Plant Hydraulic Functioning in face of Climate Change – PHYDRAUCC

Atmospheric CO2 concentration ([CO2]) has increased by 45% since 1750, the highest in the past 800 000 years. [CO2] will continue to increase as precipitation and soil moisture are modified alongside. Forest trees will hence be challenged by higher [CO2], warmer temperatures, and more frequent and/or intense summer droughts. To produce healthy trees with sustained growth under changing climatic conditions, it is imperative to determine whether elevated [CO2] will affect the structure of wood and the physiological traits conferring drought- and heat-tolerance. For the last 30 years, researchers have used Free-Air Carbon dioxide Enrichment (FACE) facilities, elaborate outdoor systems to pipe thousands of tons of carbon dioxide into forests stands to experimentally enrich the surrounding environment. Rising [CO2] has a direct positive effect on leaf photosynthesis, but potentially also an indirect negative effect on plant productivity and survival in response to the increase in stressful events such as heat waves and drought spells. Plant hydraulic traits describe the efficiency and safety of the xylem water transport system of roots, stems, branches, and leaves. Although these hydraulic traits directly link soil water availability with transpiration and photosynthesis, they are too rarely accounted for in current vegetation models, used to forecast the sequestration potential of the terrestrial biosphere under future climate change. In turn, it is still not known how growth variations under rising [CO2] and changing climate will affect those traits.
This project will combine for the first time data from multiple FACE sites to establish a clear picture of how elevated [CO2] impacts wood anatomy and its link to hydraulic conductivity and water storage capacity of roots, stems and branches, as well as their vulnerability to embolism across a range of tree species. By incorporating these results in mechanistic soil-plant-atmosphere and large-scale vegetation models, the project will ascertain whether the interplay of tree-ring formation and hydraulic functioning can lead to strong ecosystem responses to elevated [CO2], warming, and drought, that can be carried over the following years. It is, however, expected that the hydraulic traits, i.e. conductance, capacitance, and vulnerability to embolism, might not be enough to explain the strong responses to drought of some broadleaved species, but that different strategies such as for example leaf shedding must be added. The project will hence investigate the consequences of such tight bonds of water and carbon relations on the resilience of trees under future atmospheric and climatic changes, and increase our ability to forecast future environmental impacts on tree functioning and forest carbon sequestration potential.