The effects of drought, heat and elevated atmospheric CO2 on physiology and growth of Eucalyptus – Does climate-of-origin matter?**

Abstract

This thesis assessed the effects of future climate factors (i.e. [CO2], heat waves and soil water availability) on growth and physiology of Eucalyptus species originating in different climates-of-origin. The main aim was to test intra-specific variation of plant traits to climate change. Four tree species native to Australia were selected due to their national ecological and international economic importance: Eucalyptus camaldulensis, Eucalyptus grandis, Eucalyptus melliodora and Eucalyptus coolabah. I tested the response of E. camaldulensis to elevated atmospheric [CO2] (eCO2), heat and drought stress; E. grandis to eCO2 and drought stress; and the acclimation response of E. melliodora and E. coolabah to wetting and drying cycles with final drought to mortality. Phenotypic plasticity in leaf gas exchange, growth and non-structural carbohydrate (NSC) reserves was significantly different in E. camaldulensis and E. grandis when subjected to heat and/or water stress. In E. grandis, the tallest trees from cool temperatures had the largest growth reductions during stress. In E. camaldulensis, trees originating in semi-arid climates initiated leaf abscission early and conserved NSC, which led to faster stem and leaf area recovery than trees frommore mesic climates. Moreover, eCO2 ameliorated stress responses related to photosynthesis when trees were either heat-stressed or water-limited; timeto-leaf-death was extended in one provenance of E. camaldulensis in eCO2. There was no acclimation of leaf gas exchange to variable water availability during the series of droughts in E. melliodora and E. coolabah. Yet, species had contrasting water-use strategies linked to their distributional range across Australia. Eucalyptus coolabah originating in a semi-arid climate reduced its leaf area to prevent hydraulic failure, while E. melliodora originating in a mesic climate utilised NSC reserves to tolerate water limitation. These results highlight the importance of soil water availability for physiological functioning and growth, but they also show that intra-specific differences exist in response to heat and drought. In conclusion, my PhD research extends information on interand intra-specific differences in phenotypic plasticity of trees to the main and interactive effects of climate factors, which can be used to identify plantation trees for future climate regimes.