Using Patterns of Post-Fire Plant Reproduction to Inform Minimum Fire Intervals for Conservation Management in a Fire-Prone Woodland

Abstract

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. Fire intervals that support adequate regeneration 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 reproduction for woody fire-killed (obligate seeding) and resprouting species under varying rainfall and herbivory 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 had similar juvenile periods to fire-killed species (1–3.5 years for resprouting shrubs, 4.4 years for resprouting trees). Reproduction varied with rainfall and herbivory with juvenile periods at least doubling under low rainfall or high herbivory for some species. Serotinous species produced cones (woody fruits containing seeds) shortly after flowering commenced, with some evidence of seed bank decline in the oldest sites. While reproduction was clearly correlated with time since fire, plant size was a much stronger predictor. Some species form multi-cohort populations which can introduce large variation into post-fire reproductive trajectories, and this should be considered when making decisions about fire intervals that may impact species persistence. This study provides critical information to assess fire interval-related threats for Banksia woodlands and suggests that woody species of these woodlands 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.