How trees improve cities: CO2 uptake, cooling, and air quality amelioration

Using the cities of Rimini (IT) and Krakow (PL) as living laboratories, the LIFE URBANGREEN project was aimed at: 1) measuring regulation ESs provided by different woody species; 2) provide new data based on eco-physiological measurements for irrigation scheduling of urban trees; 3) assess the effect of different irrigation regimes and mulching on ESs provisioning. Two representative city transects sized 250 and 470 ha were identified in Rimini and Krakow, respectively. Transects were stratified into paved and unpaved areas. Ten woody species were selected in each city. DBH, height, and crown projection area were monitored on 230 (Rimini) and 320 (Krakow) trees from 2018 to 2021. Young, mature, and late-mature individuals were selected for physiological measurements, conducted in different seasons from 2018 to 2020 on about 1950 (Rimini) and 2300 (Krakow) leaves per year. CO2 uptake was quantified as CO2-assimilation (i.e. the amount of atmospheric CO2 converted to carbohydrates through photosynthesis) and CO2-storage (i.e. the amount of CO2 stored as organic carbon in tree woody biomass). An infra-red gas analyser was used to measure CO2 assimilation and transpiration. CO2-assimilation per unit leaf area was then upscaled to the whole canopy using both a big leaf and a multi-layer model. Transpirational cooling was calculated by multiplying the water transpired and the specific heat of vaporization of water. CO2-storage was calculated from tree volume, measured using terrestrial laser scanning and dry density. PM removal was quantified using a gravimetric method on three samples made of 200-300 cm2 leaf area each per tree, collected at different heights. Actual evapotranspiration (ETE) was calculated from transpiration measurements. Potential evapotranspiration (ET0) was calculated fitting meteorological hourly data provided by Ubimet to the Penman-Monteith equation. Landscape coefficients (KL) were calculated as the ratio of ETE to ET0. Since 2020, a subsample of 120 trees from 6 species per city was assigned to either pilot (mulching + KL-based irrigation) or control (no mulching, no irrigation) treatments. The effects of different management of ESs provisioning was assessed by monitoring plant growth and leaf gas exchange for 16 (Krakow) or 18 (Rimini) months after the imposition of treatments. Species selection affected ESs delivery: Populus nigra and Quercus robur displayed higher CO2-assimilation and transpirational cooling per unit leaf area than Aesculus hippocastanum and Acer spp. Populus nigra and Acer platanoides showed higher carbon storage than other species for trees up to about 20 cm DBH. Thereafter, Quercus robur and Ulmus laevis displayed higher storage. Species with needles (Pinus) and small hairy leaves (Quercus ilex) adsorbed more PM per unit leaf area than other species, although extrinsic factors such as honeydew can increase air quality amelioration capacity of some species.