Fire history influences reproductive traits in congeneric food tree species for a dietary specialist cockatoo

Felicity Eloise Charles, April E Reside, Patrick T Moss, Annabel L Smith

In fire-prone regions globally, plants have developed reproductive traits that enable population persistence under historical fire regimes. However, many common plant species are now threatened by a mismatch between historical and contemporary fire regimes, which also threatens animal species relying on them for food or habitat. We investigated how recent fire history (past 36 years) affected reproductive trait variation in two serotinous congeneric tree species (Casuarinaceae): Allocasuarina littoralis (an obligate seeder) and A. torulosa (a facultative resprouter). In southeast Queensland, Australia, these species are the primary food trees of the threatened Glossy black-cockatoo (Calyptorhynchus lathami, Cacatuidae). Thus, understanding responses to fire through different stages of food tree reproduction could assist conservation of this plant-animal interaction. In both tree species, germination rate (proportion of seeds germinated) increased with seed weight but was unaffected by laboratory treatments of heat and smoke. In both species, heavier seeds were associated with higher fire frequencies and greater temperature variability at seed collection sites. In A. torulosa, heavier seeds took longer to germinate, especially when seeds were collected from frequently burnt sites, possibly reflecting a trade-off between resprouting and seed production. Heat and smoke treatments slowed germination in A. torulosa but had no effect on germination in A. littoralis. Fire history did not influence reproductive output or population demographic structure for either species, but reproductive output (number of cones on female plants) was greater in sites with more woody vegetation cover. For restoration, heat and smoke treatments are unlikely to be necessary for successful germination of A. littoralis or A. torulosa once seeds are released from cones. However, local fire history and seed weight should be considered when collecting seeds for restoration programs, especially for A. torulosa. This study suggests that seed germination is influenced not only be laboratory fire treatments, but also by the recent fire history at the seed collection sites. Variation in germination response to fire of two closely related Allocasuarina tree species could influence a dietary specialist threatened cockatoo through differences in species post-fire establishment success and competitiveness.