Heatwaves are likely to increase in frequency and intensity with climate change, which may impair tree function and forest C uptake. However, we have little information regarding the impact of extreme heatwaves on the physiological performance of large trees in the field. Here, we grew Eucalyptus parramattensis trees for 1 year with experimental warming (+3°C) in a field setting, until they were greater than 6 m tall. We withheld irrigation for 1 month to dry the surface soils and then implemented an extreme heatwave treatment of 4 consecutive days with air temperatures exceeding 43°C, while monitoring whole-canopy exchange of CO2 and H2O, leaf temperatures, leaf thermal tolerance, and leaf and branch hydraulic status. The heatwave reduced midday canopy photosynthesis to near zero but transpiration persisted, maintaining canopy cooling. A standard photosynthetic model was unable to capture the observed decoupling between photosynthesis and transpiration at high temperatures, suggesting that climate models may underestimate a moderating feedback of vegetation on heatwave intensity. The heatwave also triggered a rapid increase in leaf thermal tolerance, such that leaf temperatures observed during the heatwave were maintained within the thermal limits of leaf function. All responses were equivalent for trees with a prior history of ambient and warmed (+3°C) temperatures, indicating that climate warming conferred no added tolerance of heatwaves expected in the future. This coordinated physiological response utilizing latent cooling and adjustment of thermal thresholds has implications for tree tolerance of future climate extremes as well as model predictions of future heatwave intensity at landscape and global scales.
Bibliographical noteFunding Information:
Australian Research Council Discovery, Grant/Award Number: DP140103415; New South Wales Climate Action Grant, Grant/ Award Number: NSW T07/CAG/016; Hawkesbury Institute for the Environment; Western Sydney University
We thank Burhan Amiji (Western Sydney University) for maintaining the site and for his excellent research support. We thank Alicia Cook (University of Technology Sydney) for her guidance on the T50 measurements. This experiment was made possible through a collaboration with Sune Linder and the Swedish University of Agricultural Sciences, who designed, built, and generously provided the whole tree chambers. We also gratefully acknowledge Renee Smith, Carrie Drake (Western Sydney University), and Richard Harwood (Sydney University) for their help with the whole-tree harvests. We thank Marlies Kovenock (University of Washington) and three anonymous reviewers for useful suggestions on a previous version of this manuscript. This research was supported by the Australian Research Council (Discovery, DP140103415), a New South Wales government Climate Action Grant (NSW T07/CAG/016), the Hawkesbury Institute for the Environment, and Western Sydney University.
© 2018 John Wiley & Sons Ltd
- Eucalyptus parramattensis
- climate change
- latent cooling
- thermal tolerance