Vulnerability of post-fire Eucalyptus delegatensis regeneration across stand boundaries: the importance of sapling height, maturity and fuel characteristics

Abstract

Obligate seeding eucalypts are vulnerable to population collapse from frequent canopy scorching fire events. Following a recent series of high intensity fires in the Australian Alps, dramatic ecological shifts have been observed in areas previously dominated by obligate seeding eucalypts. These events have stimulated much discussion and research on the vulnerability of these species during the post-fire regenerative period, and the likely impact of changing fire regimes on their long-term abundance. This study builds on this research to examine the potential for post-fire vulnerability in regenerating obligate seeding E. delegatensis (alpine ash) to vary across boundaries with adjacent eucalypt stands. In a management context, prescribed burning has been proposed in eucalypt stands adjacent to post-fire E. delegatensis regeneration at 14-16 years following fire, though the effect that this may have on post-fire E. delegatensis regeneration given the diffuse nature of eucalypt stand boundaries remains unclear. Similarly, there exists the possibility for uncontrolled escaped prescribed burns to affect post-fire E. delegatensis regeneration within its stand boundaries, although the ecological risk that this may pose is also unclear. The aim was to examine the effect of stand boundaries on the vulnerability of post-fire E. delegatensis regeneration at 14-years time since fire in the Cotter River Catchment, ACT. Key variables relating to instantaneous vulnerability, namely post-fire E. delegatensis sapling height, proportions of reproductively mature regeneration, and fuel characteristics responsible for determining scorch height, were measured across E. delegatensis – E. fastigata (brown barrel) stand boundaries in areas affected by either canopy scorching higher severity fire, or non-canopy scorching lower severity fire. These variables were formulated into an integrative interactions-based model which was used to assess the current vulnerability of post-fire E. delegatensis regeneration to potential wildfire and prescribed burning across its stand boundaries. At 14-years time since fire, post-fire E. delegatensis regeneration had virtually disappeared from sites previously affected by lower severity fire, and fire severity was not found to affect fuel characteristics. At sites affected by higher severity fire, post-fire E. delegatensis saplings situated outside of pre-fire stand boundaries exhibited heightened vulnerability to a repeat disturbance in the form of potential prescribed burning. This was due to significantly decreased height growth and slower rates of reproductive maturation, despite fuel characteristics responsible for scorch height being generally consistent across stand boundaries. Under wildfire conditions, all post-fire E. delegatensis saplings were vulnerable to fire due to uniformly high predicted scorch heights, although increased sapling density and maturity within E. delegatensis stands may promote resilience in the form of regenerative capacity. Variation in post-fire E. delegatensis sapling characteristics at higher severity sites was strongly correlated to canopy cover attributed to resprouting eucalypts in adjacent stands, suggesting that interspecific competition may play a key role in determining patterns of vulnerability in the species. The results provide evidence to suggest that the position of E. delegatensis stand boundaries may be reinforced by the nature of interspecific competition across them, and that stand boundaries may be expected to sharpen under a regime of more frequent fire. Regarding management, the results suggest that proposed prescribed within adjacent eucalypt stands close to E. delegatensis stand boundaries may contribute to the decline of the species in these areas. Show more