Knowing why Tasmanian Eucalyptus obliqua tall forests are so sensitive to heatwaves informs management and policy for climate change adaptation

Tasmania’s distinctive climate environment supports highly productive and carbon-dense tall eucalypt forests, but also a vulnerability to climate change. Measurements in Eucalyptus obliqua tall forest at the Warra SuperSite in southern Tasmania showed them to be very sensitive to warmer temperatures. Gross primary productivity (GPP) declines sharply when temperatures rise above the forest’s relatively low temperature optimum for GPP. Heatwaves (consecutive days of unusually high temperatures) cause the forest to switch from being a net carbon sink to a net carbon source. As the climate changes and heatwaves occur more often, the risk of severe adverse impacts to this important ecosystem increases. Our current understanding of why the Tasmanian E. obliqua tall forest is so sensitive to warmer temperatures, even if imperfect, can allow appropriate management interventions to be identified and enabling policy settings to be formulated. Tasmania’s climate environment was associated with several features of the unique response by the tall eucalypt forests to heatwaves. The high sensitivity of GPP to temperature in the forest was associated with a low temperature range – GPP sensitivity to temperature increased as temperature range experienced by the forest decreased. The decline of GPP during warm conditions was not associated with either moisture limitation or atmospheric dryness (high vapour pressure deficits), a likely reflection of the high rainfall environment in which tall eucalypt forests grow in Tasmania. Low winter radiation associated with Tasmania’s high latitude, contributes to a lack of seasonal temperature acclimation by the forest. Because the forest responses to heatwaves were associated with the local climate environment, enhanced natural selection of local genotypes is identified as the most prospective approach to help Tasmania’s tall eucalypt forests adapt to a new, warmer climate. Enhanced natural selection is only feasible to do in forests that are managed for wood production. Policy settings that allow for the continuation of native forest harvesting will be needed if enhanced natural selection is to supply future needs of climate-adapted seed for regeneration and restoration of tall eucalypt forest areas in Tasmania.