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

C3 grasses dominate: wheatgrasses (Pseudoroegneria spicata, Pascopyrum smithii, Leymus cinereus), fescues (Festuca idahoensis, F. scabrella), bluegrasses (Poa pratensis, P. compressa), junegrass (Koleria macrantha), and needlegrasses (Achnatherum nelsonii, Hesperostipa comata). The only C4 grass is three-awn (Aristida purpurea). Forbs include: Achillea millefo- lium, Lupinus spp., Balsamorhiza sagittata, Heterotheca villosa, Taraxacum spp., Sympotri- chum falcatum, and Erigeron spp. Woody species include: Symphoricarpus occidentalis, Rosa woodsii, Ericameria nauseosa, Artemisia frigida, and A. ludoviciana. Unlike many bunchgrass areas [33,40], big sage (A. tridentata) is absent, but is in nearby valleys. Four 2–4 ha. representative sampling sites (elevation: UTM) of NBR bunchgrass habitats (different dominant grasses) were respectively studied for 34, 27, 25 and 20 years: Hill (792m: 706453E 5249980N, Pascopyrum smithii/Poa pratensis); Tower 2-grassy (1380m: 708205E 5244024N, Festuca idahoensis/Hesperostipa comata); Trisky (1034m: 711317E 5242500N, Fes- tuca idahoensis/Pseudoroegneria spicata); and Triangle (832m: 713570E 5248100N, Pasco- pyrum smithii/Poa pratensis). These four sites represent an elevation gradient and the majority of data. Four additional sites were studied for fewer years (7, 3, 3 and 1 years) and supplement the data: Hill 2 (802m: 706407E 5249831N, Festuca idahoensis/Poa pratensis); North Boundary (825m: 710177E 5250023N, Pseudoroegneria spicata/Poa compressa); Tower 2-rocky (1394m:

708212E 5243906N, Festuca idahoensis/Pseudoroegneria spicata); and Pauline (797m: 705717E 5247255N, Pascopyrum smithii/Poa pratensis). All sites were fenced to exclude large herbivore grazing (Bison bison, Cervus elaphus, Ovis canadensis, Oreamnos americanus, Odocoileus hemi- onus, Odocoileus virginianus, and Antilocapra americana). Methods Field Research—permission granted by National Bison Range, Charlo, Montana - https:// www.fws.gov/refuge/national_bison_range/. Climate Daily climate data (precipitation, maximum and minimum temperature) are available since 1909. Data from 1909 –July, 1999 are available from the NBR headquarters (795m: 707570E 5249753N). Since July, 1999, the weather station was moved and data were not regularly col- lected. Therefore, data were obtained from the AgriMet Round Butte, MT station (18.9 km away, 926m: 704653E 5268696W). From 1989–2000, precipitation and temperature were highly correlated between NBR and Round Butte sites (precipitation: r = 0.94, N = 12, constant not different from 0, slope = 1.03 and p < 0.000001; temperature: r = 0.94, n = 12, constant not different from 0, slope = 0.94 and p < 0.000008). Nitrogen Nitrogen available to plants was measured in 1994–2017 at four sites (Hill, Triangle, Trisky, Tower 2-grassy). N measures were obtained twice a year (warm season: June–Sept, cool season: Oct–May) using resin bags (Rexyn © Fisher Scientific) [41]. At each site, 3 bags were randomly buried 15 cm deep. Bags were kept frozen after collection until analysis by extracting N with

2M KCl, and measuring NH4+ and NO3- with a Latchat © spectrophotometer [42]. Resin N is correlated with soil N mineralization at NBR [43]. Vegetation Vegetation measures included ANPP (annual aboveground net primary production), plant senescence, and plant species composition. PLOS ONE Bunchgrass prairie and climate change PLOS ONE | https://doi.org/10.1371/journal.pone.0243496 December 23, 2020 3 / 21