Impacts of a single fire event on large, old trees in a grass-invaded arid river system

Abstract

Background: Large old trees are keystone structures of terrestrial ecosystems that provide unique habitat resources for wildlife. Their widespread decline worldwide has serious implications for biodiversity and ecosystem integrity. In arid regions, large trees are relatively uncommon and often restricted to areas with elevated soil moisture and nutrients. Introduced grasses, now pervasive in many dryland environments, also thrive in such areas and are promoting more frequent and intense fire, potentially threatening the persistence of large trees. Here we report on the impact of a single wildfire on large river red gums (Eucalyptus camaldulensis Dehnh.) in arid riparian woodland invaded by buffel grass (Cenchrus ciliaris L.), a serious invader of desert ecosystems worldwide. In 2018, 266 trees with > 80 cm equivalent trunk diameter were mapped at six sites to provide a ‘pre-fire’ baseline. Within a year, the sites were impacted by a large, unprecedented wildfire that burnt an area of 660 km2 ha in 15 days. Sites were resurveyed in February 2019 to assess the fate of the trees. Reference to fire severity, calculated from remote-sensed imagery, is provided for additional context. Results: In total, 67 trees, 27% of all large trees at the sites were destroyed. If trees in unburnt patches are excluded, 54% of trees exposed to the fire were destroyed and the remainder lost on average 79% of their canopy. Conclusions: This severe detrimental effect of a single fire, on trees estimated to be centuries old, is indicative of tree-loss occurring across remote arid Australia in habitats where fire is now fuelled predominantly by invasive grasses. Large volumes of novel grass fuels along creeklines in combination with extreme weather events were major factors driving the spread, extent and impacts of the wildfire we report on and are causing a shift from relatively uncommon and predictable, rainfall-dependent large wildfires to large, severe fires that can occur anytime. We predict further decline in the abundance of large trees from similar fires will occur widely throughout arid Australia over the next decade with substantial long-term impacts on multiple species. New strategies are urgently required to manage fire in invaded arid ecosystems to better protect large trees and the critical resources they provide.