Impact of Phytophthora dieback on a key heathland species Xanthorrhoea australis (Asphodelaceae) (austral grasstree) and floristic composition in the eastern Otways, Victoria

Context The plant pathogen Phytophthora cinnamomi causes severe declines in susceptible vegetation, including loss of plant species, vegetation structure and fauna abundance. Grasstrees (Xanthorrhoea spp.) are keystone species that provide optimal habitat for vertebrates and invertebrates and are highly susceptible to the pathogen. Although effects in the Otway Ranges have been assessed at specific sites, there is less knowledge across the landscape on the extent of loss of Xanthorrhoea australis (austral grasstree). Aims The aims were thus to assess impacts at three Heathy Woodland sites and to determine the magnitude of loss of X. australis and susceptible species losses. Methods Floristic composition, species cover or abundance, and basal area of X. australis were recorded in quadrats within treatments (uninfested, infested, post-infested vegetation). Analyses included floristics (PRIMER v7), significant effects (ANOSIM), species contribution to similarity/dissimilarity (SIMPER). Species richness and susceptible species cover were analysed using two-way crossed ANOVAs to detect the influence of site, treatment, and interactions. Key results Species composition of uninfested vegetation was significantly different to infested and post-infested vegetation, with susceptible species more abundant in uninfested areas. Post-infested vegetation had the lowest percentage cover of susceptible species. The mean percentage cover of X. australis in uninfested vegetation (43%) was 10-fold greater than in infested areas (4.3%) and extremely low in post-infested vegetation (0.9%). Conclusions Susceptible species were subject to density declines and extirpation, and the loss of X. australis resulted in major structural vegetation changes. Implications These results have severe implications for heathy woodland communities and reliant fauna. Limiting the spread of P. cinnamomi and protecting grasstrees is critical for their security.