Growth response of an invasive alien species to climate variations on subantarctic Campbell Island

Invasive alien species (IAS) are a recognised threat to biodiversity and ecosystem services. With increasing tourism and projected 21st century climate changes across the mid- to high-latitudes of the southern hemisphere, subantarctic islands are potentially highly vulnerable to IAS, but suffer from a dearth of baseline monitoring. Here we report tree-ring measurements from a lone exotic Sitka spruce (Picea sitchensis (Bong.) Carr) on subantarctic Campbell Island to determine past growth rates and likely future response to climate changes. Though the samples were unable to resolve exactly when the tree was planted, the fast growth rate indicates it is likely to have been later than the reported date of 1901. Since at least 1941, the tree appears to have responded favourably to the relatively warm summers experienced on Campbell Island, resulting in growth more rapid than that observed in natural stands (North American Pacific Coast). Although trees of similar age are normally mature and produce cones, none have so far been observed on Campbell Island – possibly the result of the fast growth causing an extended ‘juvenile’ or pre-reproductive phase – preventing seeding across the island. Importantly, relatively dry periods are needed for cones to open and disperse seeds, conditions not recorded in the instrumental record. Examination of the Coupled Model Intercomparison Project 5 (CMIP5) outputs show increasing rainfall across the region during the 21st century under a range of emission scenarios, suggesting that even when mature, the Sitka spruce poses a limited threat to the long-term ecology of Campbell Island.