Experiments and observations together show how the distribution of Amazon trees predicts their tolerance to drought

Nowhere on Earth has more species than tropical forests, but climate change may now threaten thousands of trees here.  How can we efficiently predict which are at risk and which are safe - without doing an impossible number of experiments?

One way may be to use details of species distributions to predict their climate sensitivity. There are already some suggestions that seedlings of species from dry locations resist drought for longer than seedlings of species from wetter areas nearby. But would that be true for adult trees? And would it hold across the scale of a whole continental?

For the first time it is now possible to answer these questions. We integrate data from experiments and observations with data on the distributions of more than 100 tree groups from very dry to extremely wet areas across 11 countries in the Amazon and Central America. We found that, indeed, the distribution of tropical trees does predict their ability to tolerate droughts. The effect is larger for trees when compared to seedlings or saplings and increases with the length of the experiment.

Droughts are becoming more frequent in Amazonia. In particular, the dry seasons are becoming longer and these dry periods are also getting hotter. Our results therefore suggest that if this climatic trend persists we may observe a shift in tree species in the Amazon. This is not good news; it could presage the decline and loss of unique species across the region, and have important consequences to the services these forests provide to people too.

There are still gaps in our knowledge though. The drought experiments done to date have all taken place in areas that are already quite dry for Amazonian standards. Scientists still need to investigate what happen in the wetter – and potentially more vulnerable - areas.  And why exactly these trees respond differently is unresolved, requiring detailed studies of the physiology of wet and drier-distributed Amazon trees. For now though we already have a way to predict which plants will suffer most, and where, giving us some hope that we may be able to do something about it.

Esquivel-Muelbert et al (2017) Biogeographic distributions of neotropical trees reflect their directly measured drought tolerances, Scientific Reports 7 (1)