of woody seedlings change rank in relative growth rate between low and high irradiance? Functional Ecology 15, 145–154.
- Philipson CD, P Saner, TR Marthews, R Nilus, G Reynolds, LA Turnbull, and A Hector. (2012) Light-based regeneration niches: evidence from 21 Dipterocarp species using size-specific RGRs. Biotropica 44, 627-636.