Community-level changes in Banksia woodland following plant pathogen invasion in the Southwest Australian Floristic Region

Abstract

Question: Does the introduced pathogen Phytophthora cinnamomi change Banksia woodland α- or β-diversity and what are the implications for species re-colonization? Location: High rainfall zone of the Southwest Australian Floristic Region (SWAFR). Methods: We measured pathogen-induced floristic change along a disease chronosequence, and re-sampled historic quadrats in Banksia attenuata woodlands of the SWAFR. The chronosequence represents three disease stages: (1) healthy vegetation with no disease expression; (2) the active disease front; and (3) diseased vegetation infected for at least 15 years. Comparative data were obtained by resampling diseased plots that were historically disease-free when established in 1990. Results: β-diversity differed substantially for both chronosequence and historic data, while α-diversity was maintained, as measured by plot species richness and Simpson's reciprocal index. Species of known pathogen susceptibility were significantly reduced in cover-abundance, including the structurally dominant species; Banksia attenuata, B. ilicifolia and Allocasuarina fraseriana. Although these species remained present on diseased sites, there were overall reductions in canopy closure, leaf litter and basal area. These declines were coupled with an increase of species with unknown susceptibility, suggesting potential resistance and capacity to take advantage of altered site conditions. Conclusions: This study highlights the ability of an introduced plant pathogen to alter community floristics and associated stand variables. Species cover-abundances are unlikely to recover due to a reduced seed source, altered site conditions and pathogen persistence at the landscape level. However, maintenance of α-diversity suggests continued biological significance of Phytophthora-affected sites and the formation of novel ecosystems, themselves worthy of conservation.