Functioning of terrestrial ecosystems of the Maritime Antarctic in a warmer climate

Environmental change, as predicted by the Intergovernmental Panel on Climate Change (IPCC), will entail increases in temperature for most parts of the earth. Predictions are that the temperature increase in the Arctic and Antarctic regions will be highest of all on earth. As the mean summer temperature is around 0 ºC in Polar Regions, an increase of a few degrees, potentially may have a large effect on temperature windows in which organisms and processes can operate. Due to the proximity to populous Northern Hemisphere continents, the major emphasis of climate change research to date has focused on the polar regions of the Northern Hemisphere. However, the Antarctic Peninsula region is also one of the three fastest warming regions of the planet over the last 50 years. To better understand responses of ecosystems on earth to climate change other regions have to be investigated as well. Antarctic terrestrial ecosystems are one of the most extreme on Earth. Being the highest, coldest and windiest continent, Antarctica places severe limits on life. The organisms living here are most likely at the edge of their survival abilities. Due to the low temperatures, free water is only available during brief periods in summer when snow and ice melts. This makes water the most limiting factors for survival in the Antarctic. The severe environmental conditions for life have resulted in relative impoverished vegetation types and soil arthropod communities. Changes in temperature, especially during the summer period, may have a large impact on growth and development of soil organisms, vegetation and processes. This makes Antarctic ecosystems ideal for investigating climate change impacts. In the summer of 2003/04, Open Top Chambers (OTCs) were placed in contrasting coastal communities along a latitudinal gradient; at the Falkland Islands, dwarf shrub and grass-dominated communities, at Signy Island and Anchorage Island, moss and lichen-dominated communities were chosen. I investigated the response of soil arthropods, vegetation and decomposition to increasing temperatures generated by the OTCs. The latitudinal gradient was used as a proxy for extreme warming as a predicted result of environmental change. To better understand parts of the nutrient cycling within such communities, I investigated external nitrogen sources that might be used by the vegetation, as well as the food choice of soil arthropods. When conditions for water availability and temperature have been met, nitrogen tends to become a major limiting factor on vegetation development in the Antarctic. In chapter two, I describe potential external nitrogen sources for the three study sites in this thesis. At the Falkland Islands, I was unable to conclude whether external nitrogen sources play an important role for the vegetation. At Signy and Anchorage Island, stable isotope analyses showed that vertebrate colonies highly influence the nitrogen brought in by the wind and this was reflected in the vegetation. However, some lichen species did not appear to be using this source of nitrogen but appeared to make more use of precipitation. In the third chapter of this thesis I describe the feeding choice of the most abundant Collembola (springtail) on Anchorage Island. Preferred food choice by the Collembolan Cryptopygus antarcticus, was determined by combining food choice experiments in the lab with stable isotope analyses of field samples. The main diet appears to consist of alga and lichens. The response to warming of soil arthropods is described in chapter four. After two years of warming, the initial responses of arthropod abundance were low. However, in the lichen community on Signy Island there was a decrease in Collembolan abundance, due to a combined effect of warming and a reduction in soil moisture. The vegetation shows a similar pattern as described in chapter five. Here I found a decrease in vegetation cover in the same lichen community on Signy Island due to the warming treatment. In the grass community at the Falkland Islands, there was also a reduction in vegetation cover. A relative ‘dry’ summer resulted in a large overall decrease but the warmed plots by OTCs showed an even larger reduction in vegetation cover. The more densely vegetated communities at the three islands were apparently better at buffering the vegetation and soil community to these small increases in temperature, as they showed no response. These results illustrate the vulnerability of such open vegetation types to a small increase in temperature. In chapter six I applied different methods to investigate the response of the decomposition process to warming. The laboratory studies indicate a strong potential response to increased temperatures. The field experiments showed a lower response and this could have been a result of the relative low temperature increase by the OTCs but perhaps more importantly a reduction in soil moisture. The decomposition process shows a great potential for response to increases in temperature. If the temperature will keep rising above the ones instigated in this study, larger responses are likely to be expected but only if soil moisture will not be greatly affected. As a-biotic factors mainly govern life in the Antarctic, biotic interactions have not been extensively explored in the Antarctic. However, the biotic components in Maritime Antarctic ecosystems are likely to play some role of significance in ecosystem processes as suggested in this thesis. The main focus of this thesis was to investigate the response of different parts of terrestrial ecosystem when temperatures will keep rising in communities from the Falkland Islands and the Maritime Antarctic region. The duration of my experiment was on a very short time-scale especially considering the ‘slow’ life cycles of most Antarctic organisms. Therefore, the large differences seen between my study islands were not greatly affected by the warming treatment. These differences do indicate that there is a potential for community change above as well as below ground. In the last chapter I have brought all the findings together and provided an idea on what might happen with these Antarctic ecosystems if the temperature will keep rising in the future. Based on what is described in literature and by my own findings it is likely that open structured communities will be negatively affected by temperature increases in the near future as they have more difficulty in regulating soil moisture