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Tracking global warming in Colorado’s alpine zone

Some alpine plants in Colorado are already feeling the heat of global warming, and researchers are trying to determine where the 'tipping point' might be. PHOTO BY BOB BERWYN.

Long-term study aims to determine climate change tipping points

By Bob Berwyn

SUMMIT COUNTY — Some alpine and arctic plants — including species found in the Colorado mountains —  are showing negative effects of warmer conditions, with lower survival at the southern edges of their range, according to University of Wyoming ecologist Daniel Doak.

Together with Duke University researcher William Morris, Doak is conducting a long-term research project to determine how these species respond to climate change. In most years the impacts to plant populations are balanced by stronger growth in other areas. But in the warmest years of the six-year study, both survival and growth of the plants fell.

The study is based on the assumption that, as the Earth’s climate warms, species are expected to shift their geographical ranges away from the equator or to higher elevations. While scientists have already documented shifts for many plants and animals, the ranges of others seem stable.

When species respond in different ways to the same amount of warming, it becomes more difficult for ecologists to predict future biological effects of climate change–and to plan for these effects.

“This study illustrates the critical need for long-term research to address our most pressing ecological challenges,” says Saran Twombly, program director in the National Science Foundation (NSF)’s Division of Environmental Biology, which funded the research. “Without the temporal and spatial scales employed here, we have little hope of understanding the complex ways in which organisms will respond to climate change.”

The plant species targeted by Morris and Doak range from populations in the high mountains of Colorado and New Mexico to species growing along the arctic coastline in far northern Alaska. These regions include habitats that have undergone substantial climate change, leading to the expectation, says Doak, that–especially at the southern edge of their range–populations of the plants should be collapsing.

But the study showed a more complex pattern of responses. The opposing trends mean that under current conditions, even across the huge range of conditions Morris and Doak studied, populations of these plants are doing equally well across 30 degrees of latitude–one-third the distance from the equator to the north pole.

However, the researchers’ results don’t indicate that these plants, or other species, will be unaffected by warming conditions.

“Up to a point,” says Doak, “we may see little effect of warming for many organisms. But past a climatic tipping point, the balance of opposing effects of warming will likely cease, leading to subsequent rapid declines in populations.”

By looking at the performance of individual plants in particularly hot and cold years, they found that the compensatory effects across moderately cold to moderately warm years (lower survival balanced by more rapid growth) will not hold up with increased warming.

While this tipping point will be different for each species, responses of natural populations to gradual shifts in climate will not necessarily in turn be gradual.

“We shouldn’t interpret a lack of ecological response to past warming to mean that little or no effects are likely in the future,” says Doak.

“A key part of this approach is the need for long-term studies so we can observe and use the rare years with extreme climates to anticipate what the average future climate will bring,” Doak says.

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