Mortality and resprouting responses in forest trees driven more by tree and ecosystem characteristics than drought severity and fire frequency

Increases in tree mortality linked to drought and fires have been reported across a range of forests globally over the last few decades. Forests that resprout epicormically/aerially should be the most resistant and resilient to changes in fire regime, yet they may be at risk of increased mortality, demographic shifts and changes to species composition due to the compounding effects of drought and fire. Despite this, the synergistic effects of drought and fire frequency on resprouters has received less attention than for obligate seeder tree species. Our study examined the effects of drought severity and fire frequency on the fire resistance of eucalypts (i.e., including Eucalyptus, Angophora and Corymbia) that can resprout epicormically. Following large-scale wildfires and drought in 2013, we conducted field surveys of temperate eucalypt forests in the Sydney Basin Bioregion, focusing on two major Eucalyptus forest assemblages: dry ridgetops and wet gullies. We measured tree size, previous fire damage (i.e., fire scars), bark type, mortality and resprouting position (e.g., canopy, stem, base) of reproductive-age trees. We used a Bayesian modelling approach to derive bounded estimates of response probabilities for trees sampled in each combination of drought severity (mild/moderate versus severe) and fire frequency (low versus high), as well as bounded estimates of differences between trees with and without fire scars, different bark types and drought/fire histories. Eucalypt populations in both vegetation types were resilient to increases in mortality and changes in resprouting position under severe drought and frequent fire, and mortality and resprouting position varied substantially with tree size, fire scar presence/absence and among bark types. Tree mortality and changes in resprouting position were considerably more likely in smaller trees with fire scars. Species with non-compact bark (e.g., fibrous, stringy, rough) were less resilient to lowering in resprouting position, e.g., from canopy to stem. Populations dominated by species that have small stem diameters, thin/non-compact bark and previous fire damage are likely at elevated risk of mortality and changes in resprouting po-sition under future climate change.

Automated summary

Increases in tree mortality linked to drought and fires have been reported globally. Forests that resprout epicormically are expected to be more resistant and resilient to changes in fire regime, but they may still face increased mortality and changes in species composition due to drought and fire. This study examines the effects of drought severity and fire frequency on the fire resistance of eucalypts that can resprout epicormically.