Banksia Attenuata - Ecology

Ecology

Like many plants in south-west Western Australia, B. attenuata 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. Bearing epicormic buds and a lignotuber, B. attenuata is one of the latter group, with follicles that may open spontaneously or by fire. It is moderately serotinous, storing only one tenth the number of seeds in its seed bank as the reseeding B. hookeriana which it coexists with on sand dunes in scrub at Eneabba north of Perth. Even then, many of its follicles do not release seed after a fire, but instead after successive autumn rains. An experiment simulating wet weather following a fire saw a series of Banksia attenuata cones with follicles subjected to twice weekly immersions in water after being heated in a ring Bunsen flame to around 500–600 °C (930–1100 °F) for two minutes. Cones that had been exposed to water for more weeks had more seed released from follicles over time; around 40% released at three weeks, increasing steadily to almost 90% at ten weeks, compared with a series of controls (which were kept dry) of which fewer than 10% of seed released. Thus, the seed remains in the follicles until successive rains result in seed dispersal in the wetter winter (instead of dryer summer), increasing the chance of survival. After the follicle is split, the seed and separator are exposed to the elements. The wings of the woody separator are hygroscopic, and move together when wet, and spread and curl apart when dry. The seed is gradually drawn out by the movement with each wetting. Once released, seed germinates at temperatures between 15 and 20 °C (60–70 °F) to optimise timing with autumn and winter rains and hence maximise chance of survival. Still, many seedlings die off in the hot and dry summer months. Seedling survival for the species is lower than for banksias which regenerate by seeding over time. Despite this, the longevity of mature plants allows for maintenance of population until favourable years enable better survival of young plants. As they mature, plants are less likely to perish, and estimated to live for 300 years or more. Analyzing the seed bank and longitudinal results over fifteen years on the Eneabba sandplain showed that B. attenuata would become more abundant over time with fire intervals averaging between 6 and 20 years, peaking with intervals around 10 to 12 years, compared with longer intervals for the reseeders B. hookeriana and B. prionotes. Placed against its rivals, B. attenuata would be dominant between 8 and 10 or 11 years, but at longer intervals is outcompeted by B. hookeriana. Variability in the timing between fires allows all three species to coexist. Exaggerated good and bad weather conditions favours B. attenuata over the reseeding species, which suffer more.

Despite having relatively heavy seed, seed from Banksia attenuata has a high rate of long distance dispersal. A genetic study of populations in Eneabba showed that over 5% of plants had originated up to 2.6 km (1.6 mi) away (similar rates to Banksia hookeriana, the seed of which only weighs half as much). The mechanism for this is unclear, although Byron Lamont has proposed the Short-billed Black Cockatoo (Calyptorhynchus latirostris) as a vector; the species seeks out Banksia attenuata cones after bushfire, possibly because the large seeds and greater chance of grubs in the cone make them more nutritious. Flowering has been recorded one to two years after a bushfire.

Like many members of the family Proteaceae, Banksia attenuata is an obligate outcrossing species. Self-incompatible, inflorescences require pollinators to be fertilised and produce seed. A genetic study of seed collected near Jandakot published in 1980 showed obligate outcrossing. A field study in the Fitzgerald River National Park where inflorescences were enclosed in mesh fine enough to keep out vertebrates and invertebrates as small as the honey bee (Apis mellifera) showed that follicles still developed, indicating that small invertebrates were able to cross-pollinate the species. Banksia attenuata flowers are visited by the colletid bee Hylaeus globuliferus and bees of the genus Euhesma. Other pollinating invertebrates recorded include ants and dragonflies. An analysis of the invertebrate population found in the canopy of Banksia woodland found that mites and ticks (acari), beetles (coleoptera) and ants, bees and wasps (hymenoptera) predominated overall, with the three orders also the most common on B. attenuata along with thrips (thysanoptera). Lower overall numbers of invertebrates on Banksia species were thought to be related to the presence of insectivorous birds.

Many bird species were recorded by the national Banksia Atlas survey, including the New Holland Honeyeater (Phylidonyris novaehollandiae), Brown Honeyeater (Lichmera indistincta), Singing Honeyeater (Lichenostomus virescens), Western Spinebill (Acanthorhynchus superciliosus), Twenty-eight Parrot (Barnardius zonarius semitorquatus) and Red-tailed Black Cockatoo (Calyptorhynchus banksii). Black Cockatoos have been observed feeding upon the seed of B. attenuata, although it is not clear which species of black cockatoo was observed, the Short-billed or Long-billed Black Cockatoo (Calyptorhynchus baudinii). At a site near Jandakot, Short-billed Black Cockatoos were observed selecting immature infructescences which bore signs of infestation by the weevil Alphitopis nivea, the larvae of which tunnel in banksia spikes and eat the seed. They extract the larvae and drop the cones.

A 1978 field study conducted around Albany found the honey possum (Tarsipes rostratus) was a major pollinator of Banksia attenuata, both feeding directly on the pollen and drinking the nectar. The flower structure is suited to passing pollen onto the possum as it feeds, unlike honeyeaters whose bills are too long for this to occur readily. Coupled with the flower spike's musky odour, these findings suggest Banksia attenuata is highly adapted to be pollinated primarily by this mammal species. Furthermore, Petroc Sumner and colleagues have investigated the cone photoreceptor cells of honey possums and compared them with the colour changes of B. attenuata. They found that the possum is trichromatic (like humans and possibly many marsupials) and propose that its L (long wavelength) cones help it discern B. attenuata flower spikes, and the M (or medium wavelength) cones could help it distinguish ripe inflorescences with nectar from recently finished spikes (a difficult task for human vision). There is some evidence for other mammals as pollinators; B. attenuata-like pollen was recovered from museum skins of dunnarts (Sminthopsis spp.) and pygmy possums (Cercartetus spp.), and the sugar glider (Petaurus breviceps) visited flower spikes in captivity.

Seventeen species from several orders of slime molds (myxomycetes) have been isolated from the bark of Banksia attenuata. Over half (nine) were from the order Stemonitales, and Echinosteliales and Liceales were also common. The abundance of the first two orders may be due to the acidity of the bark. Another order, the Physarales, was unusually rare—other studies have demonstrated that the order is typically abundant on the bark of various species of tree around the world.

All banksias have developed proteoid or cluster roots in response to the nutrient-poor conditions of Australian soils (particularly lacking in phosphorus). These have been measured in Eneabba extending to a depth of 15 cm (6 in) below the soil surface. The plant develops masses of fine lateral roots which form a mat-like structure underneath the soil surface, and enable it to extract nutrients as efficiently possible out of the soil. A study of three co-occurring species in Banksia woodland in southwestern Australia—Banksia menziesii, B. attenuata and B. ilicifolia—found that all three develop fresh roots in September after winter rainfall, and that the bacteria populations associated with the root systems of B. menziesii differ from the other two, and that they also change depending on the age of the roots. Another study on root architecture of Banksia hookeriana, B. menziesii and B. attenuata found the overall structure of all three to be similar, with proteoid mats more active and growing in wetter months (winter-spring). Plants send out several sinker roots which descend to reach the water table, and the original tap root may or may not have died off. Along with B. menziesii, B. attenuata is a facultative phreatophyte. The two species are less strictly tied to the water table and hence able to grow in a wider variety of places within Banksia woodland habitat around Perth than the co-occurring B. ilicifolia and B. littoralis. A study at a rehabilitation site on a sand mine north of Perth found that the broadleaved species B. attenuata and B. hookeriana were harder to establish than the fine-leaved B. leptophylla there, due to increased impedance of the disturbed soil. Analysis of native plant species at a remnant of banksia woodland in suburban Perth which had been invaded by two herbaceous weed species (Ehrharta calycina and Pelargonium capitatum) found increased phosphorus levels in native foliage. Although B. attenuata leaves did not have increased phosphorus, they did have reduced levels of manganese—an element which is absorbed into the plant by its proteoid roots, the formation of which can be inhibited by raised levels of phosphorus.

In a 1985 study inoculating cultivated plants, Banksia attenuata showed moderate to high susceptibility to Phytophthora cinnamomi dieback; and at least some field and cultivation evidence points to it being highly susceptible. P. cinnamomi spreads from plant to plant via lateral roots, advancing at a rate of around a metre a year. The symptoms of infection in Banksia attenuata include yellowing of the leaves in the tree crown, and lesions at the base of the trunk. The red healthy roots become discoloured brown. A study of Banksia attenuata woodland 400 km (250 mi) southeast of Perth across 16 years and following a wave of P. cinnamomi infestation showed that B. attenuata populations still existed but were significantly reduced in diseased areas. Injecting a solution of phosphite into the trunks of affected B. attenuata trees at a disease front in Banksia woodland can delay morbidity from dieback for five years. Injecting and spraying phosphite also reduces the rate of spread of a dieback front for around five years. A bushfire did not influence this slowing. A 2003 study found that drenching the soil with 0.50 mM benzoic acid significantly reduced the size of P. cinnamomi lesions. Research into dieback in Western Australia has identified a new species, P. multivora, isolated from ailing eucalypts and B. attenuata in 2009.