Sabah Biodiversity Experiment

The SBE main site can be found here; but here’s some recent updates:

FOREST RESILIENCE TO CLIMATE CHANGE

The Sabah Biodiversity Experiment tests the importance of forest diversity for ecosystem functioning, stability and services. This is particularly important as much of the forest in SE Asia is predicted to suffer increased drought under climate change while selective logging has reduced levels of diversity in the remaining areas, many of them fragmented by land conversion to oil palm plantations. Many species are thought to be drought tolerant or sensitive but it would be more useful to have a quantitative trait to substantiate these observations and for prediction of the drought resilience of more poorly known species. Non-structural carbohydrates (soluble sugars and starch) have been hypothesised to play a role in drought resilience by helping break down the bubbles that can form in drought xylem, breaking the flow of water through the plant. We wanted to know if tropical forest trees (including those used in the SBE) differed in the NSC levels and if this was related to their rates of drought-induced mortality. We also wanted to experimentally test the role of NSCs in drought survival.

To test whether NSC levels vary between species and how this relates to drought survival we measured levels in seedlings of 10 tropical species (9 dipterocarps and a legume Koompasia) and looked at the relationship with mortality rates when they were experimentally droughted. Levels of NSCs varied between species and those with higher levels survived under drought for longer.

However, the species differ in many ways, not just their NSC levels so to confirm the causal link we experimentally manipulated NSC levels in seedlings of all species. This is easier said than done! From previous PhD work by Philippe Saner, we knew that seedlings in dark shade houses (~1% of incoming light) deplete their NSCs but in lighter conditions (~20-30%) can replenish their levels in a couple of months. So, we took seedlings of all ten species and grew one group of each under low light levels for about 3 months before swapping them to lighter conditions for a further three months. We did the opposite with a randomly selected matched subset. Using this light-swapping pre-treatment we produced seedlings with normal levels of NSCs (those growing under lighter conditions last) and a matched treatment with experimentally depleted NSCs (those growing under darker conditions last). Because of the aseasonal tropical climate where on average one day is much like another the order in which the seedlings experience the lighter or darker growth conditions made no difference to their size, root to shoot ratio, number of leaves etc. By using the light-swapping methodology we were then able to experimentally manipulate NSC levels while controlling for potentially confounding factors of size and morphology. We then exposed seedlings with high and low NSCs to controlled drought and confirmed that those with depleted levels of NSCs succumbed to drought more quickly, experimentally confirming the role of NSCs in resisting drought mortality.

Our results confirm a role for NSCs in the survival of drought by dipterocarps. This may allow us to predict which individuals, species and forests are more vulnerable to drought. It should also help us design mixtures of species for use in reforestation projects that are more drought resilient. Past enrichment planting schemes tend to restore areas using one or a few species. Our results suggest if these have low NSC levels they may be vulnerable to drought with potential die off across large areas. Increasing the diversity of planting mixtures to ensure a range of species with higher NSC levels should increase the resilience of these species to drought. A drought in the Danum Valley area of Borneo will allow us to test this at larger scales with older trees by comparing mortality levels in the Sabah Biodiversity Experiment plots that are planted with different mixtures of 1, 4 or 16 species.

Links

O’Brien et al. (2014) Drought survival of tropical tree seedlings enhanced by non-structural carbohydrate levels. Nature Climate Change