Identifying areas at risk of drought‐induced tree mortality across South‐Eastern Australia

Abstract

South‐East Australia has recently been subjected to two of the worst droughts in the historical record (Millennium Drought, 2000–2009 and Big Dry, 2017–2019). Unfortunately, a lack of forest monitoring has made it difficult to determine whether widespread tree mortality has resulted from these droughts. Anecdotal observations suggest the Big Dry may have led to more significant tree mortality than the Millennium drought. Critically, to be able to robustly project future expected climate change effects on Australian vegetation, we need to assess the vulnerability of Australian trees to drought. Here we implemented a model of plant hydraulics into the Community Atmosphere Biosphere Land Exchange (CABLE) land surface model. We parameterized the drought response behaviour of five broad vegetation types, based on a common garden dry‐down experiment with species originating across a rainfall gradient (188–1,125 mm/year) across South‐East Australia. The new hydraulics model significantly improved (~35%–45% reduction in root mean square error) CABLE’s previous predictions of latent heat fluxes during periods of water stress at two eddy covariance sites in Australia. Landscape‐scale predictions of the greatest percentage loss of hydraulic conductivity (PLC) of about 40%–60%, were broadly consistent with satellite estimates of regions of the greatest change in both droughts. In neither drought did CABLE predict that trees would have reached critical PLC in widespread areas (i.e. it projected a low mortality risk), although the model highlighted critical levels near the desert regions of South‐East Australia where few trees live. Overall, our experimentally constrained model results imply significant resilience to drought conferred by hydraulic function, but also highlight critical data and scientific gaps. Our approach presents a promising avenue to integrate experimental data and make regional‐scale predictions of potential drought‐induced hydraulic failure.

Understanding and predicting forest mortality in the western United States using long‐term forest inventory data and modeled hydraulic damage

“…Hydraulic models have improved prediction of water and carbon fluxes from individual plant to ecosystem scale (Eller et al, 2018(Eller et al, , 2020Mencuccini et al, 2019;Wang et al, 2019;Liu et al, 2020;Sabot et al, 2020), species distribution (Simeone et al, 2019) and tree mortality (Venturas et al, 2018;De Kauwe et al, 2020; this study). However, regional-scale prediction of mortality with HMs still remains a challenging task, owing to uncertainties associated with plant traits and heterogeneity in sub-grid scale water access.…”

Drought by CO2interactions in trees: a test of the water savings mechanism

“…The link between leaf gas exchange and soil moisture availability may be more mechanistically explained by the inclusion of plant hydraulic traits (e.g. k plant or leaf water potentials), as shown here and elsewhere (Xu et al ., 2016; Christoffersen et al ., 2016; Duan et al ., 2018; Kennedy et al ., 2019; De Kauwe et al ., 2020; Eller et al ., 2020; Sabot et al ., 2020). As such, we need to develop a parsimonious representation of plant hydraulic behaviour in vegetation models to capture the crucial effects of plant hydraulic strategies in modulating plant–soil water relations and plant biomass response to elevated CO 2 and drought (Yu et al ., 2019; Papastefanou et al ., 2020).…”

Precipitation–productivity relationships and the duration of precipitation anomalies: An underappreciated dimension of climate change

“…A famous historical example over the last century is the Dust Bowl drought of the 1930s (Knapp et al, 2020; Weaver & Albertson, 1940). Prominent contemporary examples include Australia's ~1997 to 2010 Millennium (Herberger, 2012) and 2017–2019 ‘Big Dry’ Droughts (De Kauwe et al, 2020), the California drought from ~2011 to 2015 (Goulden & Bales, 2019), and the ongoing megadrought in southwestern North America since the turn of the 21st century which, intriguingly, was preceded by the wettest 19‐year period in the last millennia for that region (Williams et al, 2020). Such prolonged wet periods seem to receive less attention than droughts but can also alter vegetation demography and NPP dynamics, from tropical forests (Esteben et al, 2020) to dryland ecosystems (Chen et al, 2017; Loik et al, 2004; Peters et al, 2012).…”