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Spatially variable recruitment response to fire severity in golden-top wattle (Acacia mariae), a thicket-forming shrub of semi-arid forests

Boyd R. Wright , Damien Andrew , Michael Hewins, Claire Hewitt, Roderick Fensham

Abstract

Context: Investigations into the life history strategies of organisms in ecosystems prone to fires are essential for effective fire impact management. In Australia, fire severity is expected to increase under anthropogenic climate change (ACC), so understanding plant responses to this fire regime element is essential for developing conservation-focused burning practices. Aims: Assess the recruitment response of Acacia mariae (golden-top wattle) to varying fire severities (high, low, and unburnt) in the semi-arid Pilliga forest in the Northern Inland region of New South Wales. Investigate seedbank dynamics and germination biology to inform post-fire recruitment patterning. Methods: Longitudinal seedbank studies were performed to understand seedbank dynamics and their influence on post-fire regeneration. A laboratory trial was conducted to assess the effects of heat shock and incubation temperature on seed germination. Field surveys were conducted at four sites to assess fire severity impacts and evaluate spatial variability in post-fire recruitment after the 2018 Gibbican Rd wildfire. Key results: Recruitment varied among sites but overall was highest at shrubs burned by high- severity (5.8 seedlings/shrub) compared to low-severity fire (0.8 seedlings/shrub) and unburnt shrubs (0.1 seedlings/shrub). Over five years, seedbank densities fluctuated markedly, peaking in 2021 following a major seeding event, but declining rapidly thereafter. Germination was optimised when seeds underwent heat shock at temperatures between 100 and 140°C and incubated at warm (. Conclusions: Acacia mariae germination is promoted by heat stimulation, explaining why high intensity burns with higher soil temperatures enhance recruitment. Differences in seedbank densities at the time of fire may account for varied recruitment across landscapes. Overall, A. mariae regenerates well after high-severity fires but poorly after low-severity fires, indicating it may be resilient to increased fire severity under ACC but struggle under current widespread low-severity prescribed management burning regimes.

BT24023  Accepted 16 December 2024

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