Traits underpinning the eco-physiological processes linking drought and flammability : A thesis submitted in partial fulfilment of the requirements for the Degree of Doctor of Philosophy at Lincoln University

Abstract

Fire is common in many terrestrial ecosystems, and shapes species composition and the distribution of biomes. Fire regimes have been altered in many parts of the world due to global climate change, with increases in drought resulting in more intense and extensive wildfires. Drought can impact fires in many ways, including by changing the flammability of plants. Recognition of this link has led to the development of the new field of pyro-ecophysiology, which, among other things, seeks to understand how plant water relations can drive changes in live fuel moisture and, therefore, plant flammability. However, few studies have investigated the relationships of drought response traits to plant flammability. The main goal of this thesis is to identify the traits which underpin the ecophysiological processes linking drought and plant flammability, and so help determine how drought affects the propensity of plants to burn. Firstly, I examined relationships between shoot flammability and drought response measures for 38 species of woody plants from New Zealand. I found that minimum leaf water potential and turgor loss point were both negatively correlated with flammability, suggesting that species with high drought tolerance were high in flammability. This suggests that while these traits are useful for the new field of pyro-ecophysiology, species with high drought tolerance should not be recommended by fire managers as low flammability plantings, as has been the case overseas. Secondly, I examined how species flammability changes throughout the year, and assessed which traits are associated with this. I measured shoot flammability and a range of physiological and morphological traits of 10 species at four different times throughout one year. For some species flammability fluctuated throughout the year, identifying species which fire managers need to be careful of when planning fire reduction measures such as green firebreaks. Three species (Griselinia littoralis, Pseudopanax crassifolius, Pseudopanax colensoi) remained low in flammability year round, suggesting that these species could be safely deployed in green firebreaks to help reduce fire spread. Changes in flammability were associated with traits such as stem water potential, moisture content, leaf relative water content, and leaf area. Thirdly, I investigated the existence of thresholds in the relationship between moisture content and shoot flammability to determine if changes in moisture content might lead to species flipping from low to high flammability. Thresholds in moisture content were found in all species and flammability variables, though threshold values were species specific. Threshold moisture content was positively related to leaf area and negatively to leaf dry matter content. As predicted by the emerging field of pyro-ecophysiology, water relations traits such as water potential and relative water content were associated with differences in shoot flammability. While both traits are useful in understanding how drought affects plant flammability, water potential is harder to measure. Leaf relative water content holds great potential as an integrative trait that can be measured rapidly, including via remote sensing, which would enable collection of real-time data on fire risk over wide areas, providing critical information for fire managers.