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

Authors: Polley, Herbert; Jones, Katherine; Kolodziejczyk, Chris; Fay, Philip

Submitted to: Plant Ecology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 12/5/2022
Publication Date: 12/31/2022
Citation: Polley, H.W., Jones, K.A., Kolodziejczyk, C.A., Fay, P.A. 2022. Reduced precipitation lessens the scaling of growth to plant N in mesic grasslands. Plant Ecology. 224(1):113-123. https://doi.org/10.1007/s11258-022-01283-0.
DOI: https://doi.org/10.1007/s11258-022-01283-0

Interpretive Summary: The amount of plant biomass that is produced on grasslands depends on both precipitation and plant nitrogen (N), rendering it difficult to predict biomass production among years. We studied precipitation effects on grassland biomass production as affected by two components of plant N, plant growth return on accumulated N and plant N concentration ([N]). We studied two grasslands located in central Texas, USA, including a grassland planted with a diverse mixture of perennial grass and forb species and a grassland planted as a monoculture of the perennial warm-season grass switchgrass. Growth and its N components were measured using remote sensing techniques at the spatial scale of 7-m diameter circular patches in each grassland near the spring peak in biomass during each of 5 years. We found that the growth rate per unit of plant biomass varied substantially among patches and years and between communities. Growth variation was more strongly correlated with differences in plant growth per unit of accumulated N than plant [N]. Reduced precipitation lessened the growth return on plant N by as much as 30% among years in both grasslands. Our results indicate that the precipitation effect on grassland production is mediated partly via change in plant growth return on accumulated N.

Technical Abstract: Grassland production is sensitive to both precipitation and plant N accumulation and utilization. Precipitation and N effects may interact, such that change in one variable influences grassland response to the second variable. We investigated effects of interannual variation (IAV) in precipitation on the response of community-level values of relative growth rate (RGR) to two multiplicative components of RGR, nitrogen productivity (NP; rate of change in biomass/plant N content) and N concentration ([N]), in two grassland communities in Texas, USA. Communities included a diverse mixture of perennial grass and forb species and monoculture of the perennial C4 grass Panicum virgatum (switchgrass). RGR and its N components were measured at the spatial scale of 7-m diameter circular patches in each community (n = 104) near the spring peak in mixture biomass during each of 5 years. We found that RGR varied substantially among patches and years and between communities. 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 simple and diverse grassland. 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 community RGR and NP. IAV in the NP-[N] correlation was linked to variation in canopy N content in mixture via a shared association with precipitation. As a result, mixture RGR responded proportionally more to increased NP during years in which both precipitation and community N content were high than low. Our results highlight an under-appreciated aspect of the pervasive role of precipitation in grassland growth that was mediated via change in the sign and magnitude of correlation between community values of NP and [N].