Changes in resource availability and variability can alter plant community composition and associated ecosystem functions. Nutrient addition can result in non-random species losses, while changes in water availability can increase species dominance. As increased rates of nutrient inputs and precipitation variability are likely to persist, it is important to understand whether past environmental changes can reduce the ability of plant communities to resist future perturbations. Here, we test whether long-term changes in nitrogen and water availability alter the resistance of plant communities to severe drought. To do this, we set up a long-term field experiment where we fertilized and irrigated, alone and in combination, a diverse plant community for a decade. After this, we set up an experimental rainfall reduction corresponding to a 1-in-100-year drought. We repeated this rainfall reduction for four years in a row. We now present plant community and productivity responses following three years of these experimental drought manipulations. Our long-term field study and factorial design have allowed us to test whether long-term increases in water and nutrient availability during the growing season have altered the resistance of these plant communities to severe drought.
Results/Conclusions
During the third year of our experimental rainfall reduction, we find no significant difference in total aboveground biomass production and find no significant higher order interactions between past irrigation, fertilization, and drought (p > 0.05). This was also true for aboveground biomass of all plant functional groups present in our experiment. However, since plots that had been receiving additional water and nutrients for a decade had higher aboveground productivity pre-drought, our results suggest that nutrient addition and irrigation decreased stability in these communities by reducing their resistance to drought (p < 0.05). In terms of diversity, we have previously reported that irrigation increased the dominance of grasses in our experiment. Post-drought, grasses continue to be the dominant, with species from other functional groups continuing to persist at low abundance. It is important to note that our plant communities started with relatively diversity, having been planted with a 32 species mix. Communities with lower plant diversity might experience even lower resistance to perturbations and climate variability. Our results provide an example of how diverse plant communities can have high resistance to perturbations, and show that environmental change can reduced community resistance to severe drought.