Loss and degradation of habitats but also increasing habitat fragmentation have caused a strong decline of plant diversity in Germany. Despite possible limitations due to their vicinity to roads, roadsides could be valuable habitats for many plant species for several reasons: They are ubiquitous, consist of different habitat types and are well connected along the road network. Thus, roadsides came more and more into focus as alternative habitats and corridors for threatened plant species in recent decades.
The aim of this study was to evaluate the value of roadsides for plant conservation in Northern Germany. We investigated the plant species richness and composition of roadside vegetation along highways and main roads. Therefore, we divided the roadside vegetation into open vegetation and shrubberies. To assess the value of roadsides for plant conservation, we compared fifty roadsides to habitats without roads in their immediate vicinity: Open roadside vegetation was compared to open linear habitats and species-rich grasslands, road shrubbery was compared to linear habitats rich in shrubs and forests.
Our results showed that roadsides and linear habitats away from roads had a similar species richness and composition. While species richness of open roadside vegetation was comparable to species richness observed in species-rich grasslands, plant species richness of road shrubberies was even higher than species richness in the selected forests. Despite the high species richness in roadside vegetation, the number of threatened and specialized species was comparatively low, whereas high numbers of ruderal species and several neophytes could be found. Specifically searched valuable roadsides also showed high amounts of neophytes and ruderal species, but could reach higher proportions of threatened and specialized species than ordinary roadside vegetation. However, based on different studies we estimate that not more than 1 % of roadsides can actually be classified as valuable.
According to these results, we conclude, that present roadsides play a minor role as alternative habitats for threatened and specialized plant species in Northern Germany, although the potential for plant species conservation could be increased through purposeful design and management.
roadside vegetation, plant diversity, threatened species