Understanding Warming Effects on Vegetation Through Geothermal Hotspots: A Case Study of the Wairakei-Tauhara Geothermal Field, Taupo, New Zealand
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Understanding and predicting warming effects on vegetation is a key challenge of the 21st century. To date, warming experiments have been limited to isolated laboratory studies, which have been criticised as too small-scale and overly simplistic because they were not conducted in a natural context. As such, understanding the future consequence of warming using a natural environment and across an effective timeframe has become a key goal of vegetation dynamic studies. Warming experiments in undisturbed settings are particularly important for understanding the complex responses of vegetation to warming within a multifunctional ecosystem. Furthermore, a natural setting approach can include other variables that will improve the current understanding of the effects of warming on vegetation. Through the course of this thesis, I developed a warming experiment to utilise natural thermal gradients in a geothermal ecosystem spanning a wide range of soil temperatures from 18 - 56 °C. First, I used historical aerial photographs from 1975 to 2016 to review the change in vegetation distribution around geothermal surface features, correlating the change with thermal infrared images captured in 2009 and 2014. This review revealed that as the geothermal features cool, vegetation growth and development is enhanced. Thus, examining the response of vegetation to changes in thermal gradients provided a means of simulating warming. Second, I used an unmanned aerial vehicle (UAV) to capture thermal infrared and standard colour imagery to address the knowledge gap in remote sensing data. I demonstrated the use of a small, cost effective UAV and highlighted techniques in sampling, processing and analysing images captured by UAV. Third, after investigating the historical response of vegetation to geothermal features, I set up a field experiment to investigate vegetation regeneration and root biomass responses to the soil temperature gradients and other geothermally associated conditions (soil chemistry). I found that although other conditions did limit vegetation establishment and growth, the thermal gradient had the most dominant effect. Given the important role that vegetation plays in the ecosystem, it is vital to understand any unpredictable changes and then to apply appropriate actions within an appropriate timeframe. This thesis addresses a critical gap in experimental soil warming, making use of a geothermal system. Warming experiments within geothermal gradients demonstrate how small-scale experiments can be used to inform future vegetation responses to warming. Using the geothermal system as the natural setting for a warming experiment also provides the opportunity to replicate similar studies around the world to improve our understanding of local area vegetation dynamics.