Sudden forest canopy collapse corresponding with extreme drought and heat in a mediterranean-type eucalypt forest in southwestern Australia

Abstract

Drought and heat-induced forest dieback and mortality are emerging global concerns.Although Mediterranean-type forest (MTF) ecosystems are considered to be resilient to drought and other disturbances, we observed a sudden and unprecedented forest collapse in a MTF in Western Australia corresponding with record dry and heat conditions in 2010/2011.An aerial survey and subsequent field investigation were undertaken to examine: the incidence and severity of canopy dieback and stem mortality, associations between canopy health and stand-related factors as well as tree species response. Canopy mortality was found to be concentrated in distinct patches, representing 1.5% of the aerial sample (1,350 ha).Within these patches, 74% of all measured stems (> 1cm DBHOB) had dying or recently killed crowns, leading to 26% stem mortality six months following the collapse. Patches of canopy collapse were more densely stocked with the dominant species, Eucalyptus marginata, and lacked the prominent midstorey species Banksia grandis, compared to the surrounding forest. A differential response to the disturbance was observed among cooccurring tree species, which suggests contrasting strategies for coping with extreme water stress. These results suggest that MTFs, once thought to be resilient to climate change, are susceptible to sudden and severe forest collapse when key thresholds have been reached.

How drought-induced forest die-off alters microclimate and increases fuel loadings and fire potentials

“…There is a strong west-east rainfall gradient across the forest, ranging from >1100 mm year -1 on its western edge (Darling Scarp) to ~700 mm year -1 in the east and north (Gentilli 1989). Further details of the study region, climate events, sites and their selection are described in Matusick et al (2013) and Brouwers et al (2013).…”

“…Beginning in February 2011, following 9 days over 35°C (Bureau of Meteorology 2011a), and after one of the driest years since comparable records (Bureau of Meteorology 2011b), tree crowns in the Northern Jarrah Forest began to rapidly discolour and die (see Matusick et al (2013) for further details). This process continued through May 2011, resulting in discrete patches of nearly complete canopy loss.…”

“…To quantify the effect of drought on fuel attributes, we analysed data at the plot scale, which required a random effect of patch (i.e. 20 patches each with n = 3 die-off plots and n = 3 control plots) and fixed effects of drought and covariates of pre-drought basal area (available from original plot measurements; Matusick et al 2013) and time since fire (years: range = 3-17 years, mean = 8.25) (Department of Parks and Wildlife records, available at http://firewatch-pro.landgate.wa.gov.au/home.php, accessed 7 December 2015). Analyses were carried out for 1-h, 10-h, 100-h and total fine fuels as well as litter depth.…”

Plants and Drought in a Changing Climate

“…While higher CO 2 concentrations may boost plant growth and simultaneously help plants conserve water, the co-occurrence of hot conditions during drought could exacerbate plant stress, and potentially lead to increased damage to tissues and higher rates of mortality [65][66][67]. This increase in plant stress could occur even if drought frequency remains constant and have major impacts on forest structure and functioning in a hotter world [68].…”

Successional changes in feeding activity by threatened cockatoos in revegetated mine sites

“…Past forestry practices in unmined forest have increased the proportion of jarrah to marri (Abbott and Loneragan 1986) and, more recently, these two species have experienced decline and die-off caused by disease, drought and extreme heat (Paap et al 2008;Matusick et al 2013). The value of revegetation as feeding habitat for black cockatoos will need to be balanced against potentially competing land uses in the jarrah forest.…”