( ent rainfall. The wettest rainforest si...)Scientists: As rainfall changes, tropical plants may acclimate

"That's the range of rainfall you might find across the entire tropics, but that would usually be over hundreds or thousands of kilometers," Schuur said. "I can visit all of these forest sites in a single day."

The scientists analyzed nitrogen isotopes in the soil and leaf samples of four plant species at each site. They learned that drier soils contained more nitrogen in the form of nitrate, while wetter soils contained more nitrogen in the form of ammonia. Isotopic analysis of the plants revealed that they switched from nitrate to ammonia "abruptly, and in unison" once the rainfall reached a certain level.

"There's an abrupt change halfway through the rainfall gradient, and they all switch to this other form for their nutrition," Schuur said.

That's a surprise partly because of the uniformity of response, he said. Such uniformity sharply contrasts the conventional notion that tropical plant species coexist by adopting widely different strategies to getting what they need. At least with regard to nitrogen uptake, all the Hawaiian plants acted the same -- and at the same time.

" ?This does not support the idea that natural selection has caused species to diverge into highly specialized niches for nitrogen consumption," the PNAS paper says.

That's a positive sign considering that as the Earth warms, some areas of the tropics are widely expected to be wetter, some drier. So, at least one of dozens of variables that will change with precipitation changes ?nutrient uptake ?might not affect tropical plants. That said, plenty of others could, Schuur said.