CO2-assimilation, sequestration, and storage by urban woody species growing in parks and along streets in two climatic zones

Abstract

Using two cities, Rimini (Italy, Cfa climate) and Krakow (Poland, Cfb), as living laboratories, this research aimed at measuring in situ the capacity of 15 woody species to assimilate, sequester, and store CO₂. About 1712 trees of the selected species were identified in parks or along streets of the two cities, and their age, DBH, height, and crown radius were measured. The volume of trunk and branches was measured using a terrestrial LiDAR. The true Leaf Area Index was calculated by correcting transmittance measurements conducted using a plant-canopy-analyser for leaf angle distribution, woody area index, and clumping. Dendrometric traits were fitted using age or DBH as independent variable to obtain site- and species-specific allometric equations. Instantaneous and daily net CO₂-assimilation per unit leaf area was measured using an infra-red gas-analyser on full-sun and shaded leaves and upscaled to the unit crown-projection area and to the whole tree using both a big-leaf and a multilayer approach. Results showed that species differed for net CO₂-assimilation per unit leaf area, leaf area index, and for the contribution of shaded leaves to overall canopy carbon gain, which yielded significant differences among species in net CO₂-assimilation per unit crown-projection-area (A_cpaML(d)). A_cpaML(d) was underestimated by 6-30 % when calculated using the big-leaf, compared to the multilayer model. While maximizing A_cpaML(d) can maximize CO₂-assimilation for a given canopy cover, species which matched high A_cpaML(d) and massive canopy spread, such as mature Platanus x acerifolia and Quercus robur, provided higher CO₂-assimilation (A_tree) at the individual tree scale. Land use (park or street), did not consistently affect CO₂-assimilation per unit leaf or crown-projection area, although A_tree can decline in response to specific management practices (e.g. heavy pruning). CO₂-storage and sequestration, in general, showed a similar pattern as A_tree, although the ratio between CO₂-sequestration and CO₂-assimilation decreased at increasing DBH.