Shrubs Matter: An Evaluation of the Capacity of Nine Shrub Species to Dissipate Latent Heat and to Remove CO2 and Airborne PM

The aim of this research was to quantify the capacity of different shrub species to remove atmospheric CO2, to adsorb particulate matter and to dissipate latent heat through transpiration. A total of 308 established plants comprising Deutzia scabra, Elaeagnus × ebbingei, Euonymus japonicus, Forsythia × intermedia, Laurus nobilis, Ligustrum vulgare, Pittosporum tobira, Prunus laurocerasus and Viburnum tinus were selected in Lugano (Switzerland) and Bolzano (Italy). Stem diameter, crown radius, Leaf Area Index, net CO2 assimilation per unit leaf area (Aleaf), transpiration, and stomatal conductance (gs) were measured during spring, summer, and fall. The net CO2 assimilation per unit of crown projection area and per plant were calculated by upscaling Aleaf using a multilayer model. Latent heat dissipation was calculated using the Penman–Monteith equation. The amount of PM trapped on leaves was measured using a gravimetric method. Differences in leaf area and leaf gas exchange among species affected their capacity to deliver specific ecosystem services. Forsythia, Pittosporum, Elaeagnus and Deutzia removed about 40% more CO2 per unit crown projection area than Laurus, Ligustrum, and Euonymus. Latent heat dissipation by shrubs was, on average, 130 W m−2, which is comparable to that of tree species. PM removal per unit leaf area was higher in species with sparse canopies and rough leaf surfaces.