Thousands of tiny seedlings taken from 48 trees and shrubs native to Panama sprouted in a greenhouse. Three thousand of the budding plants then made the journey to the understory of a tropical rain forest. Half were watered during the local dry season, the others were not. Despite the high humidity, some withered in the absence of water, and in so doing suggested that water determines the fate of plants—even in rain forests.

Applying this hypothesis to 122 forest plots catalogued by the Smithsonian Tropical Research Institute in Panama—ranging from the drier Pacific side of the isthmus to the wetter Caribbean side—plant ecologist Bettina Engelbrecht and her colleagues demonstrated that drought controls plant distribution. In other words, the droughts that will be a regular occurrence in a warmer world, according to the fourth assessment of the Intergovernmental Panel on Climate Change, could radically alter the face of tropical forests the world over.

"Drought is indeed important for species distribution, even in the humid tropics," Engelbrecht says. "Most tropical forests are exposed to one or even two dry seasons and, during these dry periods, plants can strongly suffer from the lack of water—they grow less, wilt or even dry out." And it is this sensitivity to drought that maps the various plants' success and distribution, more so than light or nutrients in the soil, even though responses to light have been observed to determine which plants quickly spring back in a clearing on the forest floor or under a gap in the canopy. "Nutrients are important for explaining the remaining variations in species distribution that is not explained by drought tolerance," she adds.

Even the 50 hectare forest on Barro Colorado Island at the center of the isthmus—where every seedling and adult plant has been identified and mapped—revealed that drought sensitivity determined which plants would thrive where. "Future changes in hydrological processes and precipitation patterns will have direct consequences for species ranges, tropical forest community composition and ecosystem function," the researchers write in Nature.

Changes in rainfall patterns that are already being observed in the tropics—a result of forest clearing and climate change—will likely remake Panama's tropical forests. "Dramatic shifts in species distributions, community composition, forest diversity and ecosystem functioning have to be expected," Engelbrecht says, "even with relatively small changes in dry season lengths of only about one month."

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