Plants' global warming dilemma: climb to escape heat or stoop for water?

For years, scientists have recorded the gradual march of plants and animals up mountain slopes and toward higher latitudes as global warming has forced them to chase their climatic comfort zones.

A new study suggests that for plants, however, a warming climate can send them downhill as well – a result several researchers say has important implications for efforts to conserve the biological richness of mountain habitats in the face of long-term global warming.

Other researchers have noted that plants can buck the general trend toward relocating at higher altitudes. But the travels often were attributed to a mix of potential factors, ranging from land-use changes to unique reactions by individual species of plants to warming.

The new study, published in the Jan. 21 issue of the journal Science, suggests that the availability of moisture – snow, rain, or fog, for instance – may override some plants' response to temperature, at least for a while, drawing them down to altitudes where more moisture is available.

At first blush, the tendency of plants to follow the water might seem obvious to any backyard gardener raising potatoes or petunias.

But in the context of global warming, the focus has tended to fall on temperature's influence on plants, rather than precipitation's, researchers say. The reason is that the researchers have higher confidence in climate models' temperature projections than they do in precipitation projections.

Indeed, the new study grew out of a desire to provide a reality check to the ecological models used to explore the possible effects global warming could have on the distribution of plants and animals, says Solomon Dobrowski, a forest ecologist at the University of Montana at Missoula.

Dr. Dobrowski and graduate student Shawn Crimmins led the team conducting the work. The study covers mountain regions that embraces roughly half the state of California. It includes the entire expanse of the state's coastal ranges north of Santa Barbara, crosses east through the southern Cascades in northern California, then reaches south along the length and breadth of the Sierra Nevada range.

Detailed data on California

Dobrowski picked California because the state hosts remarkably detailed and geographically comprehensive information on the state's plant and animal ranges gathered by naturalist Albert Wieslander during the 1920s and 30s. Few places have such rich historical information to draw on, Dobrowski says.

Mr. Crimmins examined changes in the distribution of vascular plants – which include conifers such as pines, redwoods and spruces, as well as ferns and flowering plants – comparing a five-year span in the 1930s with a similar span between 2000 and 2005. He used data from nearly 9,000 of Wielander's 33,000 plots.

Dobrowski says he was very familiar with research showing plants moving uphill. But, he recalls, Crimmins analyzed the data "and said: Wait, they are not going uphill, they are going downhill."

"I told him to double-check his numbers, because I expected things to move uphill," Dobrowski says, "because the climate data in California were pretty clear that things had warmed."

But Crimmins returned from Round Two of number-crunching with the same results, forcing the team to revisit its assumptions.

Over the 70-year span between data sets, California's average temperature has risen 0.6 degrees Celsius (1 degree Fahrenheit). Some 72 percent of the species migrated downhill during that time, compared with 28 percent marching uphill.

When the team looked for explanations, they found that the plant species appeared to be extending their habitat downhill to altitudes where water is more prevalent, even in the face of an additional, slight increases in the temperatures their new, lower locations presented.

Over the 70-year span, these species averaged a downhill slide of about 85 meters (278 feet), the team estimates.

One factor contributing to the slow, downhill march – at least along the coast – "could be temperature changes that make the fog come in further, so that downhill ends up being a little cooler," says Kathryn Thomas, a plant ecologist with the US Geological Survey who is based on Portland, Ore.

An unfortunate parting of ways

Whatever the mix of drivers, the results suggest the potential that different organisms that currently rely on each other could end up going their separate ways, to the detriment of both, Dobrowski says.

"You can envision, for example, a situation in which you have insects moving upslope, because they are more tightly linked to temperature, and vegetation moving downslope" because of its thirst for water. In effect, pollinators would move upslope while the plants they pollinate head the other direction.

The team notes that the world hosts other mountainous regions that also are projected to see long-term increases in rain and snowfall with global warming. Thus, the results may indicate a more-general pattern that additional research will need to verify.

Yet even if it were limited to the range of climatic conditions and plant species in California, this study and others like it are valuable in crafting long-term conservation plans, says Dan Porter, a biologist with the Nature Conservancy who is based in San Francisco.

The Conservancy recently completed a climate-change adaptation assessment for the southern Sierra Nevadas.

"Leading scientists fully expect species will move as the climate changes, and studies like this really help us decide where that movement and the conservation of biodiversity are perhaps most compatible."