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

0.31, 0.42), but warm season NO3-, NH4+ and total nitrogen increased over our study (Fig 3B) respectively by 200.8%, 38.6% and 94.8%. Vegetation. Over our study, ANPP averaged 191.5 (± 8.6 SE) g/m2 (among years: 100.9– 316.4 g/m2) (Fig 4A), and significantly varied among years and increased by 110.4% over our study (Table 2, Fig 4A). Grass accounted on average for 79.2% (± 2.2 SE) (among years: 41.0– 97.3%) of ANPP (Fig 4A). Grass production increased by 251%, while forb production decreased by 65% over our study (Fig 4A). Consequently, grass relative abundance increased by 54% from 61% to 96% (Table 2, P < 0.000001) of ANPP over our study. Finally, the time (Julian day) at which peak “green” biomass (>88% of ANPP) was observed (late-May–June: Fig 2) varied among years and declined by 23 days (16%) from 1985–2017 (Table 2). Summer plant senescence averaged 59.2% (± 2.5 SE) (among years: 30.1–85.6%) (Fig 4B), significantly varied among years and increased 119% over our study (Table 2, Fig 4B). Renewed fall plant production after the period of summer senescence (Fig 2) accounted on average for 3.5% of ANPP (± 0.6 SE) (among years: 0–12.6%), and significantly decreased by 67.8% over our study (Fig 4C). Plant species composition was only measured repeatedly at two sites (Hill, Triangle: 2001– 2017) with the same methodology (point frame). Because grasses are the dominant species, we examined the relative abundances of the two most common types of grass: bluegrasses and wheatgrasses, which varied in relative abundance among years (Table 2, Fig 5A). Bluegrasses

declined by 89% and wheatgrasses increased by 143% over our study (Table 2, Fig 5A). Species richness based on all species significantly varied among years and increased by 51.7% over our study (Table 2, Fig 5B) with rare dicots accounting for the increase (P < 0.0001) as grass spe- cies did not change (P < 0.54) and increased in abundance. Shannon diversity did not vary among years, but increased by 26.5% over our study (Table 2, Fig 5C). Table 2. GLMM results for soil, vegetation and plant community measurements among years and trend over the years of our study (- = decline, 0 = no change, + = increase). Significant (p < 0.05) results are in bold. AMONG YEARS OVER YEARS df F P Trend df F P TOTAL NITROGEN - Cool season 22,65 1.58 0.08 0 1.83 0.80 0.37 Warm season 23,67 1.80 0.03 + 1,86 15.13 0.0002 Annual 22,65 1.54 0.09 0 1,83 2.86 0.09 VEGETATION - ANPP 38,90 4.25 0.000001 + 1,127 37.1 0.000001 Peak biomass—day 32,77 2.41 0.00009 - 1,108 27.1 0.000001 % Grass 38,90 5.58 0.000001 + 1,127 45.6 0.000001 Senescence 31,77 3.37 0.000008 + 1,107 54.12 0.000001 PLANT COMMUNITY - COMMUNITY - Diversity 20,16 1.31 0.29 + 1,35 7.45 0.01 Richness 20,16 3.76 0.005 + 1,35 5.33 0.03 Bluegrasses 20,16 10.4 0.000009 - 1,35 50.1 0.000001 Wheat grasses 20,16 8.91 0.00002 + 1,35 23.8 0.00002 PLANT CHARACTERISTICS CHARACTERS - Drought resistance 20,16 2.35 0.04 + 1,35 23.8 0.00002 Fire tolerance 20,16 1.20 0.36 - 1,35 3.30 0.08 Invasives (%) 20,16 5.45 0.0006 + 1,35 4.77 0.04

https://doi.org/10.1371/journal.pone.0243496.t002 PLOS ONE Bunchgrass prairie and climate change PLOS ONE | https://doi.org/10.1371/journal.pone.0243496 December 23, 2020 8 / 21