Hydraulic failure and tree size linked with canopy die‐back in eucalypt forest during extreme drought

Abstract

Summary Eastern Australia was subject to its hottest and driest year on record in 2019. This extreme drought resulted in massive canopy die‐back in eucalypt forests. The role of hydraulic failure and tree size on canopy die‐back in three eucalypt tree species during this drought was examined. We measured pre‐dawn and midday leaf water potential (Ψleaf), per cent loss of stem hydraulic conductivity and quantified hydraulic vulnerability to drought‐induced xylem embolism. Tree size and tree health was also surveyed. Trees with most, or all, of their foliage dead exhibited high rates of native embolism (78–100%). This is in contrast to trees with partial canopy die‐back (30–70% canopy die‐back: 72–78% native embolism), or relatively healthy trees (little evidence of canopy die‐back: 25–31% native embolism). Midday Ψleaf was significantly more negative in trees exhibiting partial canopy die‐back (−2.7 to −6.3 MPa), compared with relatively healthy trees (−2.1 to −4.5 MPa). In two of the species the majority of individuals showing complete canopy die‐back were in the small size classes. Our results indicate that hydraulic failure is strongly associated with canopy die‐back during drought in eucalypt forests. Our study provides valuable field data to help constrain models predicting mortality risk.

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

“…The Australian climate is characterized by high interannual variability in rainfall, leading to the development of severe droughts often associated with El Niño events and a positive Indian Ocean Dipole (Abram et al, 2020; Nolan et al, 2020). From 2017 to 2020, Australia suffered one of the worst droughts on record, resulting in widespread canopy dieback in south‐eastern Australia (Li, Blackman, Rymer, et al, 2018; Nolan et al, 2021), contributing to the devastating 2019–2020 Black Summer bushfires (Boer et al, 2020; Nolan, Boer, et al, 2020). Within Australia, and globally, the role of climate change in exacerbating drought severity is becoming clearer, both in terms of altered rainfall patterns and increasing evaporative demand (King et al, 2020; Trenberth et al, 2014; Yuan et al, 2019).…”

“…Increasing temperatures, higher evaporative demand and more extreme rainfall events are expected to elevate stress for trees, resulting in increased rates of tree mortality and greater potential for widespread forest dieback (Dale et al, 2001). Broadscale forest mortality events associated with drought have been described and studied in recent years (Allen et al, 2010, 2015; Schuldt et al, 2020): the pinyon–juniper forests in western United States, boreal forests in Canada and California forests of North America (Huang et al, 2010; Michaelian et al, 2011; Young et al, 2017); Europe's Iberian peninsula, central and south‐eastern regions (Nardini et al, 2013; Neumann et al, 2017; Ruiz‐Benito et al, 2013; Schuldt et al, 2020) and Australia's southwest and southeast states (Brouwers et al, 2013; Li, Blackman, Rymer, et al, 2018; Matusick et al, 2013; Nolan et al, 2021) are all areas heavily impacted by drought and dieback events. In many of the world's tropical forests, drought has caused increased rates of tree mortality and a decline in productivity (Hubau et al, 2020; Phillips et al, 2009; Powers et al, 2020; Qie et al, 2017).…”

“…From 2017 to 2020, Australia suffered one of the worst droughts on record, resulting in widespread canopy dieback in south-eastern Australia Nolan et al, 2021), contributing to the devastating 2019-2020 Black Summer bushfires (Boer et al, 2020;. Within Australia, and globally, the role of climate change in exacerbating drought severity is becoming clearer, both in terms of altered rainfall patterns and increasing evaporative demand (King et al, 2020;Trenberth et al, 2014;Yuan et al, 2019).…”

Hydraulic variability of three temperate broadleaf tree species along a water availability gradient in central Europe

“…Two interconnected physiological mechanisms that explain reduced tree productivity and increased mortality following severe drought exposure, namely carbon starvation and hydraulic failure, have widely been discussed in the past since their introduction by McDowell et al (2008). Recent results, however, indicate that hydraulic failure is the proximate cause of drought-induced tree mortality in various biomes (Rowland et al, 2015;Anderegg et al, 2016;Adams et al, 2017;Correia et al, 2019;Hajek et al, 2020;Powers et al, 2020;Arend et al, 2021;Nolan et al, 2021).…”

“…(2008). Recent results, however, indicate that hydraulic failure is the proximate cause of drought‐induced tree mortality in various biomes (Rowland et al ., 2015; Anderegg et al ., 2016; Adams et al ., 2017; Correia et al ., 2019; Hajek et al ., 2020; Powers et al ., 2020; Arend et al ., 2021; Nolan et al ., 2021).…”