Neglected puzzle pieces of urban green infrastructure: richness, cover, and composition of insect-pollinated plants in traffic-related green spaces

Context

In cities, insect-pollinated vascular plants enhance environmental quality, support pollinators, and provide essential ecosystem services for citizens. However, floral communities associated with traffic-related green spaces are rarely considered valuable elements of urban green infrastructure (UGI).

Objectives

The main objective of this work was to assess if traffic-related green spaces in Finland possess species-rich floral communities and can assist urban ecological restoration—converting degraded areas into functionally diverse ecosystems. Thus, we evaluated richness, cover, and community composition of insect-pollinated plants (emphasizing flowering ones) on traffic islands, parking lots, and road verges.

Methods

The assessment was performed during the mean flowering phenophase of insect-pollinated plants in the European boreal zone (July and August) using a standard quadrat (1 m²) placement method. We studied plants in urban and suburban locations of three highly populated (> 170 000 inhabitants) Finnish cities—Helsinki, Tampere, and Turku. There were 90 sampling sites with 15 replicates per location type in each city and five measurement replicates per green space (habitat) type. The species richness, cover, and composition were assessed in relation to location, habitat type, city, the average daily traffic (ADT), and distance to the road.

Results

Urban locations had lower total plant species richness and fewer indicator species (characterized only by a single indicator species) compared to suburban locations (characterized by five indicator species). Species richness of plants flowering during the time of the survey did not differ among locations. Traffic islands were richer habitats for flowering plants than road verges but did not differ from parking lots. Total vegetation cover and cover of insect-pollinated flowering plants increased with an increasing distance from the road. Vegetation cover differed among habitats being higher on road verges than on traffic islands irrespective of ADT. In all habitat types, the two most common flowering species were yarrow Achillea millefolium and autumn hawkbit Leontodon autumnalis which occurred at 70.2% and 67.8% of the sampling sites, respectively. However, the mean cover of the ten most common flowering species (when present) was low and varied between 1.5 and 9.5% per m².

Conclusions

Similar richness of flowering plants (but not total plant species) in urban and suburban locations might indicate equal importance of ecosystem services provided by flowering plants in cities irrespective of location. Because traffic islands and parking lots contain rich plant communities, they should be better integrated into UGI and valued by city planners. Management intensity in terms of grass cuts should be ecologically justified. An increase in the number of insect-pollinated plant species in urban traffic-related green spaces might help to improve environmental quality in cities in terms of increasing pollinator biodiversity and reducing pollution.