Science - February 23, 2010

Drought tolerance throws light on tropical forests

The composition of tropical forests depends mainly on how trees can tolerate drought, concluded PhD student Lars Markesteijn.

Sufficient light and water are the most important requirements for the growth of trees in tropical forests. Markesteijn (Forest Ecology and Forest Management Group) carried out research in tropical forests in Bolivia to find out if the distribution of tree varieties can be attributed to their responses to light and water availability. In scientific terms, is there a trade-off between drought tolerance and shadow?
Markesteijn compared about sixty dominant varieties of vegetation in dry and wet tropical forests. Subsequently, he examined the functional and morphological characteristics of the root and the leaf systems and the hydraulic transport system in the plant. At the same time, he also carried out tests in greenhouses to see how seedlings of different varieties react to drought. 'It is generally known that a tree adjusts itself to dry situations by concentrating on the development of the root system', explains this PhD student. 'The plant puts a lot of biomass under the ground and less above it. Its way of adjusting to shadow would be the exact opposite.'
Cheap leaves
However, Markesteijn has discovered that this direct trade-off does not happen, or hardly happens in reality. 'Life's not that simple, it seems', he concludes enthusiastically. Trees have found ways to bypass this trade-off. They develop thinner and more efficient roots or 'cheaper leaves'. 'More returns with less investing in biomass. Leaves with a better price-quality ratio.' All these lead to - on a large scale - the distribution of (evergreen) varieties being dependent on drought tolerance. This knowledge comes at an opportune time for predicting the consequences of climate change on tropical forests.
Very different
The situation is very different on a smaller scale, i.e. within one forest. 'Then, drought and shadow tolerances work together', explains Markesteijn. 'And that is exactly the opposite of a trade-off. Varieties which have adjusted themselves to grow in a lot of light can only be found in places where there is a lot of water.' Markesteijn says this isn't as strange as it looks, on second thoughts. Fast growth means having a high photosynthesis level, where light and water are needed.