Maximising the resilience of grasslands to extreme precipitation events, nutrient addition, and grazing

Abstract

Grasslands cover much of Earth’s surface and support essential ecosystem services. In Australia, grasslands have been extensively modified for agriculture, with 40% of agricultural production occurring on grasslands. Understanding the drivers of grassland productivity is critical for maintaining food security while preserving other ecosystem services. Grassland productivity is shaped by interactions between precipitation, nutrient availability, and herbivory, and is increasingly threatened by global change pressures such as altered rainfall regimes, eutrophication, and grazing intensification. While global networks have studied these drivers independently, their combined effects remain poorly understood. This thesis examined how changes in precipitation patterns interact with nutrient addition and livestock grazing to influence grassland productivity, diversity, and composition in southeastern Australia. Reconstruction of spatial and temporal patterns of droughts and wet events across Australia since 1901 showed droughts have intensified in the south, while northern Australia has experienced longer, more severe wet periods. Building on this, we conducted a field experiment manipulating rainfall, nutrients, and grazing in a native pasture at the University of Sydney's Llara farm. We found that nutrient addition boosted productivity where water was available, but reduced diversity through increased dominance of Cynodon dactylon. Grazing mitigated some diversity loss under high water availability. Nutrient addition did not increase productivity under extreme drought, and instead reduced it over time, especially when grazed. This delayed green-up, shortened the growing season, and reduced C. dactylon productivity. These results suggest this system may be vulnerable to multiple, compounding global change pressures, and land managers should implement climate adaptive practices, avoiding nutrient addition and cattle grazing during periods of extreme drought.