Abstract
Members of the widespread arid Australian mulga (Acacia aneura) complex are fire-sensitive shrubs or small trees that can resprout epicormically following low-severity burning, but are readily killed by high-severity fire. The seeds of many species of mulga are stimulated to germinate by heat during burning, although post-fire regeneration rates are unpredictable. Here, we investigated whether variability in post-fire mulga recruitment relates to the relationship between fire severity and soil heating during fire, which may kill, leave unaffected, or stimulate the germination of buried seeds. This hypothesis was examined in central Australia on slender mulga (A. aptaneura), by experimentally investigating (a) seedling recruitment rates under different fire severity classes, (b) the germination and lethal temperature thresholds of seeds, (c) soil temperatures during fires of different severity classes and (d) the emergence depths of seedlings beneath high- and low-severity burnt plants. We found that post-fire recruitment was significantly lower beneath low-severity burnt and unburnt plants than high-severity burnt plants. This result was explained by the finding that maximum germinability of mulga seeds occurs after heating to between 80 and 100 °C, and that these temperatures are not achieved in unburnt patches or low-severity burns at depths where the majority of the seed bank is known to occur. Despite the increased regeneration observed after high-severity fire, post-fire recruitment was highly variable between sites, independent of fire severity. This indicates that while heat-stimulated germination may confer on mulga a risk-spreading strategy to a range of fire severities, post-burn recruitment may not always offset high adult death rates following high-severity fire.
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Acknowledgments
Grant Allan from the Northern Territory Parks and Wildlife Commission is thanked for providing dates of occurrence of all fires examined in this study. The authors also thank landholders from the Haasts Bluff Aboriginal Land Trust and from Aileron, Amburla, Bond Springs, Hamilton Downs, Narwietooma and Yamba stations for allowing access for fieldwork. Danae Moore and Joe Schofield are also thanked for enabling fieldwork to take place at New Haven Wildlife Sanctuary. Jeremy Williams facilitated access to the Mt Everard sites. Tony Auld, Ken Hodgkinson and two anonymous reviewers made helpful comments on earlier drafts of the manuscript.
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Communicated by Prof. Michael Lawes, Prof. Ross Bradstock and Prof. David Keith.
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Wright, B.R., Latz, P.K. & Zuur, A.F. Fire severity mediates seedling recruitment patterns in slender mulga (Acacia aptaneura), a fire-sensitive Australian desert shrub with heat-stimulated germination. Plant Ecol 217, 789–800 (2016). https://doi.org/10.1007/s11258-015-0550-0
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DOI: https://doi.org/10.1007/s11258-015-0550-0