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

Plant community. Plant species composition was affected by summer senescence, as bluegrasses decreased (P < 0.000007) and wheatgrasses increased (P < 0.05) with summer senescence. Diversity indices were unaffected by summer senescence (P < 0.48), while species richness increased with summer senescence (P < 0.04). Plant characteristics. Drought resistance and relative abundance of invasive species increased as summer senescence increased (respectively, P < 0.0005 and 0.01). Fire tolerance was not related to any variables. Discussion The once abundant intermountain bunchgrass prairie has not been well studied. Most past bunchgrass studies focused on plant community composition, e.g., [33,40,61]. Vegetation ANPP changes. The few estimates of bunchgrass prairie ANPP range from 50–230 g-dry/ m2, e.g., [33,62–65], did not track ANPP changes among years or with environmental vari- ables, or annual phenology. We found a much more variable ANPP (78–395 g-dry/m2) at NBR with variation among sampling sites within a year from 40–60% and variation among years within a site from 110–260%. NBR ANPP is known to increase with experimental addition of water and nitrogen fertilizer [43,60,66–70]. We observed ANPP to be positively correlated with the growing season (late-May–June) ratio of precipitation to temperature and with eleva- tion. Elevation should be positively correlated with precipitation and negatively correlated with temperature, and we also observed it to be positively related to nitrogen availability.

ANPP also increased with the previous year’s ANPP. Therefore, moisture (growing season ratio of precipitation to temperature) appears to be more important to ANPP than nitrogen. First, late-May–June (main period of growth: > 88% of ANPP) ratio of precipitation to temperature reflects moisture available for plant growth as evaporation increases at higher temperatures, so more precipitation is needed to achieve a given level of plant growth [59,60]. This ratio was experimentally demonstrated to correlate with evapotranspiration and ANPP [60]. Second, nitrogen availability from natural dynamics [29,30] and anthropogenic deposi- tion [10,30] has been shown to increase with precipitation in grasslands. We did not observe this as warm season total nitrogen was not correlated with precipitation, but increased with summer temperature, which reflects rapid mineralization [71,72]. Also, NBR is in an area of low atmospheric deposition of anthropogenic nitrogen [73]. Finally, previous year’s ANPP was positively correlated with current ANPP, which indicates that moisture and nitrogen (ele- vation) have greater impact when plants already are abundant after a year of good growth probably due to a greater plant stem density after a year of higher growth [27,74]. Most striking, NBR ANPP increased by 110% over our 40-year study as the ratio of late- May–June (main period of plant growth: > 88% of ANPP) precipitation to temperature increased due to wetter and cooler conditions. Grass ANPP over our study increased by 251%,

while dicot ANPP declined by 65%, leading to a 54% increase in grass relative abundance. The driving factor again was specific seasonal, not annual, climate change (wetter and cooler late- May–June) favoring the “cool-season” (C3) grasses that are most abundant (> 99%). Further- more, the increase in these grasses appeared to reduce forb abundance through competition, as the grasses form clumps (cespitose) that are spaced apart to avoid competition for water, as denoted by the prairie being called “bunchgrass”. Finally, ANPP changes were stronger at low elevations over our study, which may be expected as higher elevations are generally moister and cooler to begin with than low elevations. PLOS ONE Bunchgrass prairie and climate change PLOS ONE | https://doi.org/10.1371/journal.pone.0243496 December 23, 2020 13 / 21