Research recently published in Journal of Applied Ecology shows how hedgerows and road verges can host more plant species than corresponding woodland and grassland. Lead author, Thomas Vanneste and Associate Editor, Pieter De Frenne highlight what this means for managers and policymakers.
Hedgerows and road verges are important habitats across the globe. Road verges cover an estimated 270,000 km² (0.2 %) of the earth’s land surface; an area equivalent to the entire United Kingdom. Their size, wildflower abundance and other characteristics vary tremendously and depend, for instance, on management, ownership structure and road properties.
Hedgerows are also ubiquitous around the world given their [past] use as livestock barriers, markings of land property, and wood production. Hedgerow densities are region-dependent but can amount to 24 m per hectare in rural parts of northern Belgium to 120 to 150 m per hectare in Essex, England. Hedgerow densities have reduced by more than 75 % in several parts of Europe since the 1960s.
Hedgerows and road verges provide many important benefits to nature and people. In an inspiring recent Review in Journal of Applied Ecology, Benjamin Phillips and colleagues quantified road verge ecosystem services. They conclude that road verges filter air and water, support pest control and crop pollination, provide health and aesthetic benefits, reduce erosion and sequester 0.015 Gt carbon per year. Concerning the latter, hedgerows are indeed important carbon sinks: they can contain up to 300 m³ of wood per kilometre length which equates to 60 to 100 tonnes of carbon per kilometre length in the aboveground biomass alone, depending on the dominant tree species. Because of the specific shapes of hedgerow trees that have much larger crown sizes and different allometries than in forest conditions, hedgerow carbon drawdown is severely underestimated in the IPCC reports. Current state-of-the-art research using laser scanning of hedgerow trees is trying to quantify and correct this effect.
Landscapes can hugely differ in the amount of hedgerows. Detail of two neighbouring landscape windows of 5 x 5 km² in northern France studied by Valdes and colleagues. While both landscapes are comparable in terms of the total woodland area, one landscape has a low amount of hedgerows and low connectivity among woodland patches, while the other landscape has a large amount of hedgerows and high connectivity. Their configuration has important implications for species distributions and colonisation capacities in such landscapes.
In addition to providing important ecosystem services, hedgerows and road verges also harbour many plant and animal species. There have been multiple studies that assessed plant distribution patterns in road verges and hedgerows, but we have now, for the first time, surveyed plots distributed along hedgerows and road verges at the continental scale. In the summer of 2017, we travelled across Europe with our field car inventorying plants in more than 300 plots, from France in the south to Norway in the north.
Once the results came in, we found that the hedgerows and road verges tended to host more plant species than the corresponding ‘[semi]-natural’ habitat, i.e. woodlands and grasslands in this case. This is mostly due to the larger diversity of generalists in the linear elements, whereas the number of true habitat specialists was slightly lower. On average, more than half of the plant species was shared between the linear elements and forests/grasslands, including also a large proportion of habitat specialists. This suggests that hedgerows and road verges may provide a valuable habitat for many plants that are normally associated with larger forests or well-managed semi-natural grasslands.
We also noticed that the environment influences plant biodiversity in hedgerows and road verges. Most importantly, our results underpin the key role of local habitat characteristics. For instance, we found that taller hedgerows with denser and shadier tree canopies hosted more forest specialists such as wood anemone (Anemone nemorosa) and dog’s mercury (Mercurialis perennis). More soil phosphorous, on the other hand, negatively affected the number of specialist plants.
What are the implications? Land managers can manage local characteristics of hedgerows and road verges as a tool to improve the habitat quality. In turn, we expect that management actions will increase the potential establishment of (specialist) plant species along linear landscape elements. Hedgerows and road verges can harbour diverse plant communities, including also many slow colonisers and species of conservation concern.
New policies aiming to preserve and [re-]establish these linear features are thus of utmost importance, given that they will help to promote biodiversity and ensure a sustainable delivery of ecosystem services in intensive agricultural landscapes.
We therefore suggest that managers should first promote the conservation of existing species-rich hedgerows and road verges and optimise their management. Thereafter, the emergence of new linear elements should be prioritised in the proximity of ancient hedgerows/road verges or forests/grasslands, to serve as potential source populations.
Together a complex of linear structures and remnant habitat patches could form a network that facilitates the movements of species and contributes to the conservation of isolated populations.
Read the full article, Plant diversity in hedgerows and road verges across Europe, in Journal of Applied Ecology.