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

[摘要] ENGLISH ABSTRACT: Species' seasonal behaviour is of paramount importance in understanding community functioningand dynamics. Recently, plant phenology has further gained significance as a reliable indicator ofclimate change impacts. Despite the importance of understanding plant dynamics, there arerelatively few plant phenological records for the sub-Antarctic region, and where records exist theyare often not extensive. Sub-Antarctic Marion Island, typical of Southern Ocean Islands, offers auseful setting for addressing these knowledge gaps. This study documented the vegetative andreproductive phenologies (or aggregate phenological patterns) of twelve indigenous and three alienvascular plant species on the island. The phenological differences among the species and distinctseasonal groupings (e.g. early, intermediate and late species) were examined. I also investigated thephenological differences among the indigenous and alien plant species. Furthermore, the onset ofselected reproductive phenophases from the current records was compared with historical recordsfor determining the extent of climate change-related alterations in phenology. Phenological datawere 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 Crassulamoschata (n = 4), Juncus effusus (n=4) and Rumex acetosella (n=1). Sites of the same species wereseparated by at least 500 m except for the alien plant, Juncus effusus, where all four knownpopulations were selected despite two of these populations being < 500 m apart. This studyindicated that Marion Island plants grow throughout the year with no major peaks except inAzorella selago and Acaena magellanica which showed winter dormancy. However, reproductionin most plant species predominately occurred in spring and summer months. Pringleaantiscorbutica and Poa cookii were the first two species to set flower buds in September while mostspecies dispersed their seeds in summer except for Agrostis magellanica and Crassula moschatawhich dispersed in early autumn. Distinct from most temperate systems, the reproductiveseasonality displayed by Marion Island plant species is explained more by daylength than bytemperature, perhaps due to the region's typical thermal aseasonality. Interestingly, many cooccurringspecies and/or clades across the Falkland, Kerguelen, Macquarie and South GeorgiaIslands also showed similar flowering onset date to the Marion Island plants, further confirmingtheir daylength sensitivity. However, other external factors seem to come into play at later events ofreproduction. Consequently, fruit maturation time of similar species across the sub-Antarctic islandsvaried substantially despite the plants having flowered in the same month. Although plant speciesshowed similar reproductive seasonality, there were significant differences among species phenologies i.e. phenophase timing, duration and peak occurrence dates. However, using 95%confidence intervals of Generalized Linear Models weighted means, and/or one-way ANOVA(Tukey post hoc test), three homogenous sets of species (early, late, or intermediate onsets) wereidentified based on flower bud, flowering and seed dispersal phenophase onset dates. Thehomogenous species groupings observed for flower buds also remained unchanged during floweringonset except for Cotula plumosa and Callitriche antarctica which switched groups. As for the seeddispersal timing, the pattern was not consistent with that of the flower bud and flowering onsethomogenous groupings, except for Acaena magellanica and Agrostis magellanica which remainedin the early and late groups, respectively. Conversely, in the case of the timing of other phenophases(pollen release, fruit set and fruit ripening), entire phenophase durations, and peak occurrence dates,species overlapped greatly, resulting in an unbroken progression or continuum of phenology amongspecies. Similarly, the three alien plant species investigated here (Cerastium fontanum, Juncuseffusus and Rumex acetosella) showed no consistent phenological differences from the rest of thespecies. However, a widespread alien plant species on Marion Island, C. fontanum, reproduced formost of the year, although its reproduction peak was in summer months as was the case for the restof the species. This study also indicated that indigenous plant species have altered their reproductivephenologies since 1965. Although the response was species-specific, the majority of plant speciessignificantly delayed the onset of reproductive activities in 2007 by comparison with 1965.However, it is not clear if the observed species response was caused by the now drier and warmerMarion Island climate or by discrepancies in reporting in the earlier studies and/or samplingdifferences between the recent and historical records. Therefore, these results should be taken withcaution. In conclusion, this research provided a detailed phenological dynamics record for vascularplant species on the island. Over time these records may be used as a basis for monitoring andmodelling the impact of climate on plant phenology on the island.

[发布机构] Stellenbosch University