But in the still wetter terrain that characterizes some rainforests, they switch to another form of nitrogen that becomes more available in those conditions.

The findings, reported in paper set to appear this week in the online edition of the Proceedings of the National Academy of Sciences, present a notable exception to the commonly held idea that tropical plants are highly specialized in their own little environmental niches - and thus very sensitive to disturbances of those niches.

That could be good for the plants because climate change is expected to radically alter rainfall patterns in the tropics. But it comes with a caveat: Nutrient uptake is only one of many ingredients in plant life. Other unrelated changes that accompany a warming climate could still affect plant distribution and growth, such as those that hold sway over pollinators, insect predators or invasive plants.

"These plants should be able to do OK in terms of their nitrogen nutrition, even with the climate changing," said Ted Schuur, a UF assistant professor of ecology and one of four authors of the paper. "But of course, we only studied one group of organisms and one mechanism in this study" and plants depend on many different mechanisms to coexist, some of which may also change with changing rainfall.

The scientists researched plant growth at six sites on the slopes of Mount Haleakala, a volcano on the island of Maui. The sites were ideal because they share the same species, elevations and soils but have vastly different rainfall. The wettest rainforest sites receive an astonishing 196 inches of rain annually, while the driest sites in this study get about 79 inches.

"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.

earlier related report
Tropical Plants Go With The Flow Of Nitrogen
Stanford CA (SPX) May 08 - Tropical plants are able to adapt to environmental change by extracting nitrogen from a variety of sources, according to a new study that appears in the May 7 early online edition of The Proceedings of the National Academy of Sciences. By demonstrating that not all plants specialize in one specific source of nitrogen, the result turns a commonly held theory on its head.

It also provides a dose of optimism that tropical forests will be able to withstand environmental shifts in nutritional cycles brought on by global climate change.

Nitrogen is an essential nutrient that plants must absorb from the soil to survive. Most land plants outside the tropics appear to have evolved to rely on just one of three common sources of nitrogen: nitrate (NO3-), ammonium (NH4+), or dissolved organic nitrogen (DON). As a result of this limitation, they usually inhabit "niches" defined largely by the available nitrogen source. When that source crashes for any reason-often because of shifts in climate-the plants cannot adapt, with potentially disastrous consequences for natural ecosystems.

However, tropical species appear to be far more adaptable than their temperate kin when it comes to their nitrogen needs. A team of researchers has found that, when confronted with shifts in nitrogen availability, these plants simply "flip a switch" and use whatever is handy.

"When it comes to nitrogen, the tropical plants we studied behave like kids at a pizza party-they may prefer pepperoni, but if only plain cheese is available, they'll still have a slice," said lead author and postdoctoral researcher Benjamin Houlton of the Carnegie Institution's Department of Global Ecology. "This result gives a glimmer of hope that tropical ecosystems may have the capacity to adjust to certain aspects of climate change."

Working in six well-known sites with variable rainfall on Hawaii's Maui Island, the researchers measured the soil content of nitrate, ammonia, and dissolved organic nitrogen. They also determined each source's relative contribution to the growth of a variety of plant species, from small floor-dwelling shrubs, to tree ferns, to tall canopy trees.

In dry areas, nitrate was most readily available, while in wetter areas, ammonium was the dominant source. The plants made use of whichever of these two sources was most common in their native soil. Dissolved organic nitrogen was plentiful, but did not make a significant contribution to plant growth at any of the sites.

To examine the plants' nutritional response to climate change, the researchers combined new measures and models of variations in the atomic masses of nitrogen compounds that occur naturally in plants and soils. By examining these different masses, known as isotopic ratios, across different rainfall climates, they discovered an abrupt shift in the nitrogen cycle and in the nutritional strategies of entire forest communities.

"It really is quite striking; once the soil gets wet and nitrate drops below a certain threshold, the tropical plants all begin using ammonium in near-perfect unison," Houlton explains. "If these diverse plant species can be flexible in their nitrogen metabolism-thought to be non-negotiable in many temperate ecosystems-then maybe they can react to other environmental stresses just as gracefully. Still, our results will need further testing in vast areas of the tropics before we will know how well they truly represent the entire ecosystem."

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