Warmer and less variable temperatures favour an accelerated plant phenology of two invasive weeds across sub-Antarctic Macquarie Island

The great plasticity and diverse reproductive strategies of invasive alien plants are widely assumed to contribute to invasion success, even in extreme areas, often displacing native species. In this context, climate change creates new opportunities for biological invasions. Environmental variability and global warming are two of the climatic processes that may promote invasiveness, since alien species modulate their phenology to succeed under these circumstances. We monitored the phenological development (phenological stage advancement) of the two main invasive alien species: Poa annua L. and Cerastium fontanum Baumg. in the sub-Antarctic Macquarie Island during the austral summer period along an altitudinal gradient. We found that higher temperatures lead to increased plant height and accelerated phenological development than lower temperatures in P. annua but found no direct evidence of the latter in C. fontanum. However, increased temperature variability negatively affected the phenological development of both species. Interestingly, despite their different reproductive strategy (rapid and impromptu in P. annua, and more synchronic and gradual in C. fontanum), both species prolifically succeeded in producing seeds at all sites showing the great acclimation of these two alien species even in limiting conditions. Since both alien species in Macquarie Island showed larger size and faster phenology at lower altitudes (i.e. milder conditions), this would indicate a great influence of ameliorating abiotic extremes on alien plant invasive capabilities at environmental extremes. Thus, our results warn of the increasing capabilities under climatic warming scenarios for alien plants to reproduce even at such remote ranges. This highlights the need to reinforce calls for special attention to prevent the spread of these kinds of species to other similar sub-polar areas, where intensive post-introduction management may be difficult or expensive.Field research of LRP was supported by AAS 4158 project of the Australian Antarctic Science Program.LRP was also recipient of a SCAR fellowship. MMS was supported by a FPI PhD Grant (BES-2013-062910) that was funded by the Spanish Ministry of Economy and Competitiveness