Carbon and water fluxes in urban forest: improving human well - being for a more sustainable society

The constant growth of population living in urban areas creates new opportunities for urban forest to provides ecosystem services for human wellbeing such as, cooling effect, and carbon neutrality of cities. Studies related to urban forest involve transdisciplinary fields that include environmental, social, and economic aspects, at a range of different spatial and temporal scales. Nevertheless, experimental observation of carbon and energy exchange in urban forest have been so far fragmented, limited to short period of time, and never spatially distributed. While a considering amount of remote sensing and modelling studies indicates the potential cooling capacity and carbon uptake of urban forest, the impact of climatic extreme events on it is still unclear. Through multiple years of unique Eddy Covariance (EC) observations of a mature urban forest located in southern Europe we highlighted how carbon and water fluxes respond differently, almost as if uncoupled, with evaporative cooling maintained during the climatic drought and net carbon sequestration reversed. A long term EC observation, coupled with  modeling simulations, highlight   the role of urban forest as potential tool for climate and microclimate mitigation with and without drought limitations. Our results have important policy implications for urban forest management and planning and more generally for strategies, on urban forest, in relation to carbon neutrality and thermal comfort. While the urban forest had an annual net loss of CO₂ to the atmosphere, its above- and below- ground biomass and the soil represent a relevant carbon reservoir, and its summer uptake of atmospheric CO₂ enabled evaporative cooling of the microclimate. However, the impact of summer drought reduced the levels of cooling benefits compared to non-drought summers. We identified a need for drought tolerant species selection to ensure their ability to tolerate future climate and provide needed ecosystem services such as maintained assimilation rates or survival during drought. Our results represents the first long term, and continuous experimental observation to demonstrate that the urban forest cooling capacity in warm seasons can decouple from net CO₂ uptake and will be limited by the amount of water available, either from precipitation or irrigation sources.