S. J. Wooller

Regeneration by three species of Banksia on the south coast of Western Australia in relation to fire interval

The regeneration strategies of three Banksia species in relation to fire were studied over 20 years in a mediterranean heathland-shrubland on the south coast of Western Australia. Banksia baueri and B. nutans are both bushes 1-2 m high, while B. baxteri is a shrub 4 m high. All three species regenerated only from seed released from the canopy seed bank after fire. They did not start to flower until 6 years after fire and seed set took even longer. Differences between the species in age-related intensity of flowering were related to the rate at which each species accumulated seed in the canopy. Even plants over 40 years old were still increasing their overall canopy seed bank or replacing seeds that had been released or were no longer viable. The vegetation studied appeared to be little affected by humans historically and to have burnt only at intervals of 30-60 years or more. Consequently, although all three species needed fire to regenerate, management of fire regimes needs to allow adequate intervals between fires for the replacement of their canopy seed banks. Indeed, all three Banksia species studied were extinguished from one area burnt twice at an interval of 9 years. Models developed with Banksia species from the northern sand plains of Western Australia, where fires appear more frequent, may need modification to be applicable to all south-coastal species.

Seed set in two sympatric banksias, Banksia attenuata and B. baxteri

Two species of banksia, studied on the south coast of Western Australia, flowered simultaneously and had floral characteristics often associated with vertebrate pollination. The pollen of both was carried by honeyeater birds and small mammals (honey possums). Despite this, differential exclusion of vertebrates and invertebrates from inflorescences indicated that Banksia attenuata set substantial quantities of seed when visited only by invertebrates, although seed set increased with the addition of vertebrate visitors. Banksia baxteri set much seed in the absence of any animal visitors and seed set increased with invertebrate, but not vertebrate, visitation. We suggest that these differences in pollination strategies reflect regeneration by B. baxteri solely from canopy seeds released after fire, whereas B. attenuata also regenerates from seeds released between fires, as well as from lignotubers and epicormic buds.

Seed viability in relation to pollinator availability in Banksia baxteri

Seed set was examined in relation to access to inflorescences by large or small invertebrates, vertebrates, all animal visitors or none. Seed predation by insects was substantial (35-46%) and unrelated to pollinator access. The fewer the categories of animals allowed access to flowers, the greater the proportion of unpredated seeds that were aborted. Germination rates (83-96%) of filled seeds were unrelated to pollinator access, as were survival and size of seedlings at 4 months old. Overall, only 16% of seeds initiated by selfing after exclusion of all pollen vectors resulted in 4-month-old seedlings, compared with 33% of seeds initiated after access by all or larger animal visitors. Pollen and resource limitation are likely to account for only some of these findings. Rather, we suggest that preferential development of progeny fertilised by outcross pollen from other, especially distant, individuals may also play a role in reducing seed formation.

The role of non-flying animals in the pollination of Banksia nutans

Banksia nutans is a common, 1 m high, bushy shrub that flowers over summer in the sandplain heathlands on the southern coast of Western Australia. It appears to be principally pollinated by the honey possum, Tarsipes rostratus. This tiny (7–10 g) marsupial is the most abundant and widespread mammal in one large heathland where 97% of honey possums trapped near B. nutans were found to carry its pollen. An experiment was conducted that regulated access to flowers by different groups of pollinators. Exclosures around bushes removed access to flowers by flying animals, but still allowed honey possums to visit the flowers. This treatment resulted in fruit set that was not significantly different from bushes to which all animals had access. Exclusion of animal visitors resulted in significantly lower (albeit substantial) fruit set. This indicates a capacity for self-pollination that may offset the apparent reliance of Banksia nutans on honey possums for pollination.

Mixed mating in Banksia media

Differential exclusion of vertebrates and invertebrates from the inflorescences of Banksia media R.Br. on the south coast of Western Australia showed the species to be partially self-compatible. Access by invertebrates increased fruit set and additional access by vertebrates resulted in even greater fruit set. Honeyeater birds and marsupial nectarivores were abundant and widespread in the study area and most carried the pollen of Banksia media while it flowered. However, although B. media had the floral characteristics attributed to vertebrate pollination, self-pollination and pollination by insects clearly also play major roles in seed production. Banksia media regenerates solely from seed released after fire and we suggest that its mixed mating strategy is a compromise between inbreeding depression and risk of elimination from areas prone to occasional fires.