Summary Eastern Australia was subject to its hottest and driest year on record in 2019. This extreme drought resulted in massive canopy die‐back in eucalypt forests. The role of hydraulic failure and tree size on canopy die‐back in three eucalypt tree species during this drought was examined. We measured pre‐dawn and midday leaf water potential (Ψleaf), per cent loss of stem hydraulic conductivity and quantified hydraulic vulnerability to drought‐induced xylem embolism. Tree size and tree health was also surveyed. Trees with most, or all, of their foliage dead exhibited high rates of native embolism (78–100%). This is in contrast to trees with partial canopy die‐back (30–70% canopy die‐back: 72–78% native embolism), or relatively healthy trees (little evidence of canopy die‐back: 25–31% native embolism). Midday Ψleaf was significantly more negative in trees exhibiting partial canopy die‐back (−2.7 to −6.3 MPa), compared with relatively healthy trees (−2.1 to −4.5 MPa). In two of the species the majority of individuals showing complete canopy die‐back were in the small size classes. Our results indicate that hydraulic failure is strongly associated with canopy die‐back during drought in eucalypt forests. Our study provides valuable field data to help constrain models predicting mortality risk.

Shifts in the timing and frequency of climate extremes, such as drought and heatwaves, can generate sustained shifts in ecosystem function with important ecological and economic impacts for rangelands and managed pastures. The Pastures and Climate Extremes experiment (PACE) in southeast Australia used a factorial combination of elevated temperature (ambient +3 °C) and winter/spring extreme drought (60% rainfall reduction) to evaluate the impacts of increased frequency of climate extremes on pasture productivity and subsequent summer/autumn recovery. The experiment included nine species comprising three plant functional groups (C3 grasses, C4 grasses, and legumes) in monoculture and three two-species mixtures. The winter/spring drought resulted in productivity declines of up to 73% (Digitaria eriantha) during the 6-month treatment period, with nine of the twelve plantings exhibiting significant yield reductions. Functional group identity was not an important predictor of yield response to drought. Many species recovered rapidly once the drought ended, although there were carry-over effects on warm season (summer/autumn) growth for four species/mixtures, spanning all functional groups. Cool season drought translated into significant reductions in annual biomass production for four species/mixtures, ranging from 33% (Medicago sativa) to 70% (Festuca arundinacea). Additionally, warming had neutral to negative effects on productivity during both winter/spring and summer/autumn periods, resulting in annual yield declines of up to 58%, driven at least partially by indirect effects on soil water content. The combination of winter/spring drought and year-round warming resulted in net yield reductions that were either additive or less-than-additive, compared to ambient plots. This study demonstrates that predicted extreme climate conditions will have substantial negative impacts on productivity of common pasture and rangeland species.

In the face of a changing climate, research indicates that more frequent and severe drought conditions are critical problems that will constrain production of high-quality forage and influence the performance of grazing animals in the future. In addition, the duration of drought and potential trade-offs between plant morphology and nutritional composition may influence plant drought adaptation strategies across pasture species, and the consequences for forage quality are not well understood. Here we present the results of a study investigating the effects of drought on biomass productivity, dead material, leaf:stem biomass allocation and nutritional composition (whole-plant and tissue-specific) across nine diverse pasture species. For this, we conducted a field experiment exposing species to a 6-month period of simulated severe drought (60% rainfall reduction during winter and spring) and samples were collected at multiple harvests. We found that drought had different, harvest-specific effects on plant biomass structure and nutritional composition among pasture species. The severity of drought impacts on productivity, but not on nutritional quality, increased with drought duration. In general, drought strongly reduced productivity, increased the percentage of dead material and had mixed effects (increases, decreases and no effect) on leaf:stem ratio and concentrations of crude protein, non-structural carbohydrates, neutral detergent fibre and lignin. Changes in plant-level nutritional quality were driven by simultaneous changes in both leaf and stem tissues for most, but not all, species. Our findings may be especially helpful for selection of adapted species/cultivars that could minimize potential drought risks on forage, thereby optimising pasture performance under future drought scenarios.

Shifts in the timing, intensity and/or frequency of climate extremes, such as severe drought and heatwaves, can generate sustained shifts in ecosystem function with important ecological and economic impacts for rangelands and managed pastures. The Pastures and Climate Extremes experiment (PACE) in Southeast Australia was designed to investigate the impacts of a severe winter/spring drought (60% rainfall reduction) and, for a subset of species, a factorial combination of drought and elevated temperature (ambient +3°C) on pasture productivity. The experiment included nine common pasture and Australian rangeland species from three plant functional groups (C3 grasses, C4 grasses and legumes) planted in monoculture. Winter/spring drought resulted in productivity declines of 45% on average and up to 74% for the most affected species (Digitaria eriantha) during the 6-month treatment period, with eight of the nine species exhibiting significant yield reductions. Despite considerable variation in species’ sensitivity to drought, C4 grasses were more strongly affected by this treatment than C3 grasses or legumes. Warming also had negative effects on cool-season productivity, associated at least partially with exceedance of optimum growth temperatures in spring and indirect effects on soil water content. The combination of winter/spring drought and year-round warming resulted in the greatest yield reductions. We identified responses that were either additive (Festuca), or less-than-additive (Medicago), where warming reduced the magnitude of drought effects. Results from this study highlight the sensitivity of diverse pasture species to increases in winter and spring drought severity similar to those predicted for this region, and that anticipated benefits of cool-season warming are unlikely to be realized. Overall, the substantial negative impacts on productivity suggest that future, warmer, drier climates will result in shortfalls in cool-season forage availability, with profound implications for the livestock industry and natural grazer communities.