The Ecology of Hedgerows and Field Margins

John W. Dover, John W. Dover

• Publisher: Routledge
• Year: 2019
• ISBN:
  9781351355490
• Edition: 1
• Topic: Biological Sciences
• Subtopic: Environmental Conservation & Protection
• Index: Biological Sciences

Hedges and field margins are important wildlife habitats and deliver a range of ecosystem services, and their value is increasingly recognised by ecologists. This book reviews and assesses the current state of research on hedgerows and associated field margins.

With the intensification of agriculture in the second half of the last century, field sizes were increased by amalgamation and the rooting out of hedges, synthetic pesticide and inorganic fertiliser use increased, and traditional methods of hedge management were largely abandoned. The book is split into two main sections. The first deals with definitions, current and historic management, the impact of pesticides, the decline in hedge stock and condition, and new approaches to hedge evaluation using remote sensing techniques. The second section explores the pollination and biological pest control benefits provided by hedges and field margins and examines the ecology of some of the major groups that are found in hedgerows and field margins: butterflies and moths, carabid beetles, mammals, and birds. A case study on birds and invertebrates from a research farm managed as a commercial enterprise, but which attempts to farm with wildlife in mind, brings these themes together. A final chapter introduces the neglected area of hedges in the urban environment.

The book will be of great interest to advanced students, researchers and professionals in ecology, agriculture, wildlife conservation, natural history, landscape, environmental and land management.

1
Introduction to hedgerows and field margins

John W. Dover

Background

This book is primarily about the ecology of hedgerows, but we have included ‘field margins’ in the title of the book because to understand what happens in hedgerows, it is necessary to consider the influences of the adjacent land use, and these are much more complex than when Ernie Pollard, Max Hooper and Norman Moore published their seminal work Hedges in 1974 (Pollard et al. 1974). Indeed, in many cases in order to understand what is happening at the local level, it is important to consider the wider landscape-scale issues. Since the publication of Hedges, compulsory set-aside has come and gone (at least in the EU; Daugbjerg & Swinbank 2011), several classes of pesticide have been developed and been superseded by others (Jeschke et al. 2011), farmland intensification has continued with continuing loss of hedgerows either through removal (Figure 1.1) or lack of management (Carey et al. 2009a), farmland wildlife has significantly declined (Stanton et al. 2018) and responses such as agri-environment schemes have been introduced in an attempt to stem the losses (Anonymous 2009). In addition, the biodiversity and wider value of hedgerows to farming and society have been increasingly recognised through the concept of ecosystem services (Reid et al. 2005). Hedgerows are given protection in a number of countries (Baudry et al. 2000), though such protection may be partial. In the UK, for example, whilst hedgerows are designated as priority habitat (JNCC 2011), they must exhibit specific characteristics to be eligible for protection, and protection is not available for certain types of hedgerow such as those surrounding curtilages (DOE 1997; Marrington 2010).

This chapter briefly introduces readers to the different components of hedgerows and field margins and some important concepts. Succeeding chapters concentrate on particular issues; in some the emphasis is strictly on one component of the system (such as Chapter 3 on the management of hedges or Chapter 6 on softening the impact of agrochemicals on hedgerows and field margins by modifying their use at crop edges), whilst others consider the ecology of the hedgerow/field margin system as a whole in relation to a particular species or functional group. Unfortunately, whilst we cover a number of species groups, we do not have space to cover them all so, for example, we do not have a chapter on herptiles (amphibians and reptiles) despite field margins probably being important for them (Reading & Jofre 2009; Carthew et al. 2013; Mendenhall et al. 2014), especially riparian margins (Maisonneuve & Rioux 2001; Prosser et al. 2016), although perhaps not always positively (Joly et al. 2001). Our final chapter introduces a rather ignored topic – urban hedgerows. As components of urban green infrastructure, hedges deliver valuable ecosystem services, including being components of sustainable urban drainage and capturing particulate air pollution (Dover 2015), but they are also valuable as urban wildlife habitats (Chapter 14).

Figure 1.1 Hedge removal in 2009 to enlarge a field for arable production in Warburton, Cheshire, UK.

Source: © John W. Dover

What is a hedgerow?

Strictly, ‘hedge’ and ‘hedgerow’ are different terms – the hedge being the woody component of a field boundary, whilst the hedgerow includes the herbaceous component, the bank and ditch – but this distinction is often ignored and the terms used synonymously (Forman & Baudry 1984). Hedgerows can be found throughout the world including North and South America, Africa, Asia, Australia and Europe (Burel 1996; Baudry et al. 2000; Truong & Pease 2001; Breen 2017); unfortunately, the term ‘hedge’ has multiple meanings and varies with geographical location, so in the UK it has been used to describe managed or unmanaged shrubby boundaries, stone walls or turf banks (Pollard et al. 1974; Greaves & Marshall 1987); in northern France hedgerows are typically lines of trees, and the landscape as a whole is termed the ‘bocage’ (Baudry et al. 2000); in Spain, olives are often grown in dense parallel lines and explicitly termed ‘hedgerow orchards’ (Gomez-del-Campo et al. 2017). In over 100 countries, including Thailand, Venezuela, Vietnam, Bangladesh and India, vetiver grass (Vetiveria zizanioides) has been planted densely to produce ‘hedgerows’ which are used in soil and water conservation, and in bioengineering and environmental protection projects (Truong & Pease 2001). In this book, unless otherwise stated, the term ‘hedge’ is used to describe linear strips of managed or unmanaged woody vegetation (shrubs and/or lines of trees, also termed woody linear features, or WLF; Maskell et al. 2008), and other boundary structures are given names that are reasonably descriptive of their morphology, e.g. ‘stone wall’. Müller (2013) provides a classification of the field boundaries of 32 European countries that he surveyed; within it he identifies eight general types of woody vegetation (low, medium and high hedges, tree hedges, rows of trees, single trees and/or shrubs, pollard trees and high pruned trees), over 100 different planting styles of hedge and 135 management styles. A good working definition of a hedge is used in the UK Countryside Survey (Maskell et al. 2008), i.e. to be considered a hedge a WLF should have a minimum length of 20 m and be no more than 5 m wide at the base; if composed of a line of trees, it should only be one tree wide. Note that this differs somewhat from the definition used by Baudry et al. (2000) who considered that a linear feature could only qualify as a hedgerow if it was managed in some way. Also, using the Countryside Survey definition, linear features wider than 5 m would be considered to be small shelterbelts; nevertheless such features will probably function in much the same way as narrower structures, and Baudry et al. (2000) note that North American prairie shelterbelts, planted in response to soil erosion that led to the Dust Bowl in the Great Plains (Gardner 2009), are analogous to hedgerows, with many similar features and functions (Guo 2000).

Definitions and components

The nomenclature of hedges and field margins is deceptively simple yet contains a few pitfalls. As a result, we have included in this chapter a diagram of an arable field margin to illustrate the various components (Figure 1.2). It is interesting to track the evolution of diagrammatic content over time, as it reflects our increased knowledge of the ecology of field margins and the increased emphasis on managing them for biodiversity/ecosystem services rather than simply agricultural production. In Greaves and Marshall (1987), field margins were divided into three main areas: the field boundary, the boundary strip and the crop, with the latter two as fairly uncomplicated structures. In Marshall and Moonen (2002), the boundary strip became the ‘field margin strip’, a term that embraced a wide range of potential management options, and the crop now contained the term ‘conservation headland’ at its edge. In Figure 1.2, whilst I have not included a farm track in the boundary or field margin strip (it can still be present), I have added a green lane and separated the boundary/field margin strip from the crop with a sterile strip. Whilst we identify different components of hedgerows and field margins, it is probably worth remembering that whilst the different components have their own individual attributes, it is their collective value which is most important. Wolton et al. (2013) examined the relationship of 107 species considered as priority and farmland indicator species associated with hedgerows with five elements of hedgerows (trees, shrubs, hedge base, field margin and ditches). They found that 65% of species were dependent on more than one hedge component, and 35% were dependent on three or more.

Figure 1.2 Diagrammatic representation of an arable field margin to show the various components that may be present (after Greaves and Marshall 1987; Marshall and Moonen 2002). The field margin combines the field boundary, such as a hedgerow or green lane, AND any associated boundary strip. Ditches may be located on both sides of a field boundary (and would be considered to be part of the boundary) and also found either side of the track inside a green lane. WR = wheel ruts typically made by farm vehicles. The field boundary in the diagram is a hedgerow (i.e. the hedge [shrubs/trees] + associated raised vegetated bank); other common boundary features include dry stone walls, grass banks (baulks) or strips, windbreaks, and wood edges. The boundary strip is typically composed of a sown grass strip, buffer or other agri-environment strip often sown with a mix of species e.g. for pollinators or for game cover. ‘ss’ = sterile strip: an area of bare ground created either by use of a broad-spectrum herbicide or by mechanical means (rotovation). The crop margin/headland may be modified by changing agrochemical inputs to allow the growth of broadleaved plants (whilst controlling most grass species) within the crop to create ‘Conservation Headlands’.

Crop influences

The type of crop and its management have a profound impact on the flora and fauna of field boundaries. The major and most obvious division of crops is into grassland and arable. In the latter, hedgerow/field boundary removal to improve efficiency of crop production has led to some areas being largely denuded of semi-natural landscape features and others left with a much reduced stock, and the use of synthetic fertiliser and pesticides has drastically modified field boundary animal and plant communities (Robinson & Sutherland 2002; Petit et al. 2003). In grassland systems, the main division is into grassland that is used for grazing and that used for primarily for grass production (hay, silage, zero-grazing) although late-season ‘aftermath’ grazing or early spring grazing may still occur. Field boundary vegetation in permanent, extensive pasture is undoubtedly the most valuable for biodiversity, provided stocking densities are reasonable. Whilst traditional hedge management (laying) was designed to make hedges stockproof, the expense of maintaining hedges in this way has led to the wholesale use of fences. Sheep, in particular, are very good at denuding basal hedgerow vegetation and eroding hedge banks if not controlled. Unfortunately, in some cases, farmers have been known to spray out boundary vegetation around hedges to prevent shorting of poorly sited and poorly maintained electric fences. Protecting the herbaceous field boundary vegetation and palatable shrubs in field boundaries from being completely eaten-out for biodiversity conservation purposes requires protective measures (Figure 1.3). Field boundaries around intensive grassland can suff...