Fitness benefits of serotiny in fire- and drought-prone environments

Abstract

Despite the abundance of serotinous species in southwestern Australia, demonstration of the enhancement of fitness through fire, by releasing seeds onto an optimal postfire seedbed for seedling establishment, rather than following drought death, has not been explicitly explored. In this study, we measured seed release, germination and seedling recruitment following plant death after fire and simulated drought among six serotinous species in southwestern Australia. Burning or heating the cones/fruits resulted in a marked increase in follicle opening (pyriscence) compared with plant death from drought (necriscence) among Banksia but not Hakea. We recorded higher rates of germination, lower seedling mortality and better seedling growth in the post-fire environments than in unburnt sites not burnt for at least 10 years for five species (seeds of the sixth species were non-viable). Seedling mortality among the five species of 100 % occurred in the unburnt plots over the first summer, while 28–79 % of seedlings survived in the burnt plots. Soil moisture measurements indicated much lower water availability in the unburnt sites even in spring. Our study provides strong empirical evidence that serotiny/pyriscence enhances species fitness by releasing seeds into an optimal post-fire habitat in contrast to that observed from plant/crown death due to drought. In the context of the long history of fire in southwestern Australia, the greatly enhanced fitness in response to fire rather than severe drought supports the proposition that serotiny is a specific adaptation to fire.