Grassland character and communities

1.1 Introduction

Grasslands are open habitats, naturally dominated by grasses and herbaceous plants, with sedges, rushes, bryophytes and, sometimes, occasional shrubs. Most swards have a continuous cover, in some cases reaching heights of up to 1 m. Grassland vegetation extends over a broad range of soil types, usually categorised as acidic (calcifugous), neutral (mesotrophic), calcareous (calcicolous) or marshy and wet.

A further major subdivision defines the extent to which the habitat has been modified by agricultural management practices. Collectively, some 37% of the land area in the UK is classified as grassland (Countryside Survey 2009), although the vast majority of this is either improved, or semi-improved. The most 'natural' grasslands are semi-natural (sometimes referred to as unimproved), representing just 2% of the UK grassland area, a total of about 1.7 Mha (Bullock et al. 2011). These communities have long histories of traditional, usually low-intensity agricultural management. They are often species-rich, supporting only naturally occurring species; many were designated as areas of conservation concern and included in the UK Government's Biodiversity Action Plan (BAP) priority habitats (Figure 1.1). The UK BAP was succeeded in 2012 by the UK Post-2010 Biodiversity Framework, which represents a broad enabling structure for conservation action across the four countries of the UK, at country level. In contrast, most improved grasslands have been ploughed, fertilised or reseeded, usually with varieties of Perennial Rye-grass and White Clover. Transitional between these two extremes are semi-improved grasslands that have been partially modified but retain some of their original floral composition.

Soil conditions are governed by the parent material, i.e. the immediately underlying geology or drift deposits, and also by topography, drainage and land use. One of the most critical factors appears to be pH, which governs basic fertility and the solubility and availability of a wide range of elements (Hopkins 2003).

Calcium-rich limestone and chalk support calcareous grassland and some of the most species-rich communities. Base-rich igneous rocks also occasionally support calcareous grassland. Calcareous soils with pH values above 6.5 are mostly humic grey and brown rendzinas. Soils associated with eroding limestone may have high levels of free calcium carbonate, for example, supporting Sheep's-fescue–Carline Thistle grasslands, classified as calcareous grassland (CG)1 by the National Vegetation Classification (NVC). In contrast acidic, siliceous, low-calcium soils overlying sandstones, igneous rocks such as granite, or superficial deposits such as sands and gravels, support acid grassland. Dry acid grassland soils are usually free-draining, base-poor mineral soils within the pH range of 4–5.5. Acid grasslands contain tolerant species such as Mat-grass and Tormentil, known as 'calcifuges' (lime-avoiders), while the greater diversity of calcicoles (lime-lovers), such as Salad Burnet and Common Rock-rose, are specialists of species-rich calcareous grassland. Where the common factor is oligotrophic soils, a few species may occur in both acid and calcareous grasslands, for example, in chalk heaths. Soils that are neither strongly acid nor base-rich make up neutral grassland communities that can also be species-rich. Their productivity is much higher than most calcareous and strongly acidic soils, which is why so much neutral grassland has undergone agricultural improvement. These simple definitions based on soil pH imply sharp boundaries between communities on the three soil types; in reality the composition of plant species changes subtly over a continuum of soil pH. An example of this is the transition between calcareous and mesotrophic grassland vegetation, where the thin rendzina soils of a steep chalk escarpment give way to deeper soils of (upper) periglacial deposits or (lower) valley colluvium along the slope. Even at a single location, variations in different grassland communities may be found relating to natural differences in soil conditions.

Other properties of soil, such as depth and moisture levels, are also important in determining the vegetation type. Drainage has a key role in determining the species composition of British grasslands. Many communities, such as the lowland calcareous grasslands, are classified as 'dry grasslands' and are clearly associated with free-draining soils. In contrast, fen meadow and rush-pasture communities, such as the Soft Rush/ Jointed Rush–Common Marsh Bedstraw (NVC M23) and Purple Moor-grass–Meadow Thistle (NVC M24) are classified as 'wet' grasslands. In other cases, the distinction between dry and wet communities may be less obvious. In lowland England, the gradient between the Crested Dog's-tail–Common Knapweed and Meadow Foxtail–Great Burnet hay meadow communities is largely determined by fluctuations in the water table. In this book, we focus on the restoration and management of drier grassland communities, as described in Sections 1.4–1.7.

1.2 The origins of semi-natural dry grassland communities

Several classifications have been used to describe dry grassland vegetation types in Britain, all of which have their merits. The NVC, widely adopted by ecological practitioners, provides systematic descriptions of the major grassland communities (Rodwell 1992). Within these, dry grassland types may comprise 28 semi-natural communities, plus three semi-improved communities.

The origins of shade-intolerant plants in grassland communities are still not well understood. Some authorities believe it unlikely that any higher plants or animals survived in Britain during the maximum of the last glaciation, between approximately 18,000 and 14,500 radiocarbon years before present (BP) (Buckland et al. 2005). Others have suggested that the tundra of southern Britain could have supported an arctic flora of sedges, dwarf willows and dwarf birches at this time (Marren 1999), while Peterken (2013) goes further, listing a number of herbs present during early interglacial periods, many of which survived in refugia during the last glaciation. The climate then began to warm, and plants and animals are likely to have migrated north from refugia in southern and south-eastern Europe. A 'brief' return to cooler, then glacial conditions over c. 13,500 to 10,000 radiocarbon years (Buckland et al. 2005), a time known as the 'Younger Dryas', would have temporarily slowed the process of colonisation.

Pollen evidence has been used to investigate the vegetation in Britain during the Younger Dryas and early Holocene periods. Some of today's open-ground species, notably Mountain Avens, after which the Dryas was named, were present in the steppe-tundra vegetation of the Younger Dryas. The climate warmed rapidly in the early Holocene (starting around 10,000 BP), probably due to changes in ocean currents in the North Atlantic, suddenly creating very favourable conditions for the continued northward migration of plants and animals. After the retreat of the ice, the tundra vegetation in southern Britain developed into grassland, then scrub and eventually woodland; descendants of ancient lowland 'alpine meadows' may still survive today in the Pennine Dales and in some Scottish glens (Marren 1999). Marren also highlighted the herb-rich meadows found on steep and precipitous carboniferous limestone slopes in northern England. These are dominated by Meadowsweet and False Oat-grass (MG2), in which Jacob's-ladder is a constant in one subcommunity, which Pigott (1958) identified as a very rare example of 'natural' grassland.

By 9,300 years ago, as the climate continued to warm and soils developed, forest extended over most of Britain, and a period of stability continued for about 2,800 years (Rackham 1990). Historical evidence suggests the so-called 'wildwood' formed a wellconnected mosaic of forest cover, prior to any significant human disturbance. For many years, historians assumed that 'grassland communities' survived along river margins, on inland and sea cliffs, coastal dunes and above the treeline. Within the wildwood itself, they would have been confined largely to glades created by falling trees, maintained by large herbivores such as deer and Aurochs. Although relatively small in area when compared to the vast expanse of the wildwood, these open habitats may still have supported a large pool of plants, insects and other open-ground species, able to recolonise areas cleared by humans at the start of the Neolithic in Britain, roughly 4,000 BCE. At this time, the creation of pasturage, and the subsequent adoption of arable farming would have opened up the landscape, allowing shade-intolerant plants which had survived in woodland glades to flourish in the grazed landscapes, and some to become arable weeds.

The Great Orme, a carboniferous limestone headland in North Wales exemplifies the importance of coastal refuges. Cliff faces and ledges of every aspect support a range of locally rare grassland plants, together with endemic races of the Silver-studded Blue and Grayling butterflies, strongly implying that these communities may have existed since the early Holocene (Thomas 2009). Similar limestone crags across Britain would have collectively supported a diverse range of calcareous species during the peak of forest cover. Today, many rare grassland plants are confined to shallow soils on limestone, where they would presumably have avoided the heavier shading of the wildwood, even though the limestone pavement was probably sparsely wooded (Piggot and Piggot 1959). The closest present-day habitats to these prehistoric natural grasslands are those occupying harsh and inaccessible sites, such as rock ledges and outcrops where the soil is too thin and the site too exposed to have been colonised by shrubs and trees. Peterken (2013) identified just such a site on the Seven Sisters, a group of limestone towers in the Wye Valley. The grassland that occupies the tops, clefts and ledges, is species-rich and includes many rare plants. Similarly, many wetland species might have survived in the wet meadows, mires and bogs of river margins and valley bottoms.

It has been suggested that the wildwood of northwest Europe resembled a 'half-open' landscape, with large areas of grassland and heathland maintained by large numbers of large herbivores (Vera 2000). Others have concluded that while some parts of the British wildwood may have resembled modern wood pasture, the majority would probably have been closed high forest, with a mixture of temporary and more permanent glades (Kirby 2005). This was further supported by an extensive review of palaeoecological data from European primeval forests (Mitchell 2005). The smaller open areas, which undoubtedly existed, must have allowed the persistence of some plants that we would today consider as natural meadow species, and may have contained meadow-like grassland (Peterken 2009; 2013).

When people first started to fell trees in the wildwood to graze animals, new assemblages of plants must have developed under the new management systems. Species already present might have included plants which are widespread in woodland today, such as Yorkshire-fog, Common Bent, Creeping Buttercup and Tormentil. Similarly, species less common in lowland meadows, but also found in woods today, include Common Twayblade and Early-purple Orchid. Species we normally associate with 'woodland', such as Wood Anemone and Bluebell, are also meadow species in some parts of Britain. Shade-intolerant species present in woodland clearings will have been augmented by colonisation from grassland refuges outside the wildwood, such as cliffs, dunes and wetland river margins.

Evidence from pollen studies on the Isle of Skye (Birks 1973) showed that tall herb communities present on the island during the Late Glacial Maximum period contained both woodland species, such as Dog's Mercury and Herb-Paris, as well as others characteristic of modern hay meadows, such as Globeflower and Melancholy Thistle. Today, some of these species are more characteristic of woodland, others of hay meadows. Peterken (2009) identified a strong link between pre-Neolithic natural communities and historic Yorkshire Dales hay meadows, also known as upland hay meadows (predominately MG3). He describes these meadows, with their characteristic Wood Anemone, Pignut, Bluebell, Wood Cranesbill, Globeflower, Water Avens and Melancholy Thistle, as 'woodlands lacking only the trees'. Although some species were lost in the conversion and others were gained from refuges outside, it is likely that these northern hay meadows are closely associated with the original wildwood.

The link between woodlands and meadows is clearly strong, despite the fact that they are usually considered as distinct ecological communities, and treated as such by ecologists and other professionals. It is important to remember that virtually all our natural grassland has been modified at some stage by man. The landscape in Britain today, and indeed the climate, no longer resemble those during the Late Glacial or early Holocene, or the time when the wildwood covered most of Britain (Thomas 2009).

1.3 Dry grassland succession

1.3.1 Primary succession

Succession describes a series of developments in ecosystem structure and composition over time known as seral stages, which in Britain's temperate climate usually result in a relatively stable 'mature' woodland ecosystem. Succession starting with bare substrates that have not previously supported an ecological community is termed primary succession. Sand dunes represent a good example of primary succession in Britain, as it is possible to see all stages in a single location. The first dune colonists are pioneers, such as Sand Couch and Lyme-grass, which may be outcompeted by Marram as it stabilises windblown sand dunes at the 'yellow dune' stage. Marram may be accompanied by species such as Sand Sedge, Sea Bindweed, Portland and Sea Spurges, and plants typical of waste ground, such as various hawkbits, hawkweeds, thistles and ragworts. During the subsequent 'grey dune stage', Marram dies out as new sand deposition ceases and the dunes are colonised by mosses, lichens and calcareous plants, such as Viper's-bugloss, Wild Thyme and Common Centaury. Older dunes may become increasingly acidic, and characterised by species such as Gorse, Tormentil and Heather. With the build-up of humus, the soil becomes capable of supporting woody plants, and low scrubby growth dominated by Creeping Willow will eventually develop, possibly followed by woodland. Each seral stage is accompanied by different communities of plants and animals, increasing biomass and usually increasing species richness.

1.3.2 Secondary succession

Succession on cleared land such as farmland, where a soil layer has already formed, is called secondary succession. Much of the grassland we find in the modern landscape will have gone through alternate phases of arable and pasture, as the fortunes of farmers changed over hundreds of years. In the past, arable land would have been allowed to 'tumbledown' from seed sources in the wider landscape, and through subsequent grazing would gradually develop into grassland. Farmers may also have strewn barn sweepings from other parts of the farm, and animals or agricultural machinery would have carried seeds from field to field. New grassland would develop a flora similar to that of adjacent fields, from seed of local origin. Despite these alternating cycles, grassland that has not recently been improved, with communities containing a high proportion of native grasses and herbs, may still be classed as semi-natural grassland.

With the exception of rare clifftop, ledge and outcrop communities, most semi-natural dry grasslands are plagioclimax communities which depend on continued management, such as cutting, grazing or burning to prevent succession to heath, scrub or woodland. Meadows are generally allowed to grow ungrazed from late spring and cut from late June to early August, after which the regrowth is usually grazed. Pastures are grazed by farm animals throughout the summer and not cut for hay. However, many farmers do not adhere to these strict definitions. Some 'hay meadows' are grazed early in the season but still cut later in the year. Farmers may also introduce animals into a meadow in midsummer, rather than take a hay cut.

1.4 Semi-natural dry grassland types

To manage or restore semi-natural dry grassland, it is important to have some understanding of species assemblages and community types. If restoration involves reintroducing species, then these should reflect the community that would naturally occur on the site, unless changes caused by agriculture or urban and industrial development have significantly altered soil conditions. Species selection may be guided by seminatural grassland communities present locally, or so-called 'reference communities'. This section introduces the main dry grassland communities in Britain, most of which are UK priority habitats, i.e. habitats of principal importance for the conservation of biodiversity in Britain (formerly known as UK BAP priority habitats).