Species origin affects the rate of response to inter-annual growing season precipitation and nutrient addition in four Australian native grasslands

Questions: Predicted increases in temperature and changes to precipitation are expected to alter the amount of plant available nutrients, in turn, altering rates of primary production and exotic plant invasions. However, it remains unclear whether increased responses occur in wetter than average years, even in low fertility and low rainfall regions. Location: Four Australian grasslands, including sites in arid Western Australia, semi-arid Victoria, alpine Victoria and sub-tropical Queensland. Methods: Using identical nutrient addition experiments, we use 6-years of biomass, cover and species richness data to examine how rates of biomass production and native and exotic cover and richness are affected by growing season precipitation [proportion of yearly growing season precipitation (GSP) to long-term mean GSP] and nutrient (N, P, K and micronutrients) addition. Results: Rates of grassland productivity strongly increased with increasing GSP. GSP increased rates of native cover but not native or exotic richness, nor rates of exotic cover change. We detected no significant NPK effect on rates of grassland productivity, exotic cover or exotic richness change. In contrast, NPK addition decreased rates of native cover change and fertilized plots had significantly fewer native species. We did not detect a significant interaction between NPK and GSP_. Conclusions: Grassland productivity was more strongly predicted by variation in growing season precipitation than by nutrient addition, suggesting it will vary with future changes in rainfall. Response to nutrients, however, depend on species origin, suggesting that increasing soil nutrient availability due to anthropogenic activities is likely to lead to negative effects on native species richness and cover.