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

high), and computed average drought and fire resistance for the plant community (S[U.S.D. A’s species drought tolerance or fire resistance index X species proportion]). Statistics Statistics were conducted with SYSTAT 13. Climate changes over time were analyzed using nonparametric trend analysis (Mann-Kendall rank correlation). General Linear Mixed Models (GLMM) were employed to examine whether nitrogen and vegetation measures varied among years (year as categorical independent variable) and over time during the study (year as contin- uous variable). Sample sites were treated as a random variable, because they were not repli- cated within a year. All proportions were arcsine square root transformed. Climate and elevation effects (continuous independent variables) on nitrogen and vegetation were exam- ined using backward stepwise mixed-model regression, where independent variables were only included if P < 0.15 and the Akaike Information Criteria value (AIC) decreased. Results Data for various sites at the NBR from 1978 to 2017 are presented in S1 Table (Summer Weather, Nitrogen and Vegetation) and S2 Table (Plant Species). Changes over time Climate and plant phenology. Monthly average temperature and precipitation since 1909 (Fig 1A) indicate a 19.0˚C maximum temperature in July–Aug, a -2.5˚C minimum tem- perature in Dec–Jan, and a 5.7 cm maximum precipitation in May or June. As expected since 1909, average temperature increased 0.7˚C (11%, Fig 1B), and average precipitation decreased 10.9 cm (27%, Fig 1C). During our study (1978–2017), average annual temperature increased

(Mann-Kendall: z = 1.97, P < 0.02), which accounts for 63% of the overall temperature increase observed since 1909, so that average annual temperature increased at a 127% faster rate than before our study (0.018˚C/yr vs. 0.008˚C/yr). Likewise, annual precipitation decreased during our study (Mann-Kendall: z = -1.99, P < 0.02), which accounts for 21% of the overall precipitation decrease since 1909, so that average annual precipitation decreased by a 68% slower rate than before our study (0.19 cm/yr vs. 0.06 cm/yr). Fig 1. NBR climate since 1909. a) average monthly temperature and precipitation; b) trends over time in average air temperature and Mann-Kendall time series; and c) trends over time in average precipitation and Mann-Kendall time series. https://doi.org/10.1371/journal.pone.0243496.g001 PLOS ONE Bunchgrass prairie and climate change PLOS ONE | https://doi.org/10.1371/journal.pone.0243496 December 23, 2020 5 / 21