Reduced precipitation lessens the scaling of growth to plant N in mesic grasslands

Grassland production is sensitive to both precipitation and plant N accumulation and utilization, such that change in one variable influences grassland response to the second variable. We investigated effects of interannual variation in precipitation on the response of ‘community’-scale values of relative growth rate (RGR) to two multiplicative components of RGR, nitrogen productivity (NP; rate of change in biomass/plant N), an index of N utilization efficiency, and plant N concentration ([N]), in two grasslands in Texas, USA. Grasslands included a planted mixture of perennial grass and forb species and monoculture of the perennial C 4 grass Panicum virgatum that was invaded by multiple plant species. RGR and its N components were measured at the spatial scale of 7-m diameter circular patches near the spring peak in mixture biomass during each of 5 years. We found that RGR varied substantially among patches and years and between the planted mixture and monoculture. RGR variation was strongly correlated with variation in NP. Precipitation during the 3 months prior to RGR measurement mediated the RGR response to NP by altering the correlation between NP and [N] in both grasslands. Reduced precipitation led to more negative NP-[N] correlation coefficients, which reduced proportional change in RGR per change in NP by as much as 30% even in the absence of a precipitation effect on means of RGR and NP. Our results highlight an under-appreciated aspect of the pervasive role of precipitation in grassland growth that was mediated via change in the growth benefit derived from plant N.