Ecosystem services of urban trees: how can planning enhance CO2 assimilation, microclimate improvement and air quality amelioration?

To fulfil the UN Sustainable Development Goal (goal n. 11 in particular) and develop sustainable, inclusive, and resilient cities, urban green areas are rapidly gaining popularity because of their capacity to provide provisioning, regulating, and cultural ecosystem services. This resulted in aggressive planting programs across cities worldwide. It has been highlighted, however, that planting a tree is not enough to get positive ecosystem services: without proper planning, species selection, and application of science-based planting and management techniques, trees often prematurely decline, becoming source of atmospheric CO2 rather than a source of valuable benefits, and rising scepticisms on the real effectiveness of tree planting programs. The urban environment is stressful to trees, because they are subjected to a range of anthropogenic pressures, such as soil sealing, transplant and root manipulation, soil compaction which are highly specific and unpreceded before the advent of compact cities. A deeper understanding of the effects of such “novel pressures” on trees, as well as the species-specific tolerance mechanisms. Moreover, because of the urban heat island effect, cities anticipate the climate change. This impose to unravel the tolerance mechanisms of the different species to heat and drought spells, which are becoming more and more frequent in the latest years. If ecosystem services provisioning is the goal of urban forest planting programs, a quantitative knowledge of the amount of the benefit provided is required. Ecosystem services are often implicit and hard to quantify, thus the capacity of the different species to provide them has seldom been quantified and taken into account for urban green areas planning. Modelling efforts to estimate ecosystem services have been based so far using micro-meteorological or biometric models. On the contrary, physiological processes (i.e. photosynthesis) which are directly linked to ecosystem services (i.e. CO2 assimilation) are seldom included in models. Specific actions within the Life Urbangreen project have been carried out to quantify in situ CO2 assimilation, transpiration, and PM adsorption by widely used species growing in Rimini and Krakow. Benefits have been measured by integrating eco-physiological measurements of leaf gas exchange and PM adsorption to detailed biometric measurements conducted using a laser scanner. Uneven aged trees and shrubs from ten widely used species with different size at maturity and persistence of leaves were selected in paved and park areas within each city. About 20 trees per species were selected within the experimental areas. CO2 assimilation and transpiration were measured on leaves located in different portions of the canopy of sampled tree during spring, summer, and fall. Data showed significant differences among the selected species for their capacity to assimilate carbon, to adsorb particulate, and to ameliorate microclimate through transpiration. [Virtual symposium].