How can road verges contribute to biodiversity conservation?

Road verges can be rich in species of different groups of organisms and thereby contribute to biodiversity conservation. Species richness, however, differ between verges. Knowledge about which environmental factors, including management activities, that account for high biodiversity, is crucial for our possibilities to construct and maintain roads of high conservation value. In this talk, we summarize a review of such knowledge performed in a European CEDR-funded project (EPIC-Roads). We also present data on vascular plant diversity and vegetation composition from a Swedish empirical project (TRIEKOL), that analyse road verge vegetation in relation to management and other environmental variables. The aim is to provide insights in what can make a road verge biodiversity-rich.

Roadside vegetation is more or less successional and different environmental factors are important for its composition during different stages during the succession. The succession starts with construction or maintenance activities that expose bare soil, and proceeds through increasing competition between species, reduced opportunities for establishment, and raising nutrient levels due to accumulation of organic matter. The rate and progression of vegetation change is influenced by, e.g., soil nutrient levels, mowing of the vegetation, and occurrence of competitive species. Dry and nutrient poor soils can maintain high species richness of drought-tolerant species for a very long time, with limited need for mowing. Slightly more nutrient-rich soils can maintain species-rich vegetation for a long time if they are regularly mown, and domination of tall plants thereby counteracted. Rich soils, or roadsides colonised by competitive grasses or invasive species, undergo rapid succession towards species-poor tall vegetation in spite of mowing.

In addition to these environmental factors, sun exposure is important, through effects on evapotranspiration as well as on cover of bryophytes and lichens. Highly different types of vegetation can often be seen at different sides of an east-west stretching road. Which species that colonise, especially at an early stage, depends on the species pool in the surrounding landscape, mainly the adjacent habitats. Active seeding of new constructions, as well as establishment of invasive plants, strongly counteracts spontaneous colonisation and development of species-rich vegetation.

Many old roads may have been colonised from species-rich habitats that are now gone or rare, either natural habitats or habitats formed by pre-industrial agriculture, such as unfertilised pastures and hay-meadows. The species composition in verges may therefore depend on the age of the road. In particular, in transformed landscapes new roads cannot be expected to become as species-rich as old roads that were constructed when the landscape contained more species-rich habitats.