Ecology of Banksia - Response To Fire

Response To Fire

Banksia plants are naturally adapted to the presence of regular bushfires. About half of Banksia species typically survive bushfires, either because they have very thick bark that protects the trunk from fire, or because they have lignotubers from which they can resprout after fire. In addition, fire triggers the release of seed stored in the aerial seed bank — an adaptation known as serotiny. In ecological literature, the species that are killed by fire but regenerate from seed are referred to as "fire-sensitive" or "seeders", while those that typically survive by resprouting from a trunk or underground lignotuber are called "fire-tolerant" or "sprouters".

All Banksia exhibit serotiny to some extent. Most retain all of their seed until release is triggered by fire, but a few species release a small amount of seed spontaneously. Serotiny is achieved through the mechanism of thick, woody follicles, which are held tightly closed by resin. Seeds retained in follicles are protected from granivores and the heat of bushfire, and remain viable for around ten years. Follicles require a critical heat in order to melt the resin, so that the follicles may begin opening; for B. elegans, for example, this is 2 minutes at 500°C. Those species with high heat requirements typically retain their old withered florets. These are highly combustible and thus help ensure the critical heat is reached.

With some exceptions, each follicle contains two seeds plus a winged "separator". While the separator remains in the follicle, it holds the seeds in position. In some species, the separator remains in the follicle until it has cooled; once cooled, the separator loosens and falls out, and the seeds follow. In this way the separator ensures that the seeds fall onto cool ground. In other species, the separator does not loosen until it has been wet. In these species, the seeds do not fall to the ground until the first rains after the bushfire. Seed is typically released over a period of about 90 days.

Immediately after bushfire, granivorous birds move in to extract seed from open follicles, and to eat seeds that have fallen to the ground. Those seeds that escape the granivores are soon buried by wind and surface water. Nearly all buried seeds germinate.