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

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.