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

Abstract

Forest die-offs associated with drought and heat have recently occurred across the globe, raising concern that associated changes in fuels and microclimate could link initial die-off disturbance to subsequent fire disturbance. Despite widespread concern, little empirical data exist. Following forest die-off in the Northern Jarrah Forest, south-western Australia, we quantified fuel dynamics and associated microclimate for die-off and control plots. Sixteen months post die-off, die-off plots had significantly increased 1-h fuels (11.8 vs 9.8 tonnes ha -1 ) but not larger fuel classes (10-h and 100-h fuels). Owing to stem mortality, die-off plots had significantly greater standing dead wood mass (100 vs 10 tonnes ha -1 ), visible sky (hemispherical images analysis: 31 vs 23%) and potential near-ground solar radiation input (measured as Direct Site Factor: 0.52 vs 0.34). Supplemental mid-summer microclimate measurements (temperature, relative humidity and wind speed) were combined with long-term climatic data and fuel load estimates to parameterise fire behaviour models. Fire spread rates were predicted to be 30% greater in die-off plots with relatively equal contributions from fuels and microclimate, highlighting need for operational consideration by fire managers. Our results underscore potential for drought-induced tree die-off to interact with subsequent fire under climate change.

Wildfire refugia in forests: Severe fire weather and drought mute the influence of topography and fuel age

“…Drought conditions can increase the connectivity of dry fuels across the landscape, by desiccating fuels in mesic gullies and poleward‐facing slopes (Caccamo, Chisholm, Bradstock, & Puotinen, ), removing these barriers to fire spread. Drought also increases litter production (Duursma et al, ; Pook, ) and can cause partial or whole plant mortality (Collins, Bradstock, et al, ; Ruthrof et al, ), thereby increasing surface fuel loads and the amount of dead elevated fuel (Ruthrof et al, ). Drought‐related ‘pulse’ inputs of fuel have the potential to offset fuel limitations in recently burnt areas.…”

Forest fires and climate change: causes, consequences and management options

“…Ruthrof et al (2016) address the idea that changes in forest structure resulting from a climate induced die-off could lead to subsequent changes in fire behaviour. In their paper, the authors quantify changes in fuel loads and understorey microclimate following a forest die-off in south-western Australia.…”

Eucalyptus forest shows low structural resistance and resilience to climate change‐type drought

“…Accumulation of dead biomass created by the drought event may exacerbate wildfire incidence (increased continuity of fuels) and severity (higher fire intensities and lower leaf canopies; Ruthrof et al. ), resulting in additional tree mortality (Brando et al. ).…”

“…Biomass losses from drought-induced disturbance can destabilize carbon storage capacity (Ma et al 2012), and even turn some ecosystems from carbon sinks to carbon sources (Lewis et al 2011). Accumulation of dead biomass created by the drought event may exacerbate wildfire incidence (increased continuity of fuels) and severity (higher fire intensities and lower leaf canopies; Ruthrof et al 2015a), resulting in additional tree mortality (Brando et al 2014).…”