Phenology of indigenous and alien vascular flowering plants on sub-Antarctic Marion Island

Abstract

Species’ seasonal behaviour is of paramount importance in understanding community functioning and dynamics. Recently, plant phenology has further gained significance as a reliable indicator of climate change impacts. Despite the importance of understanding plant dynamics, there are relatively few plant phenological records for the sub-Antarctic region, and where records exist they are often not extensive. Sub-Antarctic Marion Island, typical of Southern Ocean Islands, offers a useful setting for addressing these knowledge gaps. This study documented the vegetative and reproductive phenologies (or aggregate phenological patterns) of twelve indigenous and three alien vascular plant species on the island. The phenological differences among the species and distinct seasonal groupings (e.g. early, intermediate and late species) were examined. I also investigated the phenological differences among the indigenous and alien plant species. Furthermore, the onset of selected reproductive phenophases from the current records was compared with historical records for determining the extent of climate change-related alterations in phenology. Phenological data were collected fortnightly on five, 5 m x 5 m permanent plots per species (except for a few species) for a full growing season. Thus the sample size is n = 5 for all plant species except for Crassula moschata (n = 4), Juncus effusus (n=4) and Rumex acetosella (n=1). Sites of the same species were separated by at least 500 m except for the alien plant, Juncus effusus, where all four known populations were selected despite two of these populations being < 500 m apart. This study indicated that Marion Island plants grow throughout the year with no major peaks except in Azorella selago and Acaena magellanica which showed winter dormancy. However, reproduction in most plant species predominately occurred in spring and summer months. Pringlea antiscorbutica and Poa cookii were the first two species to set flower buds in September while most species dispersed their seeds in summer except for Agrostis magellanica and Crassula moschata which dispersed in early autumn. Distinct from most temperate systems, the reproductive seasonality displayed by Marion Island plant species is explained more by daylength than by temperature, perhaps due to the region’s typical thermal aseasonality. Interestingly, many cooccurring species and/or clades across the Falkland, Kerguelen, Macquarie and South Georgia Islands also showed similar flowering onset date to the Marion Island plants, further confirming their daylength sensitivity. However...