Transpiration of deep soil water moderates the impacts of droughts and heatwaves on Eucalyptus species in Australia

Background/Question/Methods

The incidence and magnitude of extreme weather events is increasing with climate change. The increasing frequency and intensity of droughts and heatwaves is particularly concerning for the growth and physiology of trees. For example, impairments of hydraulic function under drought may limit the ability of trees to transpire water and regulate their leaf temperatures during extreme heatwave events. Here, e performed two separate but complementary experiments utilizing the unique infrastructure of twelve whole-tree chambers (WTCs) near Sydney, Australia. We investigated the impact of long-term warming, experimental drought, and an extreme experimental heatwave on the growth and physiological performance of Eucalyptus species. The WTCs are large cylindrical structures topped with a cone (3.25 m in diameter, 9 m in height, volume of ~53 m³) that enclose individual trees rooted in soil in field conditions. The WTCs controlled T_air, VPD, and atmospheric CO₂ concentration in the canopy air space while measuring the net exchange of CO₂ and H₂O for the entire canopy at 15-minute resolution. The rooting volume of each tree was compartmentalized with a vertical root exclusion barrier extending approximately 1 m belowground.

Results/Conclusions

We show relatively minor negative impacts of experimental drought and extreme heatwave on the physiological performance and growth of Eucalypt species. This was particularly remarkable given the intensity of our treatments, which included an extreme summer drought (3 months with no precipitation) and a record extreme heatwave (4 consecutive days with air temperatures exceeding 43 °C). We show that the facultative use of deep soil water (i.e., from >1 meter depth) was an important component of the resistance of these trees to extreme weather stressors. Continued transpiration of deep water allowed trees in the drought treatment to continue photosynthesizing and growing at 75% the rate of the trees in the control treatment. Similarly, continued transpiration of deep soil water during an extreme heatwave moderated leaf temperatures via a latent cooling effect, allowing leaves to avoid dangerously high leaf temperatures. The facultative use of deep soil water during conditions of extreme drought and heat has implications for tree tolerance of future climate extremes.