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

<p>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 strongly correlated with variation in NP.  Precipitation during the 3 months prior to RGR measurement mediated that 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.</p>