How do intervals between fires influence canopy seed production and viability?

Abstract

Abstract Canopy storage of seed (serotiny) is an important persistence strategy in fire‐prone environments. Serotinous populations can be threatened when fire intervals are shorter than the time needed to accumulate seed, or when longer than plant lifespans. Understanding how fire regimes influence canopy seedbanks requires study of the timing of plant maturity, seed output and germination traits. Research that spans plant functional types, such as resprouters or obligate seeders, helps to understand the mechanisms through which fire has influence. Using field data collected at 57 sites in a Mediterranean shrubland of southeastern Australia, we modelled how time since fire and mean fire interval influenced infructescence (hereby cone) production. Additionally, seed output, viability and germination speed were assessed in the laboratory. Reproductively mature resprouters were first observed 2 to 4 years post‐fire in the field, and reproductively mature obligate seeders 6 years post‐fire, depending on the species. Reproductively mature plants were not observed at short mean fire intervals: until 3 and 9 years for resprouters and nine and 18 years for obligate seeders, again, varying by species. When resprouter species reached maturity, we observed a decline in cone production as time since fire increased. This decline was not observed for obligate seeder species. Fire influenced the number of viable seeds produced within cones for one resprouter species, Allocasuarina paludosa, but we did not detect a relationship for other species. Germination speed was faster for species that produce few seeds per cone, and slower for species that produce many seeds, indicating a trade‐off between fire‐related seed traits. For example, Hakea rostrata, which holds two seeds per cone, germinated mostly within the first few weeks, while Callitris rhomboidea, which holds many seeds per cone, germinated asynchronously over 9 months. Short intervals between fires reduce canopy seed production for the serotinous species studied. Obligate seeders were more sensitive to frequent fire than resprouters. In the context of expected increases in wildfire frequency and droughts in Mediterranean ecosystems, our findings suggest that serotinous species' reproduction and recruitment will be differentially impacted depending on a suite of functional traits. Read the free Plain Language Summary for this article on the Journal blog.