Urban Heat Islands: differentiating between the benefits and drawbacks of using native or exotic vegetation in mitigating climate

The urban heat island (UHI) effect is one of the main weather phenomena to affect urban areas. The main cause of an UHI is the absorption of heat energy by urban structures which is then later radiated back into the environment. This slow release of heat keeps the urban area at a higher temperature compared to the surrounding rural/native habitats. The increase in temperature poses some social, economic and health related concerns. With over three billion people living in urban areas, there has been a lot of research into the effects of UHI on humans as well as the environment and methods used to ameliorate them. Vegetation based strategies are one of the most common and widely used mitigation strategies used today in almost all cities. This is mainly due to not only its ameliorating effect towards the UHI but also due to it being easy to implement in established cities as well as its additional benefits such as increasing aesthetics of an area. This thesis investigated whether there was an advantage or disadvantage of using exotic vegetation over native vegetation in green city plans for the Illawarra region. Initially I explored whether historical Landsat imagery could detect a relationship between land cover change and plant stress after a heat event along an urban gradient. While there were indications of stress along the urban gradient, the methodology did not show any correlation suggesting that further data or an alternative approach were needed. It highlighted the reduction of vegetation along an increasing urban gradient with all the indices used picking out the change. Two field based experiments were then carried out to compare native and exotic vegetation at a local level. The ameliorating effect of native and exotic trees was compared using an IR imaging camera. Surface heat under trees was measured on hot (27-35°C) and normal days (18-24°C) and compared between native and exotics. There was a difference between exotic and native vegetation in their ameliorating effect and this effect varied over season and prevailing temperature. The results suggest that there is some characteristics of trees that affect the surface heat under its canopy for example density of the shaded region under the tree. The second experiment compared whether native or exotic vegetation was better adapted at surviving increasing temperatures. This was done by comparing the spectral signatures of native and exotic vegetation in two seasons; spring (normal/average temperatures) and summer (hot temperatures). Native and exotic vegetation exhibited different responses to increasing temperatures. Exotic vegetation experienced higher water stress in response to increasing temperature and a subtle change in the composition of leaf pigments whereas native vegetation was less pronounced. The loss of water specifically for exotic vegetation was attributed to an adaptation where the tree uses water to try and buffer against the onset of extreme heat stress. Finally a lab-based experiment was conducted to identify whether vegetation undergoing heat stress expressed any diurnal recovery or any adaptations to reduce heat stress during an ongoing heat event. Forty eight trees (24 native and 24 exotic) were placed in temperature cabinets and an eight day heat event was induced (4 days of 30°C and 4 days of 35°C). On each day during the day and night spectral signatures of 2 leaves per tree were collected and analysed for stress using stress indices. What was found was for the control, t1 and R temperature there was no diurnal recovery however for T2 exotic vegetation did exhibit lower stress levels during the night. The results suggest that native vegetation have an advantage over exotic species in ameliorating the urban heat effect as well as in their ability to cope with heat stress