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The distribution of plant species in tropical forests at local and regional levels is due to differing sensitivity to drought, according to a study published in Nature this week (3 May).

The authors say understanding this will help model how changes in rainfall and soil moisture ― caused by climate change ― will affect the makeup of tropical forests, as well as inform forest conservation efforts.

Researchers from Germany and the United States linked data on the ability of 48 Panamanian tree and shrub species to survive dry conditions with data on how the distribution of these species varies across central Panama with respect to water availability.

The geography of Panama ― a narrow country with a dry Pacific side and a wet Atlantic coast ― allowed the researchers to work over a broad range of moisture levels.

"The strong rainfall gradient spanning the Isthmus of Panama provided an ideal setting for evaluating the importance of variation in water availability," said author Liza Comita, from the University of Georgia, United States.

Although correlations between water availability and species distribution already exist, the intention of the study was to determine which factors ― water, light and nutrient availability, levels of herbivores and pathogens, and shade tolerance ― are most important in determining species distribution.

The researchers found that a species' sensitivity to drought was a "significant predictor" of where the species would be found on the rainfall gradient across the Isthmus.

They found that species that occur more often in drier forests to the south of the country are more drought-tolerant than those found in wetter forests in the north, near the western Atlantic coast.

A species' drought sensitivity also played a role in determining where it would be located locally within those sites.

This is the first time, say the researchers, that the mechanism behind the local and regional distribution of a species can be fully explained by its vulnerability to the availability of water.

This research is important, says Comita, because global climate change will bring substantial shifts in rainfall patterns in tropical forests. 

"If we want to understand the consequences of such shifts, we have to understand the direct role of rainfall and drought periods in shaping tropical forests," Comita told SciDev.Net.

The research was sponsored by the US Smithsonian Tropical Research Institute.

Link to full paper in Nature

Reference: Nature 447, 80 (2007)