Will Global Warming Cause Sagebrush Ecosystems To Shrink?

By Jen Schripsema

The high plateaus bounded by the Sierra Nevada and Rocky Mountains, called the Great Basin, are home to sagebrush ecosystems that define the landscape of much of the American West. But vegetation models developed by Northwest researchers predict that global warming may cause the area covered by sagebrush to shrink to a fraction of its current size.

These findings were presented in the fall at the North American Wildlife and Natural Resources Conference by Ronald Neilson, an Oregon State University professor, and Dominique Bachelet of the U.S.D.A Forest Service. Using seven future climate scenarios, they computed which plant communities would survive in each. In all scenarios, the sagebrush ecosystems shrunk as woodlands took over; the warmest scenario reduced the area currently dominated by sagebrush by 80 percent.

The Great Basin is characterized by hot, dry summers and cold winters, which provide most of the sparse precipitation. This semi-arid climate favors drought-tolerant and deep-rooted sagebrush species. The freezing winter temperatures currently confine most tree species to the south.

According to the climate change scenarios the researchers have utilized, "The American West is going to get warmer and it's going to get wetter," says Neilson, "generally leading to an expansion of forests, more vegetation overall, and significantly more fire."

Neilson and Bachelet specialize in designing model simulations for climate change, nutrient cycling, and other ecosystem processes. They developed future climate change scenarios using output from several general circulation models for the period 2000-2100. The predictions for vegetation distribution are based on these climate scenarios using two types of models, a static biogeography model MAPSS and a dynamic vegetation model MC1.

The biogeography model generates a map showing the most likely vegetation in any given climate, using the average over a 30-year interval. This model, MAPSS, can simulate the competition between individual trees, shrubs, and grasses. Researchers test the accuracy of the MAPSS model using historical data. "We take an average of 1961 to 1990 monthly rainfall and monthly temperature and so on and so forth. And if the model is any good, it'll give you a pretty good representation of what was actually out there," Neilson says.

"The MAPSS model will give you map of the vegetation distribution now and redraw and give you different map for what the vegetation distribution might look like at the end of the 21st century, but it won't tell you how you get from here to there," Neilson says. So, the dynamic vegetation model MC1 was designed to simulate vegetation distribution based on a similar logic to the one included in MAPSS and to use this information to simulate plant growth, death, and decomposition using algorithms derived from the biogeochemistry model Century, written by colleagues from Colorado State University.

Researcher James Lenihan contributed the fire component of the dynamic vegetation model MC1. Using climate and vegetation data, the model predicts when a fire will occur and how large it will be. The model shows that the increased fuel load provided by the encroaching woodlands will lead to larger, more frequent fires. Fire suppression can be turned on and off to see the effect of fire management policies.

But climate change is notoriously complex. It can be difficult to tease apart human-caused changes from natural short-term patterns like El Nino and medium-term patterns that alter ocean currents for decades. Changes in ocean currents can alter the regional weather patterns. "These short-, medium-, and long-term climatic changes all interact with each other, sometimes offsetting impacts on temperature and precipitation, and sometimes compounding them," Neilson says. Strong evidence suggesting that long-term global warming will lead to increasing volatility in short-term weather patterns further complicates the predictability of climate changes.

Also, their vegetation models cannot consider all factors that could potentially affect the Great Basin ecosystems. Most notably, they cannot predict changes in land use, which until now has been the largest factor contributing to the decline of the sagebrush ecosystems. Since European settlement, the pressures of grazing, development, and agriculture have already reduced the ecosystem to half its former size.

Still, their predictions have important implications for policies on conservation, fire management, and greenhouse gas emissions. Bachelet says that, given the uncertainty associated with model projections, we should carefully consider these implications "with a large grain of salt."

Jen Schripsema earned a B.A. in biology from Colorado College. She is currently pursuing a Master's degree in technical communication at the University of Washington.

Image at Top

Sagebrush ecosystem east of the Sierra Nevada mountains in California. Photo: Marc Hoshovsky