Exposure of trees to drought‐induced die‐off is defined by a common climatic threshold across different vegetation types

Abstract

Increases in drought and temperature stress in forest and woodland ecosystems are thought to be responsible for the rise in episodic mortality events observed globally. However, key climatic drivers common to mortality events and the impacts of future extreme droughts on tree survival have not been evaluated. Here, we characterize climatic drivers associated with documented tree die-off events across Australia using standardized climatic indices to represent the key dimensions of drought stress for a range of vegetation types. We identify a common probabilistic threshold associated with an increased risk of die-off across all the sites that we examined. We show that observed die-off events occur when water deficits and maximum temperatures are high and exist outside 98% of the observed range in drought intensity; this threshold was evident at all sites regardless of vegetation type and climate. The observed die-off events also coincided with at least one heat wave (three consecutive days above the 90th percentile for maximum temperature), emphasizing a pivotal role of heat stress in amplifying tree die-off and mortality processes. The joint drought intensity and maximum temperature distributions were modeled for each site to describe the co-occurrence of both hot and dry conditions and evaluate future shifts in climatic thresholds associated with the die-off events. Under a relatively dry and moderate warming scenario, the frequency of droughts capable of inducing significant tree die-off across Australia could increase from 1 in 24 years to 1 in 15 years by 2050, accompanied by a doubling in the occurrence of associated heat waves. By defining commonalities in drought conditions capable of inducing tree die-off, we show a strong interactive effect of water and high temperature stress and provide a consistent approach for assessing changes in the exposure of ecosystems to extreme drought events.

Carbon uptake and water use in woodlands and forests in southern Australia during an extreme heat wave event in the “Angry Summer” of 2012/2013

“…It is therefore crucial to better understand ecosystem responses to climate extremes. The role of climate extremes could be critical in shaping future ecosystem dynamics (Zimmermann et al, 2009), but the sporadic and unpredictable nature of these events makes it difficult to monitor how they affect vegetation through space and time (Mitchell et al, 2014).…”

“…Australian forest and woodland ecosystems are strongly influenced by large climatic variability, characterised by recurring drought events and heat waves Mitchell et al, 2014). Eucalyptus regnans ecosystems in southeast Australia, for example, have an exceptional capacity to withstand drought and the ability to recover almost instantly after heat waves (Pfautsch and Adams, 2013).…”

Review of broad-scale drought monitoring of forests: Toward an integrated data mining approach

“…For instance, the Palmer Modified Drought Index (PMDI) is less volatile than the PDSI, and can more accurately capture a linear combination of temperature and precipitation effects across broad geographic regions (Heddinghaus and Sabol, 1991). The Standardized Precipitation Index (SPI) uses only precipitation data to characterize moisture conditions during multiple, well-defined time windows (McKee et al, 1993); unfortunately, this could be a limitation in the face of increasing recognition that high temperatures magnify drought impacts on trees, and that these impacts are likely to worsen due to climate change (Breshears et al, 2005;McDowell et al, 2008;Allen et al, 2010Allen et al, , 2015Williams et al, 2013;Mitchell et al, 2014;Mildrexler et al, 2016). The Standardized Precipitation Evapotranspiration Index (SPEI) is calculated for multiple time windows like the SPI, but also incorporates PET estimates (VicenteSerrano et al, 2010).…”

Innovation in the forest industry: ready or not!

“…Despite this outlook, Australia's plantation estate is currently shrinking rather than growing (e.g. net area decreased by 12 800 ha in 2013-14) (ABARES 2015), and the long-term viability of some plantation regions is under threat from climate impacts such as drought, pests and diseases , with probable increasing pressure from these elements into the future (Mitchell et al 2014). Future climate variability and increasing competition for suitable land, associated with megatrends such as population growth (Hajkowicz et al 2012), mean it will likely be difficult to maintain, let alone expand, the existing plantation footprint within Australia without substantial silvicultural or genetic intervention.…”