Changes in structure of over‐ and midstory tree species in a Mediterranean‐type forest after an extreme drought‐associated heatwave

Abstract

Worldwide, extreme climatic events such as drought and heatwaves are associated with forest mortality. However, the precise drivers of tree mortality at individual and stand levels vary considerably, with substantial gaps in knowledge across studies in biomes and continents. In 2010–2011, a drought‐associated heatwave occurred in south‐western Australia and drove sudden and rapid forest canopy collapse. Working in the Northern Jarrah (Eucalyptus marginata) Forest, we quantified the response of key overstory (E. marginata, Corymbia calophylla) and midstory (Banksia grandis, Allocasuarina fraseriana) tree species to the extreme climate event. Using transects spanning a gradient of drought impacts (minimal (50–100 m), transitional (100–150 m) and severe (30–60 m)), tree species mortality in relation to stand characteristics (stand basal area and stem density) and edaphic factors (soil depth) was determined. We show differential mortality between the two overstory species and the two midstory species corresponding to the drought‐associated heatwave. The dominant overstory species, E. marginata, had significantly higher mortality (~19%) than C. calophylla (~7%) in the severe zone. The midstory species, B. grandis, demonstrated substantially higher mortality (~59%) than A. fraseriana (~4%) in the transitional zone. Banksia grandis exhibited a substantial shift in structure in response to the drought‐associated heatwave in relation to tree size, basal area and soil depth. This study illustrates the role of climate extremes in driving ecosystem change and highlights the critical need to identify and quantify the resulting impact to help predict future forest die‐off events and to underpin forest management and conservation.

Underappreciated plant vulnerabilities to heat waves

“…For example, a heat‐wave‐and‐drought‐induced forest die‐off in southwestern Australia triggered significant loss of live standing biomass (49.3 t carbon ha −1 ; Walden et al, 2019). That forest die‐off event resulted in adult mortality and altered regeneration of a key midstorey species, Banksia grandis , indirectly affecting an important food source of an endangered cockatoo (Steel et al, 2019). There have also been shifts in rhizosphere microbial communities following heat‐wave‐triggered forest die‐off (Hopkins et al, 2018).…”

Avian community changes following drought-induced canopy collapse in a Mediterranean-type forest

“…The study sites formed part of long-term monitoring project to assess recovery and ongoing changes to drought-affected forest (Matusick et al 2013). Severe drought-affected sites were characterised by significant loss of canopy cover (Matusick et al 2013) and increased mortality of both canopy and understorey species (Steel et al 2019). Droughtaffected sites had more fine coarse woody debris and standing dead trees (fig.…”

“…The lower numbers of nectarivore reported in droughtaffected jarrah forest sites is likely related to reduced availability of food resources due to reduced flowering effort as well as death of the plants. Mortality of flowering plants, including marri (7%), jarrah (19%) and bull banksia (59%) trees, was greater in severely affected jarrah forest drought sites (Steel et al 2019), resulting in fewer food resources available at these sites for nectarivores. In the northern jarrah forest, Western wattlebirds and Western spinebills forage almost exclusively on bull banksia (Abbott and Van Heurck 1985), a plant species that was present principally in healthy plots in the present study.…”

Rapid assimilation and analysis of a suit of remote sensing data for predicting extreme events and their impact on ecological-human systems

“…Systems, playing a central role in water, carbon and nutrient cycling (Lade et al, 2020, Norris et al, 2020, Ruckelshaus et al, 2020, Carpenter et al, 2009. The consequences of hydrological extremessuch as droughts and flooding -are measured by their impact on ecosystems such as permanent plant die-off events or species alternation (AghaKouchak et al, 2020, Trotsiuk et al, 2020, Steel et al, 2019, Ruthrof et al, 2018. To represent such events and their ecosystem impacts, ESMs rely on parametrization often supported by intensive spatially-limited in-situ observations.…”