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

Seasonal values for precipitation and temperature trends during our study were also exam- ined (Table 1). Like the annual trends, Water Year (Oct–Sep) precipitation tended to decrease and temperature to increase. Seasonally, precipitation tended to decrease in spring, summer and recharge period (winter + spring), increase in fall and remain unchanged in winter. Sea- sonally, temperature tended to increase primarily in summer, and remain unchanged in other seasons. Because the majority (> 88%) of plant growth (peak “green” biomass) occurs at NBR from late-May–June (Fig 2), the decrease in Water Year, recharge period and spring, like annual, precipitation suggests that annual plant production should decline over our study. Climate changes at more specific times during vegetation phenology were examined. Dur- ing the main period of plant growth (late-May–June: Fig 2), temperature tended to remain unchanged (Mann-Kendall: z = -0.52, P < 0.30), as found for spring in general, while precipi- tation tended to increase (Mann-Kendall: z = 1.23, P < 0.11), opposite the spring seasonal Table 1. Seasonal precipitation and temperature trends during our study (1978–2017) based on Mann-Kendall trend series. PRECIPITATION: TEMPERATURE: Direction & Mann-Kendall z P Direction & Mann-Kendall z P Water Year (prev. Oct–current Sept) -1.22 0.11 +1.21 0.11 Winter (Jan–Mar) +0.47 0.32 +0.27 0.39 Spring (Apr–Jun) -1.41 0.08 -0.65 0.74 Summer (Jul–Sept) -2.33 0.01 +3.54 0.0002 Fall (Oct–Dec) +3.04 0.001 +0.48 0.31 Recharge (winter + spring) -0.51 0.30 0.00 0.50

https://doi.org/10.1371/journal.pone.0243496.t001 Fig 2. Average monthly “green” plant biomass from the Hill site as an example. https://doi.org/10.1371/journal.pone.0243496.g002 PLOS ONE Bunchgrass prairie and climate change PLOS ONE | https://doi.org/10.1371/journal.pone.0243496 December 23, 2020 6 / 21