Global change factors such as drought, warming and heatwaves have important impacts on ecosystem processes both individually and through their interactions. Sustained periods of drought and elevated temperatures can prompt seasonally-dependent ecosystem responses with opposing consequences for ecosystem function due to seasonal shifts in resource limitations. For managed grasslands the seasonally-explicit consequences of interacting global change factors such as drought and warming can have massive economic implications and affect their long-term sustainability for year-long maintenance. Our Pastures in Climate Extremes (PACE) experimental facility in eastern Australia uses a factorial manipulation of elevated air temperature and winter + spring extreme drought (60% reduction) to examine pasture susceptibility and sustainability under future climate scenarios. The experiment includes twelve pasture types with a range of species traits represented; including native and exotic species, C3 and C4 grasses, temperate legumes, and pastures comprised of species mixtures. All pastures are continuously monitored using mounted phenocams to track canopy development over time and in response to simulated grazing (biomass harvests) or weather events (e.g. frost and heatwaves). Canopy greenness, or Green Chromatic Coordinate (GCC), is calculated based on RBG indices derived from images and can be used to track pasture susceptibility and recovery associated with the experimental conditions.
Results/Conclusions
Our treatments resulted in substantial pasture-specific effects on canopy GCC that demonstrate variety in susceptibility and recovery to extreme climate conditions. For pastures exposed to both warming and drought, a C3 grass showed an additive response to the treatments such that warmed*droughted plots had the least GCC, while native mixed species pasture was greener in heated plots during winter and spring, indicating positive effects of warming on cool-season growth. For drought some species showed high sensitivity, such as C3 Rye grass that experienced a decline in greenness within two weeks of drought initiation. In contrast, two C4 grasses did not show a drought effect during the same period. Pasture canopy greenness can be a useful and almost immediate metric to indicate ecosystem responses to local weather patterns promoting increased growth or die-backs and therefore available forage for grazers. When paired with site level measurements of productivity and potential drivers of canopy changes, such as stem density and leaf area index, we can develop a diverse range of mechanistic drivers of pasture resilience. These relationships can then be used to link data from plot-scale images to landscape-scale implications for grassland systems responding to extreme climate conditions.