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

S. Bokhorst

    Research output: PhD ThesisPhD thesis

    Abstract

    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.
    Original languageEnglish
    Awarding Institution
    • VU University Amsterdam
    Supervisors/Advisors
    • Aerts, M.A.P.A., Promotor
    • Huiskes, A.H.L., Co-promotor
    • Convey, P., Co-promotor, External person
    Award date07 Sep 2007
    Place of PublicationAmsterdam
    Publisher
    Publication statusPublished - 07 Sep 2007

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