Banksia Prionotes - Ecology and Physiology - Response To Fire

Response To Fire

Like many plants in south-west Western Australia, B. prionotes is adapted to an environment in which bushfire events are relatively frequent. Most Banksia species can be placed in one of two broad groups according to their response to fire: reseeders are killed by fire, but fire also triggers the release of their canopy seed bank, thus promoting recruitment of the next generation; resprouters survive fire, resprouting from a lignotuber or, more rarely, epicormic buds protected by thick bark. B. prionotes is unusual in that it does not fit neatly into either of these groups. It lacks a lignotuber or thick bark, and so cannot be considered a resprouter; yet it may survive or escape some fires because of its height, the sparseness of its foliage, and because it occurs in dune swales where fire are cooler and patchier. On the other hand, it is not a typical reseeder either, because of its relatively low fire mortality rates, and because it is only weakly serotinous: although fire promotes seed release, seed release still occurs in the absence of fire.

The actual degree of serotiny and fire mortality in B. prionotes varies with latitude, or, more likely, climate. Observations suggest that it is always killed by fire in the north of its range, which is relatively hot and dry, and where individual plants are usually smaller, but may survive fire in the cooler, moister, south. Moreover, it is essentially non-serotinous in the south, since all seed is released by the end of the second year, but seed retention increases steadily to the north, and at the northern end of its range, it typically takes around four years for a plant to release half of its seed in the absence of bushfire, with some seed retained for up to 12 years.

A number of other characteristics of B. prionotes can be understood as secondary responses to weak serotiny. For example, winter flowering ensures that seed is ripe by the beginning of the bushfire season; this is very important for weakly serotinous species, which rely heavily upon the current year's seed crop. Another example is the deciduous florets of B. prionotes. In strongly serotinous species, the old florets are retained on the cones, where they function as fire fuel, helping to ensure that follicles reach temperatures sufficient to trigger seed release. In B. prionotes, however, seed release is triggered at relatively low temperatures: in one study, 50% of follicles opened at 265 °C (510 °F), and 90% opened at 330 °C (625 °F); in contrast, the closely related but strongly serotinous B. hookeriana required 340 °C (645 °F) and 500 °C (930 °F) respectively. Floret retention would therefore be to no advantage, and might even prevent seed from escaping spontaneously opened follicles.

Seed release in B. prionotes is promoted by repeated wetting of the cones. The seed separator that holds the seeds in place is hygroscopic; its two wings pull together then wet, then spread and curl inwards as it dries out again. In doing so, it functions as a lever, gradually prying seeds out of a follicle over the course of a wet-dry cycle. This adaptation ensures that seed release following fire is delayed until the onset of rain, when germination and seedling survival rates are higher.

Because of its higher susceptibility and lower reliance on fire for reproduction, the optimal fire interval for B. prionotes is higher than for other Banksia species with which it occurs. One simulation suggested an interval of 18 years was optimal for B. prionotes, compared to 15 years for B. hookeriana and 11 years for B. attenuata. The same model suggested that B. prionotes is quite susceptible to reductions in fire intervals. On the other hand, it shows little susceptibility to increases in fire interval: although senescence and death are often observed in plants older than about 30 years, healthy stands have been observed that have escaped fire for 50 years. These stands have a multi-aged structure, demonstrating the occurrence of successful inter-fire recruitment.

Fire response may also furnish an explanation for the evolution of this species. The differences in fire regime between dune crests and swales would have created different evolutionary pressures, with plants on crests adapting to frequent hot fires by becoming strongly serotinous, and plants in swales adapting to patchier, cooler fires with weaker serotiny. Speciation would be made possible by the much reduced genetic exchange between crest plants and swale plants, although evidence suggests that there was some introgression at first. Eventually, however, the need for weakly serotinous plants to produce ripe seed by the bushfire season would have brought forward its flowering season until the flowering seasons no longer overlapped; thus a phenological barrier to exchange was erected, allowing the two populations to drift independently of each other.