Does the rare Banksia goodii have inferior vegetative, reproductive or ecological attributes compared with its widespread co‐occurring relative B. gardneri?

Abstract

Banksia goodii (rare) and B. gardneri var. gardneri (widespread) are closely related rhizomatous evergreen subshrubs of south western Australian scrub‐heath and woodland. B. goodii is restricted to sixteen populations, most of which are remnants on roadsides due to landscape fragmentation from agricultural activities. Bioclimatic profiles indicate that B. gardneri spans a wider range of climates than B. goodii. However, their post‐fire survival and resprouting vigour, rates of seedling growth and soil penetration and susceptibility to seedling predators are similar. Both have the potential for geographic range extension, especially B. goodi. Seedlings of both species survived for at least 3 years at sites with 2 (but not 3) months shorter and 1 month longer growing seasons than experienced by natural B. goodii populations. Seedling establishment did not occur in the absence of fire and was low (1.3% of released intact seeds for B. goodii) even following en masse release of seeds after fire. Sowing of intact seeds, especially in cleared areas and with protection from herbivores, increased this to 35%. Fire survival among adults of both species was 100% following two fires. B. goodii, which roots along its rhizomes, produces ramets, while B. gardneri, with a lignotuber instead, does not. Seed production, pre‐dispersal granivory and canopy seed storage are low but similar for both species, although B. gardneri has a superior level of germination. The biology of B. gardneri thus provides few insights as to why B. goodii is rare. There were no differences in topo‐edaphic attributes, community structure, shading or litter cover between localized sites where either species occurred alone or together. Differences in present or prehistoric fire regimes, clearing (habitat fragmentation), susceptibility to generalist fungal pathogens, or impacts of alien invasives, cannot explain the distribution of B. goodii relative to B. gardneri. The rarity of B. goodii may be a result of its recent origin, habitat specialization (dense woodland), and the impacts of habitat fragmentation within its current range, rather than inferior vegetative, reproductive and/or ecological attributes.