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 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.

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.…”

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 find that the native grass T. triandra had the most resistant xylem than the nonnative species, this may reflect the fact that the nonnative species used in this study have been selectively bred for increased performance under managed pasture systems, while the 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). Hence, this may provide a better indicator of hydraulic vulnerability and drought response strategy under natural conditions than previous studies, which have primarily been conducted on short‐lived, pot‐grown individuals under greenhouse conditions.…”

“…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.…”

Drought is a stronger driver of plant morphology and nutritional composition than warming in two common pasture species

“…We collected field soil (sandy-loam texture with a soil pH ~5.6) from the Pastures and Climate Extremes (PACE) field experimental facility, also located on the Western Sydney University Hawkesbury Campus (33°36'S,150°44'E) (Churchill et al, 2020). The soil was sieved (5 mm), air-dried and mixed with quartz sand (7:3, v/v) then 3.9 kg of soil was placed into each plastic pot (3.7 L, 150 mm diameter, 240 mm height).…”