Abstract
Globally, combinations of drought and warming are driving widespread tree mortality and crown dieback. Yet thresholds triggering either tree mortality or crown dieback remain uncertain, particularly with respect to two issues: (i) the degree to which heat waves, as an acute stress, can trigger mortality, and (ii) the degree to which chronic historical drought can have legacy effects on these processes. Using forest study sites in southwestern Australia that experienced dieback associated with a short-term drought with a heatwave (heatwave-compounded drought) in 2011 and span a gradient in long-term precipitation (LTP) change, we examined the potential for chronic historical drought to amplify tree mortality or crown dieback during a heatwave-compounded drought event for the dominant overstory species Eucalyptus marginata and Corymbia calophylla. We show pronounced legacy effects associated with chronically reduced LTP (1951–1980 versus 1981–2010) at the tree level in both study species. When comparing areas experiencing 7.0% and 11.5% decline in LTP, the probability of tree mortality increased from low (<0.10) to high (>0.55) in both species, and probability of crown dieback increased from high (0.74) to nearly complete (0.96) in E. marginata. Results from beta regression analysis at the stand-level confirmed tree-level results, illustrating a significant inverse relationship between LTP reduction and either tree mortality (F = 10.39, P = 0.0073) or dieback (F = 54.72, P < 0.0001). Our findings quantify chronic climate legacy effects during a well-documented tree mortality and crown dieback event that is specifically associated with an heatwave-compounded drought. Our results highlight how insights into both acute heatwave-compounded drought effects and chronic drought legacies need to be integrated into assessments of how drought and warming together trigger broad-scale tree mortality and crown dieback events.
Highlights
Compound climate events, including the combination of drought and atmospheric warming, are recognized as an important driver of widespread tree mortality events worldwide (Breshears et al 2013, Mitchell et al 2014, Allen et al 2015), and have implications for tree species distributions, forest composition, structure, and functioning (Mathys et al 2016)
Climatological drought and warming events are categorized in a variety of ways based on their duration, frequency, and intensity, and these have varying effects on trees and forests (McDowell et al 2008, Barbeta and Peñuelas 2016)
The Mediterranean-type climate of southwestern Australia is characterized by long, dry summer periods followed by cool, wet winters, with nearly 80% of rainfall occurring from April to October (Bates et al 2008)
Summary
Compound climate events, including the combination of drought and atmospheric warming, are recognized as an important driver of widespread tree mortality events worldwide (Breshears et al 2013, Mitchell et al 2014, Allen et al 2015), and have implications for tree species distributions, forest composition, structure, and functioning (Mathys et al 2016). Short-term drying and heating events, such as acute drought and heat waves, cause severe acute water stress in trees, typified by rapid loss of stem conductivity and leaf turgor (Anderegg et al 2014, Bader et al 2014), leading to tree crown dieback, tree mortality, and widespread forest mortality in severe cases (Anderegg et al 2015). Of particular interest are the interacting and compounded effects of chronic and acute drought and heat events on trees and forests, since both climatic averages and extreme events are predicted to change for many areas of the world over the coming decades (Collins et al 2013)
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