Post-fire plant reproductive trajectories under varying rainfall and herbivory in Banksia woodlands

The time interval between fires is a critical component of the fire regime that affects plant species persistence in fire-prone ecosystems. Fire intervals that are too short or too long may not support regeneration from seed banks or resprouting. The range of fire intervals that support adequate seed production and accumulation may also vary with other factors such as climate, herbivory, and population structure.

Using field data on flowering and canopy seed banks, we modelled post-fire reproductive trajectories for woody fire-killed (obligate seeding) and resprouting species under varying rainfall and herbivory conditions along a 35-year fire age chronosequence in Banksia woodlands in southwestern Australia. We found that fire-killed species attained reproductive maturity rapidly after fire with predicted juvenile periods (time to 50% flowering) of 1.5 – 2.3 years for shrubs and 4 years for trees. Resprouting species returned to reproductive maturity with similar juvenile period timings to fire-killed species (1 – 3.5 years for resprouting shrubs, 4.4 years for resprouting trees). Reproduction varied with rainfall and herbivory where juvenile periods at least doubled under low rainfall or high herbivory conditions for some species. Serotinous species produced cones (woody fruits containing seeds) shortly after flowering commenced, with only weak evidence of canopy seed bank decline in the oldest sites. We also tested the mechanistic relationship between reproduction and plant size to assess how well time since the last fire reflects reproductive status. While patterns of reproduction were clearly correlated with time since fire, plant size was a much stronger predictor of reproduction. Some species in the region have the tendency to form multi-cohort populations which can introduce large variation into post-fire reproductive trajectories.

This study provides critical information to assess fire interval related threats for Banksia woodlands and suggests that woody species are generally tolerant of a wide range of fire intervals. Only the slowest-maturing, fire-killed species (Banksia prionotes, Proteaceae) may require fire intervals > 10 years to reduce immaturity risk under the least favourable growing conditions, and this species often occurs in discrete patches in the landscape such that fire management can be tailored accordingly.