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In arid Australia, fires in different seasons often result in markedly different recruitment rates of woody plants, and this can have important and long-lasting impacts on post-fire community structure and composition. The current study investigated ecological mechanisms behind this effect in four widespread arid Acacia species: Acacia aptaneura, Maslin & J.E. Reid Acacia kempeana F.Muell., Acacia maitlandii F.Muell. and Acacia melleodora Pedley. Specifically, the study examined whether increased recruitment after summer compared with winter burning relates to (1) higher soil temperatures during summer fires having increased capacity to overcome dormancy of seeds with heat stimulated germination; (2) warmer soil temperature conditions after summer fires providing more favourable incubation temperatures for germination; and/or (3) summer fires coinciding with seasonal fluxes in seedbank densities, which may occur in response to granivory and/or rapid rates of seed deterioration after dispersal. Results supported hypotheses (1) and (2), as germination of all species responded positively to heat shock and warmer incubation temperatures. Some support was also demonstrated for hypothesis (3), as seedbank densities were shown to fluctuate seasonally, most likely in response to strong granivory. This multi-species study provides insights into mechanisms behind enhanced shrub recruitment after arid zone summer fires; and reinforces the notion that season of burn can be a strong determinant of regeneration trajectories of woody plants after fire. [ABSTRACT FROM AUTHOR]

PREMISE OF THE STUDY: Fire typically triggers extensive regeneration of plants with heat-stimulated germination by causing short periods of intense soil heating. If plants with heat-stimulated germination are also subject to seed predation and display mast-seeding cycles, postfire recruitment may be contingent on the seedfall density of prefire masts, and on whether granivores are satiated at the time of fire. METHODS: We conducted a longitudinal seedbank study and a mensurative field experiment in central Australia to examine whether fire and the variation in seedfall density across sites in a mast year interact to influence recruitment of slender mulga (Acacia aptaneura), an iteroparous masting shrub with heat-stimulated germination. KEY RESULT: The seedbank study showed seedbank pulsing after masting, with mean seed counts in the upper 4-cm soil layer being 132.8 seeds/m2 12-mo after a dense seedfall, but only 3.8 seeds/m2 following a year with no seed production. Consistent with this, recruitment increased postfire at sites where denser seedfall had occurred during the preburn mast year. Conversely, little recruitment occurred at unburnt populations, irrespective of prefire seedfall density. CONCLUSIONS: We attribute our findings to: (1) elevated soil temperatures during fires stimulating germination of heat-cued seeds; and (2) granivore satiation following masting facilitating assimilation of seeds into the soil seedbank. These results highlight the importance of rare seed-input events for regeneration in fire-prone systems dominated by masting plants, and provide the first example from an arid biome of fire interacting with masting to influence recruitment. [ABSTRACT FROM AUTHOR]

Fire intensity is recognised as an important driver of regeneration dynamics in forest and shrubland vegetation types, but its effects on recruitment processes in arid grasslands are poorly understood. Here, we used fire severity as a surrogate measure of fire intensity, and compared post-fire seedling recruitment from arid Triodia pungens (soft spinifex) plots burnt by low-severity experimental fires against those burnt by a high-severity wildfire. To explain T. pungens recruitment patterns, we also: (1) conducted a longitudinal depth distribution study of soil seedbank densities, and (2) carried out a lethal-temperature experiment on T. pungens seeds. High-severity burning was associated with reduced T. pungens recruitment compared with low-severity fire, and this relationship was attributed to the low lethal temperature thresholds of seeds and the adverse impacts of elevated soil temperatures during high-severity fire on the shallow soil seedbank. Among other species in the community, there were varied recruitment responses to fire severity. Overall, our results indicate that a strong link exists between fire severity and recruitment in arid spinifex grassland, though the association is variable among species, and dependent on the seedbank attributes, germination biologies and lethal-temperature thresholds of seeds of individual species. [ABSTRACT FROM AUTHOR]