Forest dynamics under climate change: the dual impacts of elevated carbon dioxide and hot-dry events on tree carbon and water relations

Forests are the primary terrestrial carbon sinks on Earth due to trees' unique capacity to absorb and store atmospheric carbon dioxide (CO2) through photosynthesis. However, increasing extreme hot-dry events, significantly contribute to global forest mortality. Severe soil drought can lead to xylem embolism, while heat can increase leaf transpiration and impair trees' capacity to cool their leaves, possibly leading to large-scale canopy mortality. Both extreme heat and water scarcity can accelerate widespread tree dieback and shift forests from carbon sinks to carbon sources. On the other hand, the continual rise in atmospheric CO2 from human activities may enhance plant net photosynthesis while reducing tree transpiration. Under elevated CO2 (eCO2) conditions, water use efficiency improves, which could, in turn, reduce the sensitivity of trees growing under elevated CO2 to depleted soil moisture levels. While the individual effects of heat, drought, and eCO2 have been studied, there is still a lack of critical data on how mature forests respond to the combined stress of eCO2 and hot droughts (both atmospheric and soil droughts). In this talk, I will review our current understanding and aim of identifying key knowledge gaps on the individual and combined impacts of heat, drought, and elevated CO2 on tree physiological responses. A better understanding of these interactions will improve the accuracy of current climate-vegetation models in predicting forest carbon dynamics under climate change.