Urban greenspace soils can provide a multitude of ecosystem services (ESS), such as retention of storm water, sequestration of carbon and nutrients, and de-toxifying harmful, urban-derived substances. Similarly, urban vegetation is - at a city level - reported to purify air by capturing and absorbing combustion-derived gases and particles in its foliage. However, it is largely unknown (i) whether different vegetation types modify urban soils and thus provide ESS differently, and (ii) whether urban air quality is locally – e.g. at park scale - affected by urban vegetation.
We studied the influence plant type (grass/lawn, evergreen trees, deciduous trees) on soils and their ecosystem services in 41 parks of varying ages in two cities in Finland. Soils were sampled for physical-chemical-biological properties and heavy metals close to each plant type up to 50 cm depth, and zero-tension lysimeters were placed in plant rhizospheres to collect water and elements in the leachates. Air quality was monitored in urban greenspaces in near-road environments in tree-covered and open areas. We hypothesize that 1) plant types, especially conifers, modify the soils differently resulting in divergent ESS between plant types, and (2) that air is cleaner under tree canopies compared to open areas.
Our results imply that, despite the ability of trees to reduce concentrations of road dust and other coarse-sized (>PM2.5) particles, urban trees seem ineffective (PM2.5), and even increase the levels of traffic-derived gaseous pollutants (NO2, VOCs, PAHs). This inefficiency is likely due to trees and other vegetation reducing wind speed and weakening the dilution of air pollutants, thus worsening local air quality at the height of where most humans are breathing.
Urban vegetation had a clear effect on soil quality in older parks: trees, particularly evergreens, lowered soil pH and soil water content and increased soil organic matter, carbon and nitrogen sequestration compared to lawns. This clear plant-derived influence was also reflected in (i) metal and nutrient pools and (ii) their mobility in the park soils, but these plant type effects depended on park age. In many cases the vertical flow of water was the most influential factor controlling the mobility of metals and nutrients in the soil profile.
Our study suggests that urban trees can be important in ESS provision, but that managing urban greenspace for better ecosystem service provision requires specific knowledge on e.g. plant types and the spatial structure of urban greenspaces within urban landscapes.