Fire interval and fire seasonality effects on woody plant demography in Banksia woodlands

Russell G Miller

Fire regimes impose a strong evolutionary pressure on plant species and have shaped the structure, function, and composition of ecosystems over millennia. Fire frequency, interval, seasonality, and intensity are key factors defining a fire regime and can have varying impacts on species depending on their traits and vital demographic rates, such as survival, reproduction, and seedling recruitment. Abrupt, human-induced changes to fire regimes can erode resilience and lead to changes in species composition and population abundances. Identifying fire regimes that support species persistence is crucial for the conservation and management of fire-prone ecosystems and is a key goal of the research presented in this thesis. This thesis investigates the response of a range of woody plant species to varying fire intervals and seasonality in Banksia woodlands on the Swan Coastal Plain of southwestern Australia. This includes research that explores what contemporary population cohort structures and other demographic attributes reveal about the historical fire regime, quantifies reproductive dynamics across a range of fire intervals, identifies global mechanisms of fire seasonality impacts, and experimentally tests two of these fire seasonality mechanisms for Banksia woodland species. The species selected for study in this thesis represent dominant plant functional types in the region, capturing variation in post-fire regeneration strategy, growth form, and seed storage mode. Observed variation in demographic attributes across species with different growth forms, lifespans, and post-fire regeneration strategies was consistent with known ecological strategies to cope with high variation in fire intervals in a mixed-severity fire regime. All the study species showed strong post-fire resprouting or seedling recruitment, and short-lived obligate seeding shrubs and weakly serotinous trees also developed multi-cohort populations via inter-fire seedling recruitment – inferred as a bet-hedging strategy to cope with the occasional long absence of fire. All species matured rapidly after fire with minimum tolerable fire intervals of 2 – 9 years under average conditions based on the juvenile period approach, however, post-fire maturation was delayed under low rainfall or high herbivory conditions. Seven mechanisms of fire seasonality effects were identified from the global scientific literature (an eighth was identified by other authors). These were framed around key demographic processes of adult plant survival, propagule (seed) availability, and post-fire seedling establishment. Experimental results demonstrated the potential for unseasonal fire to inhibit seedling recruitment in Banksia woodland species via impacts on pre-germination seed persistence and post-fire seedling establishment. Collectively, this research provides important evidence of the tolerance of model plant species to varying fire regimes. The findings highlight the dominant role of fire regimes in shaping the ecology of Banksia woodlands. The insights gained through this research can help inform conservation and fire management for species persistence and ecosystem resilience.

Fire interval and fire seasonality effects on woody plant demography in Banksia woodlands

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