Tropical plants may adapt more easily to climate change than thought

As rainfall patterns change due to global warming, tropical plants may acclimate more easily than commonly thought, new research shows. The findings are highly important for countries in the South who are set to become biofuel producers, relying on tropical energy crops, and for the livelihoods of millions of smallholders in the developing world who make a living from agro-forestry.

Scientists from Princeton University and from the University of Florida have found that plants in tropical Hawaii have the ability to adapt to big changes in rainfall in at least one major respect - how they get nutrients. The plants largely rely on one form of the vital nutrient nitrogen in moist areas. 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.

The results of the study could be good news for the plants because, according to the scientific consensus of the IPCC's Working Group II on the impacts of climate change on the environment, global warming is expected to alter rainfall patterns in the tropics (earlier post). 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.

Flexibility in nutrient uptake
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, the researchers found. When confronted with shifts in nitrogen availability, these plants simply "flip a switch" and use whatever is handy.

"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:
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"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.

More information:
Eurekalert: Scientists: As rainfall changes, tropical plants may acclimate - May, 7, 2007.

Eurekalert: Tropical plants go with the flow ... of nitrogen - May 7, 2007.