Accounting for Nature: Assessing habitats in the UK countryside
4.   Boundary and Linear Features

Changes in habitat condition –hedges and roadside verges

4.26 In addition to information about the stock and change of Boundary and Linear Features, CS2000 can provide further insights into the ecological character of these habitats though the analysis of the vegetation plot data collected as part of the field survey.

4.27 As described in Chapter 1 several different types of plot were located within the field survey squares to assess the range of vegetation conditions found there. Two of these plot types are particularly valuable for assessing the Boundary and Linear Features, namely those located along the hedgerows themselves, known as ‘hedge plots’ and those sampled along roadside verges, the so called ‘verge plots’. These plots can be used to understand conditions in the narrow strips of semi-natural vegetation associated with these widespread linear habitats.

4.28 When reviewing the data from the hedge and verge plots, it is important to note that judgements about the condition of vegetation, and the way it is changing over time, must take account of the landscape context in which they occur. In some areas, particularly those that are intensively farmed, linear habitats can be important reservoirs of biodiversity, even though the area covered may be small.

Figure 4.3: The composition of hedge plots sampled in Great Britain in both 1990 and 1998 in terms of the major vegetation types.

[Click here to reveal the underlying data]

Hedgerow condition

4.29 Chapter 2 has described how vegetation data can be used to assess aspects of habitat condition. Two hedge plots, measuring 10m x 1m, were located in each survey square in which hedges were found. They record the vegetation growing along the bottom of the hedges as well as the shrubs in the hedge itself. The same plots were surveyed in 1990 and 1998. The hedge plots were not surprisingly dominated by ‘lowland wooded’ and ‘tall grass and herb’ vegetation types (see Figure 4.3). The proportion of hedge plots comprising the tall grass and herb vegetation type increased between 1990 and 1998.

4.30 Results for changes in vegetation condition for hedges characterised by tall grass and herb vegetation and lowland wooded vegetation in England and Wales are shown in Figure 4.4. The hedge plots comprising lowland wooded vegetation in 1990 showed few significant trends in the condition indicators between 1990 and 1998. There was an overall reduction in competitive species and an increase in ‘ruderal’ species typical of disturbed ground, perhaps an indication of management impacts on these wooded hedges.

4.31 The hedge plots that were characterised by tall grass and herb vegetation in 1990 showed a statistically significant, 12% decline in species richness. There were reductions in the frequency of plants that are important food sources for butterflies and ‘ruderal’ plants that are able to tolerate more disturbed conditions. The frequency of shade tolerant species increased, indicating a general increase in the size of hedge canopies and undergrowth, possibly because of less intensive management of the hedges. These changes were concentrated in the eastern lowlands of England (Environmental Zone 1) in hedges adjacent to both the Arable and Improved Grassland Broad Habitats. There was also some evidence of increasing nutrient status, but only in hedges adjacent to Improved Grassland in the western lowlands of England and Wales (Environmental Zone 2).

4.32 In Scotland the number of hedge plots was small, and no significant changes were detected.

4.33 The results suggest that the value of hedgerow vegetation as a habitat for wildlife is generally stable but there is some evidence for decline in plant diversity of tall grass and herb vegetation associated with hedgerows. The analysis completed so far does not enable an assessment of whether the balance of planting and removal, and restoration and degeneration, described above (see paragraph 4.21) has had an affect on the overall ecological condition of hedges.

4.34 A further set of hedgerow vegetation plots were sampled for the first time in Great Britain by CS2000. These plots were intended to establish a baseline for assessing the diversity of tree and shrub species in hedges. In each sample square with hedges, up to ten 30m lengths of hedge were surveyed and the woody species were recorded. Diversity of woody species in a standard 30m length of hedge is used in the definition of species-rich hedgerows in the BAP Habitat Action Plan and in the Hedgerow Regulations. The preliminary results of this survey show that in the sample as a whole about a quarter (26%) of hedges had five or more woody species and would therefore be defined as a species-rich hedge (Figure 4.5).

Figure 4.5: Diversity of woody species in hedgerows. The chart shows the cumulative frequency of plots with different numbers of woody species per 30m length. Thus the chart shows that 86% of plots had two or more woody species and that over a quarter (26%) had five or more woody species. One plot had eleven woody species. Nearly 2500 plots were included in the analysis. The preliminary results presented here are not weighted according to the area of the sampling strata or length of hedges.

[Click on the graph to reveal the underlying data]

Roadside verges

4.35 Roadside verge plots are characterised by a wide range of vegetation types in different environmental situations throughout the country (see Figure 4.6). The most widespread are the tall grass and herb, grassland and infertile grassland vegetation types. Road verges provide an important refuge for unimproved, species-rich infertile grassland, which has generally declined in enclosed farmland due to agricultural intensification. Between 1990 and 1998 there were minor net shifts from acidic upland vegetation to infertile grassland, from infertile grassland to tall grass and herb vegetation and from tall grass and herb to wooded vegetation. This resulted in increases to the overall proportion of verges comprising tall grass and herb and lowland wooded vegetation.

Figure 4.6: The composition of roadside verge plots sampled in Great Britain in both 1990 and 1998 in terms of the major vegetation types.

[Click the graph to reveal the underlying data]

4.36 Some of the changes in condition recorded by CS2000 for the roadside verge plots are shown in Figure 4.7. For ease of presentation only the changes in species-richness and fertility for the three most widespread vegetation types are shown.

4.37 Several different statistically significant trends can be seen in the verge plots at national scales. In England and Wales, species richness declined by 9% in tall grass and herb and by 7% infertile grassland verges. Species richness also declined by 11% in infertile grassland verges but only in the westerly lowlands (Environmental Zone 2). In tall grass and herb verges, vegetation change favoured species associated with less fertile conditions whereas the reverse was true in infertile grassland verges. In the Scottish lowlands (Environmental Zone 4) vegetation changes in infertile grassland road verges favoured plants associated with increased nutrient levels, more disturbance and an overall increase in species richness.

4.38 The implication of increasing nutrient levels and local reductions in species-richness in some infertile grassland verges is a particular cause of concern. Such roadside verges are an important refuge for unimproved grassland species in intensively farmed landscapes. The losses in plant diversity in tall grass and herb and fertile grassland verges were also associated with reductions in the availability of food plants for butterfly larvae.