How will the main and interactive effects of elevated [CO2] and elevated temperature affect tree response to drought and drought-induced tree mortality?

Worldwide forest die-off events have been observed in a number of forest biomes due to severe droughts, rising global temperatures and associated increased vapour pressure deficit (VPD). If drought duration or severity increases with rising temperatures and increased VPD, all forest biomes may be increasingly vulnerable to drought-induced mortality. Despite the importance of forests in the biosphere and the significant potential consequences of forest die-offs, the mechanisms underpinning drought-induced tree mortality are poorly understood. In the context of climate change, elevated temperature has often been reported to exacerbate drought stress and accelerate the time-to-mortality in plants exposed to prolonged drought, while elevated [CO2] has been proposed as a mitigating factor because it can reduce stomatal conductance (gs) and thereby reduce water loss. Rarely have these three environmental factors (elevated [CO2], elevated temperature, and drought) been studied in combination to generate a more complete assessment of the wide-ranging, long-term effects of climate change on trees. Therefore, my PhD thesis was designed to investigate the main and interactive effects of elevated [CO2] and temperature on tree response to drought and subsequent mortality in four species representing different taxa and functional groups: Eucalyptus globulus Labill. (relatively isohydric, angiosperm), Eucalyptus radiata Sieber ex DC (relatively anisohydric, angiosperm), Pinus radiata D. Don (relatively isohydric, gymnosperm) and Callitris rhomboidea R. Br (relatively anisohydric, gymnosperm). My goal was to use these tree species to generate improved understanding of tree physiological responses to drought and its interactions with elevated [CO2] and temperature. This PhD research addressed the main and interactive effects of elevated [CO2] and temperature on tree response to drought and drought-induced tree mortality, by linking water relations and carbon dynamics in four tree species representing different taxa (angiosperms and gymnosperms) and functional groups (relatively isohydric and anisohydric). The study confirmed that hydraulic failure was the dominant mechanism underpinning tree mortality during severe droughts regardless of species or stomatal response strategy. Increasing temperature (ambient + 4 °C) and consequent higher VPD exacerbated drought stress and led to more rapid mortality through hydraulic failure in most species in this study. Rising [CO2] (ambient + 240 μl l-1) ameliorated moderate drought stress in E. globulus, but the positive effects of rising [CO2] were eliminated by increasing drought intensity. Further, elevated [CO2] did not ameliorate drought stress in E. radiata, P. radiata and C. rhomboidea or delay the time-to-mortality. These results suggest that elevated [CO2] may not ameliorate drought or temperature stress in these tree species, particularly when drought is prolonged and severe. Elevated [CO2] partially offset the negative effects of elevated temperature during moderate drought stress in E. globulus, but did not ameliorate drought response to elevated temperature in the other three species in this study. This study suggests that rising temperatures and associated higher VPD may be the predominant contributing factors to drought-induced mortality. Global forests maintain very narrow hydraulic safety margins and these findings raise concern that under future climate scenarios, characterised by rising temperatures and changing drought frequency and intensity, forests will be increasingly vulnerable to large scale mortality events with associated changes in the cycle of mass and energy with ecosystems and the provision of vital ecosystem services