By Evelyn Boswell
MSU News Service

BOZEMAN – The Greater Yellowstone Ecosystem has gotten drier over the last four decades, causing a long-term decline in grassland productivity, according to Montana State University researchers.

By monitoring a mountain meadow northeast of Bozeman – analyzing plants and examining regional climate records – Jack Brookshire and Tad Weaver documented a sustained decline of more than 50 percent in native grassland productivity. They blamed it on increasing aridity, particularly too little rain late in the summer.

Summarizing more than 40 years of their own work and that of their students, the two researchers published their findings in the May 14 issue of the scientific journal “Nature Communications.” The journal is affiliated with the prestigious international journal “Nature,” and covers topics in physics, chemistry, earth sciences, and biology.

Brookshire is assistant professor of ecosystem biogeochemistry in MSU’s Department of Land Resources and Environmental Sciences in the College of Agriculture. Weaver, who co-founded the study in 1969 and was the sole investigator until Brookshire’s arrival in 2009, is professor emeritus in the MSU Department of Ecology in the College of Letters and Sciences.
Weaver said he was especially surprised by two outcomes of the study: the long-term decline in yield, and the fact that late-season rains strongly affected those differences. The first was shown to be a result of increasing aridity.

“While control of production might have been expected to relate to growing season precipitation, it was more affected by late summer/autumn rainfall, as if production were determined by resources stored to support growth in the following year,” Weaver said.

From 1969 to 2012, MSU researchers visited a U.S. Forest Service meadow on a windswept ridge in the Bangtail Mountains northeast of Bozeman. The predominant grass at the site is Idaho fescue, and they measured the meadow production in unmodified plots, and in experimentally snow-supplemented plots. While elk and deer have access to the meadow, cattle haven’t grazed it since the 1930s, Brookshire said.

The researchers examined long-term climate records such as precipitation and temperatures for more than four decades. They also considered regional snowpack chemistry and long-term patterns in carbon dioxide concentrations and nitrogen deposition from the atmosphere.

“Our results demonstrate lasting consequences of recent climate change on grassland production and underscore the importance of understanding past climate-ecosystem coupling to predicting future responses to changing climate,” the scientists wrote.