Nutrients matter in tropical forests

Don’t blame a lack of rain: A tropical forest of dry, bare-branched trees might be that way because of soil chock full of phosphorus.

In Panama’s dry season, leafy woodlands stand starkly next to forests of naked trees. Scientists had thought that rainfall caused the contrasting growth patterns because different soils there have different abilities to hold water. Narrow bands of rocky outcrops shoot through Panama’s soil, and rocky soils often hold little water.

But when ecologist Richard Condit of the Smithsonian Tropical Research Institute in Panama City and his colleagues analyzed samples from 72 forest sites across Panama, they found that soil moisture levels between neighboring leafy and leafless trees didn’t differ as much as they had expected.

Instead, phosphorus levels seemed to drive the growth patterns of leafy versus leafless trees, the researchers report online February 25 in the Proceedings of the National Academy of Sciences. The leafy trees, Condit says, evolved to live in low-phosphorus soils. Even in dry seasons, these trees hold onto their leaves — losing water — because nutrients are scarce, and making new leaves is expensive. Where phosphorus is abundant, however, trees can afford to dump their leaves, and grow new ones when the soil is wet.

The results surprised Condit. “I didn’t expect nutrients to be so important.”