This research aims to extend the existing knowledge on air quality improvement by the arboreal–shrub heritage. The PM accumulation (PM_10–100, PM_2.5–10, and PM_0.2–2.5 (µg·cm⁻²)) was measured with consolidated gravimetric techniques during spring, summer, and fall for 2160 leaf samples belonging to the basal, median, and apical part of the crown of 17 species located in the streets and parks of 2 European cities (Rimini and Krakow). On the same samples, the deposition (PM₁₀ and PM_2.5 (µg·cm⁻²·day⁻¹)) was evaluated according to a model based on the wash-off rain effect. Quercus ilex accumulated more PM_x than the other species in Rimini, while in Krakow, the highest accumulators were Pinus nigra for PM_10–100, Tilia cordata for PM_2.5–10, and Populus nigra for PM_0.2–2.5. Only in Krakow was the capture capacity of some species affected by the street or park growing condition. The basal leaves showed greater PM_10–100 accumulation than the median and apical ones. In Rimini, the total PM accumulation tended to increase throughout the year, while in Krakow, the opposite occurred. However, as the accumulation increased, the deposition decreased. The PM accumulation was reduced by rainfall and enhanced by the air PM concentration, while the wind speed effect was opposite, depending on the city. These findings are useful for directing decision makers in the design of greener, healthier, and sustainable cities.

The demand for saplings has risen in recent years as a consequence of massive planting campaigns targeted at increasing canopy cover. To test the hypothesis that seaweed extract can improve root biomass and length, an experiment was carried out at the ERSAF Regional Forest Nursery in Curno, Italy. The seeds of 5 woody species were planted in trays using a substrate amended with 0×, 1×, 2×, or 3× the label dose of a pure Ascophyllum nodosum extract. After germination, 6,400 seedlings were arranged according to a randomized complete block design with 10 blocks. After 1 growing season, plants were transplanted into 1.7-dm³ forest containers for 1 additional growing season. Root, stem, and leaf dry weights, total leaf area, total root length, and specific root length were measured over an 80-week period. Leaf gas exchange and greenness index were monitored for 78 weeks using an infrared gas analyser and a SPAD meter. Species differed for growth rate, biomass allocation to roots, and specific root length. The algal biostimulant increased stem and whole plant dry weights for 1 year only when applied at 3x the label dose. Significant effects on leaf gas exchange were found only at the highest dose and were mostly due to higher leaf greenness index than to lower diffusional limitations to photosynthesis. Results suggest that substrate amendment with Ascophyllum extracts may have short term positive effects on plant growth, likely due to a nutritional boost. However, they did not trigger structural changes in plant traits that can enhance transplant tolerance in the long run.

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 (A_leaf), transpiration, and stomatal conductance (g_s) were measured during spring, summer, and fall. The net CO₂ assimilation per unit of crown projection area and per plant were calculated by upscaling A_leaf 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.