Global changes are simultaneously affecting multiple resources that limit the biomass of plant communities. A few experimental studies have examined how water and nutrients co-limit grassland productivity, and they have observed that nutrient availability can constrain the ability of grassland biomass to respond to water addition. However, we lack general understanding of these interactions across diverse ecosystems. How consistent are these responses across grasslands with very different ambient precipitation, community composition and nutrient status? With a model of multiple limitation of biomass, we hypothesise that nutrient addition should increase drought sensitivity of biomass, and that this effect will be higher in sites where biomass is more strongly limited by nutrients.
We use long term data collected in a multi-site distributed grassland experiment (the Nutrient Network), to examine how nutrient addition influences the sensitivity of grassland biomass to natural variation in rainfall. From 2007 onwards standardized nutrient additions (N,P,K and micronutrients) have been applied at 12 grassland sites across the US, along with standardized measurement of biomass and plant cover. We analysed how live biomass related to changes in precipitation, specifically during three droughts in Texas (2011), the midwest (2012) and California (2012-2015).
We find that nutrient enrichment increases the sensitivity of live biomass (measured as change in biomass per unit change in precipitation) to drought as compared to control plots. Across 5 sites in California, sites that receive lower mean precipitation have a higher sensitivity to drought, though that trend doesn't hold across sites in the midwest. Sites that were more limited by nutrients (which showed a greater increase in biomass upon fertilization), also had a lower sensitivity to drought. There is also significant site level variation in these responses. We are currently analysing how variation in community structure (richness, evenness and functional composition) can contribute to explaining this diversity in drought responses.
These results suggest that a model of multiple, sequential limitation of resources can explain the biomass response of different grasslands to drought. We seek to further examine the variation of these results across sites and through time, to build and test theoretical models of nutrient and water colimitation at the community scale.