Tree Physiology

To cope, or not to cope: tropical rainforest tree leaves show little response to drought

Drought will become more severe in tropical rainforests as climate change continues. This could cause widespread tree mortality in these forests, unless the trees are able to acclimate and produce leaves that can tolerate dry conditions (structures known as xeromorphic leaves). However, there are very few studies of tropical tree responses to drought due to the difficulty of establishing and maintaining field sites in tropical rainforests. In a recent article in Tree Physiology, Binks and colleagues do just that: using a 12-year long through-fall exclusion experiment to reduce water availability, they studied how leaf structure changed under drought in the lowland Amazon Rainforest.

Caxiuanã National Forest Reserve in the lowland Amazon Rainforest.
Caxiuanã National Forest Reserve in the lowland Amazon Rainforest. Photo: Patrick Meir

What they found was surprising: even after 12 years of drought stress, leaves produced in the tropical trees were no more xeromorphic than leaves at the beginning of the experiment – drought stress had very little influence on leaf anatomy.

Why is this? The authors hypothesize that the availability of water during the trees’ evolutionary history may have selected against phenotypic plasticity (the ability to acclimate and change physiology in response to environmental conditions) for drought tolerance. However, we do know that tropical rainforests are hyper-diverse, and this diversity may have promoted environmental niche specialization in the trees studied by Binks and colleagues. If most species in tropical rainforests are specialized to a narrow environmental niche, then climate change-induced drought could cause massive shifts in community species composition.

This study makes it clear why it is so important to study tropical forests – the hyper-diversity of these forests means that much of what we assume about tree responses to environmental change may not apply to these ecosystems.

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