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 precipiation 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 grassslands in Texas, USA.  Grasslands included a planted mixture of perennial grass and forb species and a monoculture of the perennial C4 grass Panicum vigatum 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 biomss 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..., We used remote sensing techniques to calculate relative growth rate (RGR), nitrogen productivity (NP), and plant N concentration ([N]) at the scale of 7-m diameter circular patches (n = 104) in each of two grassland types (mixture of perennial grass and forb species, planted monoculture of the grass switchgrass).  'Community'-scale values RGR and its N components (NP, [N]) were calculated near the spring biomass peak in each of 5 years.  We examined correlations among spatial variation in RGR, NP, and [N] in each grassland as influenced by interannual variation in precipitation.,