The reproductive ecology of vascular plants on subantarctic Macquarie Island

Southern Ocean islands have long been sites of scientific interest, due to their isolation, climate, geology and biology. This study investigated reproductive ecology of vascular plants on subantarctic Macquarie I. The persistence of plant species in a community relies on life history strategies which ensure population growth and sustainability. This can be via either vegetative or sexual reproduction. Many studies have shown that for alpine and high latitude plants sexual reproduction is not an important life history trait. The study identified that 26 species flowered, 22 dispersed seed and 12 produced seedlings over the study period on Macquarie I. This represents 72%, 61 % and 33% of the angiosperm flora of the island, respectively. There was a strong seasonality with almost all species flowering and fruiting in spring and summer. Overall sexual reproduction coincided with an increase in day length in summer. Seed rain of several plant communities was measured across eight sites of differing environments. At most sites seed rain was representative of standing vegetation, with the most widespread and abundant species1 on the island being the most widespread and abundant species present in the seed rain. The study has shown that several species (particularly Pleurophyllum hookeri and Stilbocarpa polaris) produced large numbers of seeds (over 13, 300 seeds `m^-2`) which are greater seed rain densities than previously recorded in arctic and alpine studies. Both species of large leaved megaherb (P. hookeri and S. polaris) are abundant on the island. S. polaris and P. hookeri allocated resources similarly as both species were found to allocate over 50 % of their total biomass to leaf tissue and a substantial proportion of biomass (20+%) to sexual reproduction. The study identified that seed germination occurred for megaherbs across a range of altitudes. Seedling densities recorded for both species were extremely high (up to 10, 000 `m^2`). Despite high seedling mortality during the study many seedling were observed to survive to become juvenile plants. P. hookeri population demography was investigated across four different populations. All were found to be dominated by large old plants. Despite some demographic variation between populations, seedlings were recruited into the population at most sites. Alien species were found to have deleterious impacts on megaherb species through seed predation, seedling herbivory, and site disturbance. This study has shown that plants on subantarctic Macquarie I. allocated considerable resources to sexual reproduction which is an important plant life history strategy for plants growing in this environment.