Year

2004

Degree Name

Doctor of Philosophy

Department

School of Biological Sciences - Faculty of Science

Abstract

Climate change is expected to affect the high latitudes first and most severely, rendering Antarctica one of the most significant baseline environments for the study of global climate change. The indirect effects of climate warming, including changes to the availability of key environmental resources, such as water and nutrients, are likely to have a greater impact upon Antarctic terrestrial ecosystems than the effects of fluctuations in temperature alone. Water availability is the focus in this thesis for two main reasons; firstly, there is a wealth of evidence to suggest water is currently limiting to Antarctic plant distributions and productivity, and secondly, availability of this key resource is predicted to change with the onset of climate change. Nutrient availability is a second variable considered in this work, as there is evidence to suggest that nutrients also play a role in determining plant species distributions, and changes to nutrient balance and turnover rates are also expected in response to climate change. This work was conducted in the floristically important Windmill Islands region of East Antarctica, with the three Windmill Islands moss species Bryum pseudotriquetrum, Ceratodon purpureus and Grimmia antarctici forming its focus. A combination of field ecology, ecophysiology and laboratory studies were used to determine fine-scale patterns of present species distributions and their relationship to naturally occurring water and nutrient resource gradients, the impact of increased water and nutrients on a range of cryptogamic communities, and tolerance of desiccation biological profiles. A survey-based approach was used to determine species level patterns in bryophyte species distributions, and identify correlations with resource availability. Ten replicate transects, along community gradients, from pure bryophyte stands, through transitional moribund bryophyte zones, to lichen-dominated communities, were surveyed at two sites. The physical environment at each site was characterised by measurement of site soil properties, along with individual transect aspects and slopes. To determine the relationship between resource gradients and community patterns water and nutrient availability, along with a range of plant biochemistry measures indicative of the growth environment, were measured using a series of quadrats along each transect. Percent abundance for each species and/or vegetation category was determined for each quadrat by microscopic examination of field samples. The community gradient, covering the entire ecological range of past and present bryophyte occupation, was found to be accompanied by resource gradients that operated in opposing directions. Pure bryophyte communities existed under conditions of high water availability and low nutrient availability. Crustose lichen-dominated communities persisted under opposing environmental conditions, of low water availability and high nutrient availability. Grimmia antarctici dominated the wettest habitats but its distribution extended into the dry moribund zones, albeit in low levels of abundance. Bryum pseudotriquetrum occurred in consistent levels of abundance across the entire gradient, whilst C. purpureus was restricted to the driest habitats. Live bryophyte material was found to occur in moribund turf, supporting the potential for bryophyte regeneration under a future wetter climate. Regenerating turf showed potential to support high species diversity, as all four bryophytes survive in this zone. To investigate the likely impacts of a wetter climate on Antarctic terrestrial communities, four cryptogamic communities, pure bryophyte, moribund bryophyte, crustose and fructicose lichen-dominated communities were subject to a multi-season manipulative field experiment. Within each community type, eight replicate quadrats received increased water and/or nutrient availability over two consecutive summer seasons. A range of physiological and biochemical measurements were conducted in order to quantify the community response to the treatments and determine the extent of any nutrient and water limitation. Few multi-season manipulative field experiments have been conducted in continental Antarctica. Whilst an overall increase in productivity in response to water and nutrient additions was supported, productivity appeared to respond more strongly to nutrient additions than to water additions. Pure bryophyte and fructicose lichen communities also showed stronger positive responses to additions, identifying some communities that may be better able to adapt and prosper under the ameliorating conditions associated with a warmer, wetter future climate. Using a range of morphological, biochemical and physiological techniques, biological profiles related to desiccation tolerance were developed for the three bryophyte study species, providing measures of relative abilities to avoid, tolerate, recover from and survive desiccation. Ceratodon purpureus showed good desiccation avoidance characteristics, its photosynthetic efficiency remained high at low water contents and it was lipid rich, suggesting that this species is well adapted to survive a drying climate. Bryum pseudotriquetrum is also likely to survive drier conditions, as this species showed good desiccation avoidance, had a plastic response to desiccation, and contained stachyose, which is likely to assist in its survival of desiccation events. Conversely, G. antarctici showed poor desiccation avoidance, as photosynthetic efficiency required highest water contents and it contained few protective substances, this species is therefore least likely to survive a drying environment. This study provides a baseline from which future changes to the Windmill Islands cryptogamic communities can be monitored. A baseline incorporating fine-scale bryophyte species patterns is particularly useful, as this component of the cryptogamic community is likely to be highly sensitive to even small shifts in water availability and detection of change is likely to be more sensitive at fine- rather than broad-scales. Both water and nutrient resource availability was found to underlie regional bryophyte species distributional patterns. The dynamics of the Windmill Islands flora is therefore likely to shift in response to climate change as the availability of these key resources is altered. Under a wetter future climate, productivity is overall likely to increase but only certain cryptogamic communities are likely to thrive. Regeneration of moribund bryophytes appears likely only if the future moisture regime creates consistently moist conditions. Bryum pseudotriquetrum is most likely to survive both a drying climate and also a fluctuating climate, which is a highly likely scenario for the region. Under a drier climate, the Antarctic endemic, G. antarctici, is likely to be most adversely affected, as it dominates only the wettest communities and generally shows poor tolerance of desiccation. Conservation issues are therefore raised for this species, if the current drying trend continues, creating overall biodiversity concerns.

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