Pastures and Climate Extremes: Impacts of Cool Season Warming and Drought on the Productivity of Key Pasture Species in a Field Experiment

Abstract

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.

High safety margins to drought‐induced hydraulic failure found in five pasture grasses

“…F I G U R E 4 Linear regression model comparing (a) the point at which stomata are 50% closed (P gs50 ) and water potential at which 12% of xylem are embolized (P X12 ), (b) P gs50 and water potential at which 50% of xylem are embolized (P X50 ), (c) P gs50 and water potential at which 88% of xylem are embolized (P X88 ), (d) the point at which stomata are 90% closed (P gs90 ) and P X12 (e) P gs90 and P X50 (f) P gs90 and P X88 , (g) water potential at loss of 50% of effective hydraulic conductance (P K50 ) and Px12, (h) P K50 and P X50 (i) P K50 and P X88 (j) water potential at a loss of 90% of effective hydraulic conductance (P K90 ) and P X12 (k) P K90 and P X50 and (l) native grass has not. Another reason for the high level of xylem embolism resistance we observed in our species is that we measured hydraulic traits on long-lived individuals (>12 months of growth) grown under field conditions that included numerous exposures to low soil moisture availability (as shown in Churchill et al, 2020).…”

“…We investigated five widely grown perennial pasture grasses: two nonnative, temperate C 3 species, Festuca arundinacea and Phalaris aquatica ; two nonnative, tropical C 4 species, Chloris gayana and Digitaria eriantha ; and one native C 4 species, Themeda triandra . This study consisted of two experiments: the first experiment was to analyse the vulnerability of leaf xylem to embolism using the OV technique on field‐grown plants extracted from the Pasture and Climate Extremes (PACE) facility at the Hawkesbury Institute for the Environment (Churchill et al, 2020); the second experiment determined the response of stomatal conductance ( g s ) and effective plant hydraulic conductance ( K leaf ) to decreasing leaf water potential in a pot dry‐down experiment using plants sourced from the PACE field plots.…”

Root trait shifts towards an avoidance strategy promote productivity and recovery in C3 and C4 pasture grasses under drought

“…Statistics for these figures are shown in Table 2. The ANPP data shown in panel (a) have been reported previously (Churchill et al, 2022) and are reproduced here for convenience. drought (t = −2.29, p = 0.02) treatments, and were consistently higher across species under drought treatment (Figure 4f).…”

Belowground carbon allocation, root trait plasticity, and productivity during drought and warming in a pasture grass

“…Statistics for these figures are shown in Table 1 . The ANPP data shown in (A) have been reported previously ( Churchill et al , 2022 ) and are reproduced here for convenience.…”