OOS 17-4 - Spatial predictions of future regeneration failure in obligate seeding Eucalypt forests

Climate mediated changes in fire regimes may depend on what constrains fire in an ecosystem. Australia contains a wide range of environments and ecosystems that fall along a continuum of fuel-limited— having insufficient fuel biomass to burn most years, such as in arid grasslands—to climate-limited—where cool, moist climatic conditions mean that fuels are not available to burn in most years.
The tall forests in temperate south-eastern Australia fall closer to the climate-limited end of the continuum and are often dominated by obligate seeding eucalypts. These forests have historically very long inter-fire intervals (>100 years) but have recently been exposed to recurrent, high intensity wildfires between 2000 and 2020. Alpine ash (Eucalyptus delegatensis) and Mountain ash (Eucalyptus regnans- the worlds tallest angiosperm) are particularly vulnerable to reductions in fire intervals. Both species take approximately 20 years to reach reproductive maturity, and are at risk of local extinction if young, regenerating forest is burned again during this vulnerable phase (‘immaturity risk'). Contracting fire return intervals, therefore, are a major risk to this forest type.

Our research suggests that future fire intervals are likely to reduce in climate-limited systems as there is the potential for a greater availability of fuels to burn through increased aridity. However, climate change may also be influencing the demographic traits of these keystone species. Together, these processes describe the concept of ‘Interval squeeze’—the reduction in the fire interval window that is compatible with species persistence due to the compounding impacts of a changing fire regime alongside reductions in recruitment, growth, and survival.

In this study we investigate the spatial variation of regeneration failure driven by changes in the fire regime alongside potential shifts in key demographic traits such as time to reproductive maturity.
We find that that alpine and mountain ash are at risk of local population extinction under changing fire regimes. However, the level of risk is not uniform across the landscape and is associated with certain landscape characteristics. Although shifts in some demographic rates could increase risk, it is the changes in the fire interval that will have the greatest impact on landscape population persistence for both species.