Spatial and temporal variation in damage and dieback in a threatened subantarctic cushion species

A decline was observed in the subantarctic Macquarie Island endemic cushion, Azorella macquariensis, during the summer of 2008–2009, resulting in the listing of the species as critically endangered in 2010. Photographs of A. macquariensis in the period 2009–2013 were used to (1) identify types of damage, (2) determine the likely causes of three distinct types of damage, (3) establish whether dieback was spreading from affected to unaffected sites and (4) find out whether dieback was associated with the expansion of Agrostis magellanica. Grey damage occurred on the most wind-exposed parts of cushions and on the most wind-exposed sites. Speck damage occurred in the opposite situations and was consistent in its location, attributes and timing with rabbit grazing. Yellow dieback was sporadic in its occurrence. Its symptoms were consistent with those of a pathogen. Yellow damage expanded between spring 2009 and autumn 2010, with neither grey nor speck damage increasing. Yellow damage was associated with a marked decline in live cushion cover in plots between 2010 and 2013. The cushion was not eliminated from any plots, despite increased cover of A. magellanica in plots with dead cushions. Only one site not affected by yellow damage in 2010 had become affected by 2013. Given these results, and given that yellow damage has been observed in the past, 2008–2010 may have been an infrequent extreme outbreak of a pathogen and/or a response of a pathogen to ongoing climatic change.