Habitat changes in British lowland landscapes and their
implications for bird populations

by Rob Fuller

4. Issues concerning forestry, woodland and scrub

There were large changes in the regional extent and management of woodland in lowland Britain during the last century. Large-scale coniferous afforestation took place in several regions, many broad-leaved woods were planted up with conifers, traditional coppice and wood-pasture systems were superseded by high forest management and the structure of many woods became increasingly modified by burgeoning deer populations. The century also saw large-scale expansion of scrub in many places that had previously been open grassland, fen or heath.

Before considering these issues individually, it is important to point out that several woodland bird species are in decline and the reasons are poorly understood. Among the species concerned are Willow Tit, Marsh Tit, Spotted Flycatcher, Lesser Spotted Woodpecker, Woodcock, Tree Pipit and Lesser Redpoll (Baillie et al. 2002). The causes undoubtedly differ from one species to another and there is potentially a large number of factors involved (Vanhinsbergh et al. 2002). These factors may include deterioration of woodland edges due to surrounding land management and the impacts of deer.

For further information on the issues discussed below see Avery & Leslie (1990), Petty & Avery (1990) and Fuller (1995).

4.1. Afforestation of open habitats in the lowlands

The most extensive areas of new forestry in Britain have been created on moorland habitats in the uplands. However, in the lowlands there have also been some large-scale plantings of conifers especially on heaths and coastal dunes. The largest lowland man-made conifer forest is Thetford Forest in Norfolk and Suffolk, which was planted mainly on heathland and low-grade farmland. Large lowland plantations occur on dune systems at Newborough Warren on Anglesey and at Culbin and Tentsmuir in eastern Scotland.

These plantings took place mainly in the immediate pre and post-war decades. The overall effect has been the loss of habitat for some open country bird species. For example, afforestation on the southern heaths has probably resulted in some loss of Dartford Warbler habitat and the creation of Thetford Forest destroyed large areas of Stone Curlew habitat. Some heathland species have, however, adapted to these new forests, in particular Woodlark and Nightjar which both occur in large numbers in the young open stands of trees in Thetford Forest. Other relatively scarce species that have benefited from afforestation include Goshawk, Long-eared Owl and Firecrest, though many of the latter appear to be in conifer stands planted within broad-leaved woods.

The forests have created large areas of habitat for several relatively common woodland species. The early stages, before canopy-closure, provided habitats for Tree Pipits, Willow Warblers, Yellowhammers, Linnets, Lesser Redpolls and other birds that require open scrub-like vegetation. The canopy-closure period provided habitats for Song Thrushes, Turtle Doves and many other common woodland birds. As the stands matured further they became increasingly dominated by Chaffinches, Coal Tits and Goldcrests. The ranges and national abundance of Siskin and Crossbill have also increased directly as a result of afforestation both in the lowlands and uplands. Some of these conifer plantations have changed considerably in character since they were planted. The first-generation forests tended to be relatively uniform in terms of the ages of trees they contained. As these trees have been harvested, the opportunity has been taken to 'restructure' the forest into smaller and often irregular shaped compartments containing stands of different ages. The consequence is that these forests now tend to contain more internal edges and a greater diversity of growth stages than was the case in the 1960s and 1970s. It is possible that the field layer vegetation of the second-generation forests is different from that of the early forests with consequences for the birds. This effect is known to exist in upland forests but appears not to have been recorded in lowland forests. Though the effects of forest restructuring have not been studied in detail, it is likely that this has diversified the bird life of most forests by ensuring a more even representation of the different growth stages and their distinctive bird assemblages.

4.2. Planting of broadleaved woods with conifers

From 1945 until approximately 1980, many stands of broadleaved woodland were replaced with conifers, often non-native species. Planting conifers into lowland broadleaved woodland is now rare, especially within ancient semi-natural woodland. In fact there has been a tendency to restore broadleaved trees in such places rather than to create second-generation conifer stands.

Several differences between these replantings and afforestation are relevant in the context of their effects on birds. Whilst the replanting of existing woods with conifers has been widespread, it has generally involved individual stands that are small relative to those in the new conifer forests described above. Furthermore, the conifers are often placed within a woodland setting. Site conditions also tend to be different. Whereas afforestation has often taken place on relatively poor sandy soils, replanting in woodland has frequently been on richer soils with the consequence that the developing vegetation has been more luxuriant and diverse. Though these effects have not been studied directly, it seems that the bird communities in replanted sites tend to be more diverse and the densities of birds are higher than in afforested sites containing trees of similar age.

4.3 Decline in traditional woodland management

Actively managed coppiced woodlands became progressively scarcer during the 20th century. Many coppiced woods were either cleared for agriculture or planted with conifers. Others were simply neglected and are gradually turning to some form of high forest structure. Working coppice is cut on a rotation that can be as short as 7 years or as long as 40 years depending on the tree species and the intended use for the harvested stems. Following cutting, the vegetation undergoes a transformation with the development of dense low growth, which becomes rapidly shaded out once the canopy closes. Canopy closure typically occurs between 5 and 12 years after cutting depending on factors such as the spacing of the coppice stools and pressure from deer browsing (see section 4.4). These early stages of active coppice often hold high breeding densities of birds, including both resident and migrant species. Coppice can provide an exceptionally good habitat for several warblers notably Garden Warbler, Blackcap, Willow Warbler and Chiffchaff. The young growth in some actively coppiced woods also holds breeding Nightingales. These species all avoid the older stages of coppice, which are typically devoid of dense undergrowth.

Old coppice holds low densities of birds and numbers of species are few, the main ones tending to be Robins and various tits. This type of bird community persists for several decades after canopy-closure. The implication is that coppice abandonment has impoverished the bird communities of many woods. However, in the absence of further intervention, these woods will eventually improve as bird habitats but this may take many decades. As the trees in these woods grow in stature so more nest sites will become available for hole-nesting birds. The gradual opening of the canopy and the occurrence of treefalls will help to regenerate an understorey which will benefit birds such as warblers that nest or feed within the shrub layer.

For examples of studies of bird communities in coppice and information relevant to management of coppiced and formerly coppiced woods see Fuller and Henderson (1992), Fuller (1995), Fuller and Green (1998).

4.4. Increases in deer

4.5. Successional development of scrub and woodland

The broad pattern of change in bird communities is similar to that associated with large-scale afforestation (see section 4.1), with an ongoing turnover of species as the open habitat is gradually invaded by scrub. Linnets, Yellowhammers, Tree Pipits and Whitethroats are typical of the very open scrub. As the bushes thicken they are replaced by Willow Warblers, Garden Warblers and Blackcaps; the densities of these warblers - and more locally of Nightingales - can be extremely high in mosaics of grass and dense scrub. Soon after the canopy has closed, these species diminish and the community becomes dominated by common resident species such as Chaffinch, Robin and Wren. The recent history of scrub expansion has, therefore, resulted in the local increase of several summer visitors. Whilst many of these sites still hold large populations of these birds, this is essentially a temporary phenomenon because tall rank scrub or young woodland does not provide a suitable habitat for most of these scrub specialists. These successional changes in scrub habitats have probably been especially beneficial for Nightingales, indeed scrub is now the most important habitat for the species in England (Wilson et al. 2002).

4.6 Creation of new woods on farmland

Incentives for planting new woods on farmland have been in place for several years through the Farm Woodland Scheme and the Farm Woodland Premium Scheme. These schemes have stimulated the planting of many small woods, the majority of which are less than 6 ha in size. One of the reasons for planting new woods on farmland is habitat creation for wildlife. A recent study of the birds using these recently established woods (Vanhinsbergh et al. 2002) found that they were used by a high diversity of species, though most were too young to attract woodland specialists such as woodpeckers, Nuthatches and Marsh Tits. As one might expect, the larger woods held more species. Less expected was the finding that woods connected to hedgerows with trees held more bird species than woods connected to treeless hedgerows. The creation of new small woods will eventually diversify the bird life of many areas of farmland, especially those with few hedges and existing woods. It should be noted, however, that the bird communities found in hedgerows and woodland are essentially complementary (Fuller et al. 2001). Whilst many species make use both of hedges and woods, some species are woodland specialists but others are hedgerow specialists. Habitat creation initiatives in lowland landscapes should ideally embrace both hedgerow and woodland restoration.