The influence of aspect, soil compaction and management on the grassland species composition of roadside verges, in relation to improving slope stability

Abstract

Roadside verge vegetation is a complex, unusual community, poorly understood and researched and a typical example of a “novel ecosystem”. The land has been altered by humans, the topsoil introduced from elsewhere, a seed mixture added and a management regime implemented. Vegetation is a cost effective, sustainable method for improving slope stability on steeper roadside slopes. On newly constructed roadside verges its rapid establishment prevents soil erosion and provides an aesthetic cover. The standard grass seed mixture sown includes grass species which are quick growing, vigorous and competitive. The belowground root system increases soil strength and the aboveground canopy reduces surface erosion, although soil compaction can restrict root and shoot growth. Morphological differences between grasses and wildflowers allow the root systems to utilise different soil strata and the aboveground canopy to fully exploit aboveground niches. Therefore maybe a species-rich grassland seed mixture should be sown to improve slope stability? The BIONICS Embankment at Nafferton Farm was used to investigate aspect and soil compaction on the establishment of a species-rich grassland. A second smaller Mesocosm experiment investigated the effects of aspect, soil compaction and cutting regimes on the aboveground composition and biomass, and the belowground rooting depths of two different seed mixtures. Also a survey of the plant communities and environmental characteristics of grass cuttings was carried out along the A303 and A38 in Wiltshire and Devon. On the BIONICS embankment 14 generalist plant species germinated from the seed mixture and arable and injurious weeds colonised. More wildflowers, e.g. Lotus corniculatus, grew on the south-facing slopes while more grasses and some herbs, e.g. Ranunculus repens, grew on the north-facing slopes. Ellenberg Indicator Values for fertility and moisture were higher on north-facing slopes, with light values greater on south-facing slopes. Lolium perenne, Medicago lupulina and Achillea millefolium were associated with the less compacted subsoil. In the Mesocosm experiment, L. perenne, Cynosurus cristatus, Phleum bertolonii, Daucus carota, Leucanthemum vulgare, Plantago lanceolata, Rumex acetosa and Sanguisorba minor were common. More wildflowers were found on the south-facing plots, such as L. vulgare and P. lanceolata, whereas the grasses L. perenne and C. cristatus preferred the north-facing plots. Frequent cutting increased species richness and diversity, favoured C. cristatus and L. 4 vulgare and reduced L. perenne and P. lanceolata. Grasses such as L. perenne and P. bertolonii were more reduced by compaction than the wildflowers such as Rhinanthus minor and Trifolium pratense. Plots sown with a mixture of grasses and wildflowers had greater above-ground biomass than the grasses-only plots and on the north-facing aspect had greater biomass than the flat and south-facing plots. Plots just sown with grasses had higher biomass on the flat plots. R. minor did not germinate well in the plots with high biomass but survived in regularly cut plots. Plant roots in the Mesocosms grew down the whole soil profile in all treatments (40 cm). The total root mass was greater in the grasses-only plots in comparison to the grass and wildflower plots. However, the grass roots were smaller and finer, whereas the wildflower roots were bigger, yet lighter. Although the grasses-only plots had greater root mass, these plots had lower aboveground biomass, while the grass and wildflower plots had lower root mass, but greater aboveground biomass. Bulk density (BD) and Penetrometer resistance (CI) was greater in the compacted treatment, although this did not reduce root mass, or reduce aboveground biomass. However, CI was greatest in the flat and south-facing plots where root mass was less, and CI was lower in the north-facing plots where root mass was greater. The roadside survey showed that natural colonisation could produce species-rich habitats over time with 116 species found. Arrhenatherum elatius was the commonest grass, Cirsium arvense and Senecio jacobaea were frequent and scrub encroachment was widespread. These roadside verges were not cut frequently enough to prevent succession into scrub and woodland. Centaurea nigra, L. vulgare and D. carota had higher cover on south-facing slopes, contributing to the higher diversity on this aspect. Signs of bareground and desiccation were greater on south-facing slopes. Grasses were more abundant on the north-facing slopes leading to mesotrophic communities on this aspect. Many sites had a unique and unusual combination of species that didn’t fit into the National Vegetation Classification (NVC). Roadside embankment construction does not hinder the establishment of a species-rich seed mixture, although a regular cutting regime is required of at least once a year to prevent scrub encroachment and to reduce the spread of injurious weeds. A grass and wildflower seed mixture containing a range of functional types will have greater aboveground biomass, with a denser canopy and heterogeneous root system, which will help to prevent surface erosion and runoff and increase soil strength.