Fire, drought and productivity as drivers of dead wood biomass in eucalypt forests of south-eastern Australia

Abstract

Dead wood, including dead standing trees (DST) and coarse woody debris (CWD), is a critical component of forest ecosystems that provides habitat and refugia for fauna, flora, and microbial communities and plays a key role in carbon and nutrient cycling. However, few studies have modelled the long-term dynamics of dead wood, limiting our ability to predict how the abundance and composition of dead wood may change with climate change or altered fire regimes. Here we analyse DST and CWD data in 884 plots encompassing multiple field campaigns and forest types of varying canopy cover and species composition across the State of Victoria in temperate south-eastern Australia. We use boosted regression tree modelling to examine the relative influence of disturbance history and tree functional traits on dead wood biomass while accounting for the influence of environmental and climatic factors and stand attributes across a broad productivity gradient. We modelled absolute and relative dead wood biomass by size (‘small’ 100 < 200 mm diameter, ‘medium’ 200 < 500 mm diameter, ‘large’ ≥ 500 mm diameter) and decay classes (sound to advanced decay) to evaluate the consistency of predictor effects among different components of dead wood. We found that live tree basal area and mean annual precipitation were influential predictors of both DST and CWD biomass, indicating an over-arching effect of forest productivity on dead wood biomass. Fire history was also an important predictor, with DST biomass decreasing and CWD biomass increasing with time since last wildfire. The proportion of large DST biomass increased with increased tree mortality as a result of fire interval and time in drought. DST biomass also increased and CWD biomass decreased with increasing wood density, and this was relatively more important than the other functional traits we examined (heartwood nitrogen content and bark type). Our study suggests that forest productivity, fire history, drought and wood density are important determinants of dead wood, as they influence dead wood inputs and outputs. Our study reveals the broad-scale drivers of dead wood biomass, and the potential for altered fire regimes and changing climate to influence live- to dead wood dynamics and associated ecosystem functions.