Living on the edge: A continental‐scale assessment of forest vulnerability to drought

Abstract

Globally, forests are facing an increasing risk of mass tree mortality events associated with extreme droughts and higher temperatures. Hydraulic dysfunction is considered a key mechanism of drought‐triggered dieback. By leveraging the climate breadth of the Australian landscape and a national network of research sites (Terrestrial Ecosystem Research Network), we conducted a continental‐scale study of physiological and hydraulic traits of 33 native tree species from contrasting environments to disentangle the complexities of plant response to drought across communities. We found strong relationships between key plant hydraulic traits and site aridity. Leaf turgor loss point and xylem embolism resistance were correlated with minimum water potential experienced by each species. Across the data set, there was a strong coordination between hydraulic traits, including those linked to hydraulic safety, stomatal regulation and the cost of carbon investment into woody tissue. These results illustrate that aridity has acted as a strong selective pressure, shaping hydraulic traits of tree species across the Australian landscape. Hydraulic safety margins were constrained across sites, with species from wetter sites tending to have smaller safety margin compared with species at drier sites, suggesting trees are operating close to their hydraulic thresholds and forest biomes across the spectrum may be susceptible to shifts in climate that result in the intensification of drought.

Small and slow is safe: On the drought tolerance of tropical tree species

“…Consequently, Ψ min ‐based HSM and TLP‐based HSM are closely associated across species (Figure 2f), which indicates that they quantify the same drought tolerance responses. This result parallels a recent study showing that both HSM definitions have comparable average values in an Australian tropical rainforest (Peters et al, 2021). Our results suggest that TLP‐based HSM quantifies drought exposure and risk of drought‐induced hydraulic failure in tropical species (Powers et al, 2020).…”

Decoupling between ecosystem photosynthesis and transpiration: a last resort against overheating

“…This behaviour, which can be described as a decoupling between photosynthesis and transpiration under heat stress, can be hypothesised to be a plant strategy to reduce the adverse effects of too high temperatures on the plant, cooling the canopy air space via evaporation (Drake et al 2018). However, hydraulic traits exhibit substantial differences among species (Peters et al 2021). The 'transpirational cooling' strategy can thus vary strongly from species to species, and it relies on the access to water (O'sullivan et al 2017).…”

Eucalyptus obliqua tall forest in cool, temperate Tasmania becomes a carbon source during a protracted warm spell in November 2017

“…The decline in GPP during the warm spell at Warra was different. The E. obliqua tall forests at Warra are among the most mesic of Australia's forests 20 and as a result have sacrificed hydraulic safety 21 to maintain high rates of photosynthesis across the range of leaf water potentials typically encountered 22 . Soil moisture was not limiting at Warra www.nature.com/scientificreports/ during the warm spell; VPDs did not reach high values and canopy conductance remained comparable with the much milder conditions during the comparison period.…”