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.
Summarizing more than 40 years of work by the authors and students, Jack Brookshire and Tad Weaver published their findings in the May 14 issue of the scientific journal, Nature Communications. Brookshire is assistant professor of ecosystem biogeochemistry in the Department of Land Resources and Environmental Sciences in the College of Agriculture. Weaver, who co-founded the study in 1969 and soon became the sole investigator until Brookshire’s arrival in 2009, is professor emeritus in the Department of Ecology in the College of Letters and Sciences.
By monitoring a mountain meadow northeast of Bozeman--analyzing plants and examining regional climate records-- the two scientists 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, and used snow addition experiments to confirm their findings. They also showed that while increased carbon dioxide in the atmosphere may have slightly offset the effects of increasing aridity, any changes in nitrogen deposition had no effect.
“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 duo said in their newly published paper.
Weaver said he was especially surprised by two outcomes of the study, specifically by the long-term decline in yield and by the fact that late-season rains strongly affected the yield differences. The first was shown to be driven by increasing aridity.
And second, Weaver said “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.”
The researchers said unique long-term records from the meadow, climate and snowpack experiment made their study possible.
From 1969 to 2012, MSU researchers visited a U.S. Forest Service meadow on a windswept ridge in the Bangtail Mountains. They measured the meadow production in unmodified plots and in experimentally snow-supplemented plots. They saved samples so they could later analyze their nutrient and isotope content. The predominant grass is Idaho fescue. While elk and deer have access to the meadow, it hasn’t been grazed by cattle since the 1930s, Brookshire said.
The researchers related these data to potentially controlling environmental factors. They examined long-term climate records such as precipitation and temperatures from 1969 through 2012. They considered regional snowpack chemistry and also considered long-term patterns in carbon dioxide concentrations and nitrogen deposition from the atmosphere.
The researchers used three approaches to determine how climate variation and atmospheric changes affected the long-term dynamics of grassland production. First, they examined the degree to which production and climate are coupled to explain long-term declines in production. Second, they conducted a 44-year experiment where they used snow fences to double and quadruple the amount of snow on embedded plots as one measure of seasonal effects. The extra snow buffered the ground temperatures, postponed the growing season by two to four weeks and changed the plants dominating the vegetation. Third, the researchers analyzed the plants to determine the influence of carbon dioxide and nitrogen enrichment.
“Our long-term results of declining grassland production contrast with the results of some models and short-term experiments. We find that increasing dryness over the last several decades is outpacing any potential growth stimulation from increasing atmospheric carbon dioxide and nitrogen deposition” Brookshire said.
Nature Communications is a scientific journal affiliated with the prestigious international journal Nature. Nature Communications covers all topics in physics, chemistry, earth sciences and biology.
This is the third time this spring that MSU researchers have published articles in the journal. Solar physicist Robert Leamon was part of a team that discovered that the sun, like the Earth, has seasons and published its findings on April 7. Ed Schmidt, a professor in the Department of Microbiology and Immunology, published a paper on March 20 that described an antioxidant system that helps sustain the liver when other systems are missing or compromised.
To read the new paper by Brookshire and Weaver, go to http://www.nature.com.proxybz.lib.montana.edu/ncomms/2015/150514/ncomms8148/full/ncomms8148.html.
Evelyn Boswell, (406) 994-5135 or firstname.lastname@example.org