Limits to post-fire vegetation recovery under climate change.

Record-breaking fire seasons in many regions across the globe raise important questions about plant community responses to shifting fire regimes (i.e., changing fire frequency, severity and seasonality). Here, we examine the impacts of climate-driven shifts in fire regimes on vegetation communities, and likely responses to fire coinciding with severe drought, heatwaves and/or insect outbreaks. We present scenario-based conceptual models on how overlapping disturbance events and shifting fire regimes interact differently to limit post-fire resprouting and recruitment capacity. We demonstrate that, although many communities will remain resilient to changing fire regimes in the short-term, longer-term changes to vegetation structure, demography and species composition are likely, with a range of subsequent effects on ecosystem function. Resprouting species are likely to be most resilient to changing fire regimes. However, even these species are susceptible if exposed to repeated short-interval fire in combination with other stressors. Post-fire recruitment is highly vulnerable to increased fire frequency, particularly as climatic limitations on propagule availability intensify. Prediction of community responses to fire under climate change will be greatly improved by addressing knowledge gaps on how overlapping disturbances and climate change-induced shifts in fire regime affect post-fire resprouting, recruitment, growth rates, and species-level adaptation capacity.

Short-interval wildfires increase likelihood of resprouting failure in fire-tolerant trees.

Epicormic and basal resprouting promote tree survival and persistence in fire-prone regions worldwide. However, little is known about limits to resprouting effectiveness when severe wildfires increase in frequency. In the extensive fire-tolerant mixed-eucalypt forests of temperate Australia, we examined the effects of one and two high-severity wildfires within six years on relationships between tree size (stem diameter) and resprouting (epicormic and/or basal), and on seedling regeneration. The diameter of eucalypts likely to be topkilled (no epicormic recovery) by high-severity fire increased from ∼15 cm after the first wildfire to ∼22 cm after the second. Middle-sized stems (22-36 cm diameter) were likely to resprout both epicormically and basally after one wildfire, but short-interval wildfires eroded this dual capacity, thereby reducing the probability of survival. Seedling abundance also decreased after two successive fires. Our study indicates that short-interval wildfires increased tree 'escape size', and eroded resprouting success particularly of middle-sized trees, which were too large for basal resprouting but too small for epicormic recovery. This, in combination with reduced seedling recruitment, portends structural and demographic challenges for even the most fire-tolerant forests under emerging fire regimes.