Recruitment failure of keystone trees in Phytophthora infested forest

Abstract

AbstractInvasive plant pathogens cause considerable forest loss worldwide. However, little is known about how tree populations recover post‐invasion. In Western Australia, Phytophthora cinnamomi ‘dieback’ affects jarrah (Eucalyptus marginata), with historical records of mass deaths in this keystone tree species due to infestation, resulting in permanent loss of standing forest. Historical observations of marri (Corymbia calophylla), a co‐dominant keystone jarrah forest tree, suggest it is resistant to P. cinnamomi. Impacts of P. cinnamomi on the population dynamics of jarrah and marri are unknown. We recorded densities and stem diameters of jarrah and marri in a peri‐urban forest to examine their population demographics in >60‐year‐old P. cinnamomi infestations compared with adjacent non‐infested forest. While seedling (<0.5 cm stem diameter, ~ 2 –15 cm tall) densities were not significantly different, total tree densities were 43% (jarrah) and 48% (marri) lower for infested sites. Densities of jarrah 0.5–9 and 20–29 cm stem diameters were lower within infestations. Survival probabilities of jarrah seedlings were significantly greater in P. cinnamomi infestations, but those of mature jarrah were not different. While average marri stem density was 49% greater within infestations, densities of small marri (<20 cm stem diameter) were lower within infestations. Survival probabilities for marri seedlings were significantly decreased in infestations, while there was greater survival probability for mature marri within P. cinnamomi infestations. Our data suggest recruitment of jarrah and marri has likely reduced since P. cinnamomi invasion. This is the first documented impact on marri, which indicates that we have been overlooking the risk of decline due to P. cinnamomi for this species. We stress the need for continued management and highlight the necessity of planting seedlings ~6 months old, and assisting their survival with phosphite and fertilizers, to bolster local tree populations.