Climate change and primary production: Forty years in a bunchgrass prairie

abundances of plant species that differ in their ability to respond to changing temperature, pre- cipitation and nitrogen regimes [11–14]. Grasslands are particularly important to humans by providing a disproportionate amount of agricultural and livestock production, as well as wildlife habitat [9]. As grasslands are often found in drier and warmer climates, they may be particularly susceptible to declining primary produc- tion and changing vegetation as climate changes [15]. Few long term data sets examine grassland responses to climate change, e.g., [8,9,16–28], and most examine large spatial scale responses to changes in annual temperature and precipitation. However, Craine et al. [23–25] indicate that seasonal shifts in temperature and precipitation may be more important at local spatial scales. While changes in temperature and precipitation may directly affect grassland primary pro- ductivity, there may also be changes in nitrogen availability, the most common limiting nutri- ent. First, some studies found precipitation and nitrogen availability in grasslands to be positively correlated, which makes it difficult to distinguish between water and nitrogen limita- tion for primary productivity [29,30]. Second, if anthropogenic nitrogen in the atmosphere increases, then more will be deposited with increased precipitation [10]. Therefore, correlating precipitation changes with primary production may be due to water or nitrogen availability. We examined how climate and nitrogen changes may be affecting an intermountain bunch-

grass prairie. Intermountain bunchgrass prairie, one of the most endangered ecosystems in North America, once covered ~11.7 million hectares of intermountain valleys from the Rockies west to the Cascades and from southern Alberta/British Columbia south to northern Utah/Nevada, but now is restricted to less than 100,000 hectares by agriculture and overgrazing (<1% of original area) [31–34]. Bunchgrass is overlooked in grassland studies, as U.S. Great Plains and desert grass- land/steppe are often the focus [8,9,35]. Bunchgrass receives much less precipitation than the Great Plains as surrounding mountains create rain shadows, and little precipitation occurs in summer (July–Sept) given prevailing Westerlies [36]. Furthermore, unlike the Great Plains, bunchgrass has a lower wildfire incidence due to less frequent lightning [36,37]. Finally, bunch- grass may be especially sensitive to climate change, as its precipitation and temperature falls within a narrow range between that for cold desert and Great Plains mixed grass prairie [32,38,39]. The bunchgrass prairie region is expected to become warmer and drier over time with anthropogenic climate change [8,9]. If drying is observed, it should lead to greater water limita- tion, and possibly greater nitrogen limitation, as nitrogen mineralization and atmospheric depo- sition may decline with less precipitation. Whether water or nitrogen is limiting, primary production should be declining over time, and drying should be increasing plant senescence

over the normal summer dry period. However, vegetation responses may depend on more than annual changes over time; therefore, we also examined changes over time in seasonality that may better explain vegetation responses. In addition, climate change may affect the vegetation’s spe- cies composition, which impacts drought tolerance, fire resistance, and susceptibility to invasive species. Finally, warming and drying might convert bunchgrass prairie into cold desert steppe. We report on a bunchgrass prairie (National Bison Range, MT, USA) with a 109-year cli- mate record (1909–2017), 40 years of primary production measures (1978–2017), 26 years of plant composition observations (1978; 1980–1983; 1997–2017), and 24 years of soil nitrogen measures (1994–2017) for several grassland habitats. Study site The National Bison Range (NBR) is one of the largest remaining tracts (~9000 ha) of inter- mountain bunchgrass prairie. It was never farmed and has been protected since 1908 as a U.S. F.W.S. refuge for bison (Bison bison). Prior to this, it was a cattle/bison ranch for less than 15 years. NBR bunchgrass prairie occurs at ~800 to 1400 m in elevation. PLOS ONE Bunchgrass prairie and climate change PLOS ONE | https://doi.org/10.1371/journal.pone.0243496 December 23, 2020 2 / 21 gov (GHIPM program ended in 1994); Utah State University Agricultural Experiment Station (1992- 1996) - https://uaes.usu.edu/; USDA/ARS and USDA-CSREES/NRICGP (00-35101-9267). The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript.

Competing interests: The authors have declared that no competing interests exist.