Heatwave sensitivity of Tasmanian Eucalyptus obliqua tall forests: diagnosis and management implications for adaptation to climate change

Abstract

Tasmania’s distinctive climate environment supports highly productive and carbon-dense tall eucalypt forests. Measurements in Eucalyptus obliqua tall forests at the Warra SuperSite, southern Tasmania, showed them to be very sensitive to high temperatures. Heatwaves (three or more consecutive days of unusually high temperatures) caused these forests to switch from a net sink to a net source of carbon because ecosystem respiration exceeded the diminished carbon uptake. Therefore, as the global climate warms, the risk of emissions increasing from these important ecosystems is rising. After reviewing observations of their responses to the climate, this study made a putative diagnosis for the temperature sensitivity of the E. obliqua tall forests. A stable climate environment characterised by moisture being rarely limiting; a narrow temperature range; and a winter radiation limitation was associated with the high temperature sensitivity of the forest. Local genetic variation in E. obliqua, which provides an opportunity for natural selection of local genotypes that are better adapted to a warmer climate, was identified as the most prospective option for management. Natural selection from periodic wildfire may not be sufficient for this species to adapt to rapidly changing conditions. Enhanced natural selection could be an alternative. This will require a coincidence of the early forest regeneration phase with selection events (periods of high temperatures), a combination that occurs with greater regularity in forest areas that are harvested for wood production and regenerated. Seed can be selected from sites that were successfully regenerated and maintained rapid growth during high temperature events. It is proposed that a network of dendrometers is installed in tall eucalypt forests across Tasmania to determine the extent of impact of high temperatures on growth and the degree of variation in growth response that can support enhanced natural selection. Policy should enable continuation of management intervention in tall eucalypt forests dedicated to wood production to accelerate selection of genotypes that are better adapted to hotter conditions.