Box 101 Connectivity in a fragmented landscape

A good example of woodland fragmentation and connectivity is provided by communities of the maritime juniper Juniperus oxycedrus ssp. macrocarpa, a Mediterranean species tolerant of saline spray along the south-west coast of Spain, which are both vulnerable and ecologically important. Though now greatly reduced in area, it seems that these formerly extended from the El Rompido cliff in Huelva to the west all the way to Gibraltar in the east. Studies by Munoz-Reinoso (2004) are designed to form the basis for future research and restoration policy in an area where these communities have been gravely diminished by both the planting of Mediterranean stone pines Pinus pinea, and the increasing urbanization of the coast. Here the tendency is for the sizes of previously large communities of maritime juniper to be greatly reduced. There are now many small sites where regeneration is often poor, indeed of a total population estimated at c. 24 500 individuals, 93.6% were present at just three locations. High proportions of young plants were found in large and protected populations such as that at the Donana National Park. Adult individuals dominate the smaller communities under the extensive plantations of Pinus pinea. These communities have been subjected to fragmentation and are adversely affected by the deposition of pine needles, deficient pollination and invasion by inland plant species.

Besides the landscape value of its woodlands, which harbour several endangered plants including the labiates Sideritis arborescens and Thymus carnosus, maritime juniper is itself listed as endangered in this area of Spain. Moreover, fruiting individuals are an important food source for vertebrates such as badgers, foxes, wild boar and rabbits. If typical communities of maritime juniper are to be restored, it is important that they be large enough to support adequate populations of associated plants. The communities themselves show considerable variation; climate and soil texture largely control the amount of water available to plants and thus separate xeric from mesic communities. On a smaller scale, coastal physiography (dunes and cliffs), together with such factors as soil calcium carbonate content and sand mobility, also result in differences in floristic composition.

woodlands of native hardwoods are also being planted to provide stepping stones to promote connectivity between small fragments of ancient woodland.

In addition to reducing connectivity, fragmentation also leads directly to species loss. Island biogeography theory (Section 11.4.1) predicts that smaller forest fragments will hold fewer species, and more isolated fragments (i.e. more distant from other areas of forest) will also have fewer species. The effect of small fragments is made worse by the number of forest interior species that require large tracts of unfragmented forest to persist and which can be sensitive

I increased wind disturbance I elevated tree mortality invasion of disturbance-adapted butterflies I altered spp. composition of leaf-litter ants I invasion of disturbance-adapted beetles I altered spp. composition of leaf-litter invertebrates

I altered abundance and diversity of leaf-litter invertebrates I altered height of greatest foliage density I lower relative humidity

I faster recruitment of disturbance-adapted trees I reduced canopy height I reduced soil moisture I lower canopy-foliage density I increased air temperature

■ increased temperature and vapour pressure deficit

■ reduced understorey-bird abundance

■ elevated litterfall

I increased photosynthetically active radiation in understorey I lower relative humidity increased number of treefall gaps higher understorey-foliage density increased seedling growth invasion of disturbance-adapted plants

■ lower leaf relative-water contents

■ lower soil-moisture content

■ higher vapour pressure deficit

■ higher leaf conductance

■ increased phosphorus content of falling leaves

■ invasion of disturbance-adapted plants

■ increased recruitment of Cecropia spp. I reduced density of fungal fruiting bodies

I—i—i—i—|—i—i—i—|—i—i—i—|—i—i—i—|—i—i—i—|-

0 100 200 300 400 500

Edge penetration distance (metres)

Figure 10.12 Penetration distances of different edge effects into forest fragments in central Amazonia. These data are part of the Biological Dynamics of Forest Fragments Project. Started in 1979 and covering 1000 km2, this is the longest running and largest study on habitat fragmentation. (Redrawn from: Laurance et al., 2002. Conservation Biology 16.)

to a road dissecting an otherwise large pristine forest. Most primates fall into this category. The edge effect, the change in conditions which permeate into a forest from the edge, makes the apparent forest fragments even smaller. Figure 10.12 illustrates some of the biological and physical changes that occur at a tropical forest edge where it is usually lighter, less humid and with higher extremes of temperature than in the forest interior. The figure shows that some of these effects are felt 300-400 m into the forest. In conifer forests of Wyoming, Reed et al. (1996) found that the landscape area affected by clear-cuts and roads was 2.5-3.5 times their actual area.

Fragmentation into smaller units aided by the edge effect can have a profound influence upon forest species. For example, taking birds, Burke and Nol (2000) working in the mixed forests of southern Ontario found that most bird species bred successfully in large forest fragments (mean size 121 ha) but less well so (below replacement levels) in small fragments (mean of 7.8 ha).

Jones et al. (2000), using results of the National Breeding Bird Survey for the mid-Atlantic region of the USA in a GIS (geographic information system) model, found that of all landscape features affecting species richness of birds, forest edge was the most important. Generally, forest specialist birds were negatively correlated with forest edge and generalist birds were positively associated. In addition to differences in environmental factors and vegetation, there is evidence that bird differences between interiors and edges are influenced by greater nest predation and particularly greater incidence of brood parasitism by the brown-headed cowbird Molothrus ater (Chalfoun et al., 2002). Like the European cuckoo Cuculus canorus, the cowbird lays its eggs in other birds' nests and the chick thrives at the expense of the hosts. This parasitism is greatest near forest edges since it is closer to the open land favoured by the cowbird. However, not all forest edges are detrimental. A study of the birds in hardwood and mixed forests of Vermont by Ortega and Capen (2002) found that four of the 18 forest interior species had lower abundance adjacent to roads but two species had higher abundance and 12 were unaffected. Of the six edge species, four were more abundant near roads and two were unaffected. A number of likely reasons could be put forward based on the characteristics of this particular forest or the ecological requirements of the birds, but the important point is that forest systems are so complex that conclusions that apply to one forest may not apply to others.

Fragmentation in tropical forests may have severe consequences since many tropical species are rare across their entire range so each forest fragment is less likely to hold a viable population. Since tropical forests are dense by nature, large openings can act as significant barriers. Even unpaved roads only 30-40 m wide in Amazonian forests act to inhibit the movement of many species. But even here edges are not completely bad news; many species that can utilize more open conditions thrive in the forest edge including termites, light-loving butterflies, hummingbirds and fruit-eating bats (Laurance et al, 2002). In fact some animals such as frogs and small mammals have a greater species richness in forest fragments than in intact forest (Gascon and Lovejoy, 1998).

Forest edges can differ from interior areas not just in their fauna but also in their plant species. In the hardwood forests in the Piedmont area of North Carolina, USA there are noticeable differences in tree species composition up to 5 m into the forest. This may seem a small distance but comparison with the third bar up from the bottom in Fig. 10.12 shows that invasive plants establish best near to an edge. The edge zone is more open and so readily invaded in the USA by non-native trees such as the tree-of-heaven Ailanthus altissima. It seems likely that once established these trees can provide a large seed source with potential to spread into gaps further into the forest (Rob McDonald, pers. comm.). In the central Amazonian rain forests, lianas are more important in forest edges than in interior forests. Along edges (defined as a 100-m wide strip) Laurance et al. (2001) found that though there is not a greater biomass of lianas along edges, they are more numerous with a higher proportion of small lianas (2-3 cm diameter) and relatively few bigger ones (4 or more cm diameter). More trees are infested and tree growth along the edge is reduced due to competition above and below ground. Laurance et al. suggest that since lianas are very prone to damage by fire and wind, and because they inhibit the invasion of trees, they create a positive feedback loop by promoting and exacerbating subsequent disturbance, making the gaps larger. On the positive side, these abundant lianas with some new growth of pioneer species can partially 'seal' the edge of the forest within 5 years, reducing the extent of the edge effect until a disturbance opens it again.

Roads can produce particularly large edge effects. In the Rocky Mountains in Wyoming, Reed et al. (1996) found that the edge effect of roads penetrated 1.54-1.98 times further into the conifer forest than along the edge of a forestry clearcut. Forman (2000) estimates that 15-22% of total land area of the contiguous US states is ecologically affected by roads, based on an edge effect of 200 m from secondary roads, 305-365 m from primary roads in forests and grassland (with 10 000 vehicles per day) and 810 m for busy roads in urban areas (50 000 vehicles per day). In addition to chemical pollution and difficulties in crossing roads, birds can suffer from acoustic masking of their songs, reducing their ability to attract mates. Birds with high-pitched song with frequencies well above those of traffic noise are less susceptible to noise pollution. In tropical forests, presence of roads is one of the main factors leading to increased deforestation and further fragmentation (Laurance et al., 2002). Reviews on the ecological effects of roads have been produced by Forman and Alexander (1998) and Spellerberg (1998).

10.7.3 Conservation and biodiversity in a multi-purpose landscape

Although in some instances a particular species is so rare that its needs are paramount, a major objective of conservation is normally the promotion or preservation of biodiversity. As we have already seen, biodiversity can be high in exotic woodlands, but these lack the integrity and uniqueness of ancient communities, whose species and surroundings can tell us much about the past, as Rackham (2003) demonstrates.

Throughout this chapter we have indicated that the philosophy of forest management in developed countries has been changing from concentrating purely on timber production towards multi-use forests managed sustainably.

We have also seen how fragmentation of forests and the need for connectivity to allow movement within metapopulations demonstrates the need to view forests within the wider landscape. This is the domain of landscape ecology which involves the study of patterns within the landscape, how patches (e.g. areas of forest) interact within a landscape mosaic, and how these patterns and interactions change over time.

The use of forests for timber production and biodiversity conservation has been considered under multi-use forests (Section 10.2) but a large number of battles have been, and are being, fought on the use of different forests in the landscape. This is especially true when pristine forests, which are increasingly seen as strongholds of biological diversity, are threatened with logging. This is exemplified by the bitter conflict over plans to fell old-growth Douglas fir forests Pseudotsuga menziesii in north-western North America which are the home of the northern spotted owl Strix occidentalis caurina. At its most polarized, loggers feel that valuable large timber in these stands should not be wasted while conservationists uphold the rights of the owl to exist unmolested. Compromises are suggested - harvesting some of the timber in certain areas and leaving others untouched - but it is a truism that good compromises leave everyone unhappy. Much of the argument over this and many similar cases in temperate forests revolve around the long-standing view that biodiversity is best preserved in reserves from which logging is excluded.

Landscape ecology approaches are showing, however, that this is often not sufficient. Reserves are without doubt very important, but in most types of temperate forest these typically make up less than 10-15% of the land area so much of the biodiversity remains outside the reserve. Even large reserves are likely to be dependent upon populations outside the reserve because the species concerned are migratory or, as in the case of carnivores, they have large ranges. In other instances there is a need to supplement small populations; here the need to conserve forest species in the wider landscape, albeit at lower levels, also becomes important. This is where multi-use forests are valuable in providing some biodiversity conservation. As noted above, in a very hostile landscape (such as forest fragments in a matrix of intensive-use agricultural land), wooded corridors, such as provided by fencerows/hedgerows and wooded stepping stones (which reduce the distance organisms need to move in one go to colonize new areas) are increasingly valuable. These need to be placed in the landscape, if there is a choice, to maximize their use; much research is currently directed at this.

Conservation in the wider landscape has been taken to heart in the design of multi-use forests, particularly in plantations. Plantation design has changed considerably over recent decades. Emphasis is often put on visual landscaping, based on the principle that people prefer forests that look natural and mature. Fortunately, many of the resulting features are also useful for biodiversity conservation (see the section above). Lindenmayer and Franklin (1997), Peterken (1999) and Hartley (2002) suggest a number of measures to increase the natural features that should be built into forest design and management.

(1) Species composition. Use native species and mixed-species stands if possible. Where this is not feasible, native species should be left as mature retention trees in openings in the stands and new trees should be encouraged around the perimeters, in wet areas and as an open understorey below the crop.

(2) Site preparation. Retain as many components of the original forest as possible to maintain biodiversity - acting as a lifeboat: the soil should be disturbed as little as possible; standing dead snags and the maximum amount of dead wood should be left (see Section 7.7 for a discussion on the value of dead wood).

(3) Open space. Leave up to 15% of the plantation as open space, which should be concentrated along roads and in areas intrinsically unfavourable to tree growth such as damp hollows. Retain 10-15% cover within the cut areas to enhance connectivity.

(4) Treeline. Leave a more natural irregular treeline along upper margins of plantations in hilly areas, possibly planting scrub above this.

(5) Tending. Fell trees in large irregularly shaped areas, thin some areas heavily to encourage the understorey, leave others unthinned to create dense thickets, or better still move to group selection, shelterwood or continuous cover forestry systems (see Section 10.3.3). Divide the forest into short- and long-rotation areas to help simulate a more varied age structure with many young trees but also areas of older trees. Leave unfelled or native vegetation buffer zones around the cut areas to reduce the edge effect.

The Wyre Forest on the Shropshire/Worcestershire border, central England is an excellent example of a multi-use forest. It resembles many others, including tropical forests - as we now realize - in having been managed for its timber for an exceedingly long time, in this case for over 900 years at the very least. The ecology, management and history of its 2430 ha are described in a symposium volume (Packham and Harding, 1995) which clearly illustrates the large number of factors responsible for its present biodiversity in terms of its vascular and lower plant communities, invertebrates, amphibians, reptiles, birds and mammals. The way in which ancient forest records and present evidence help us to unravel the patterns of the past is of particular interest (Hobson, 1995a, 1997). Figure 1.4 illustrates the way in which many of the oaks have the basal curve typical of trees which have regrown from coppice stools, a relict of the times when coppicing by the charcoal burner took place regularly. Baskets, brooms, rustic garden furniture and fencing were all made

Bezszypu Kowy Rysunek

Figure 10.13 Mixed conifer-broadleaved area of the Wyre Forest, England. Douglas fir Pseudotsuga menziesii (unlabelled) planted by the Forestry Commission is regenerating naturally from seed, though suffering losses from windthrow in some places. Sessile oak (QM, Quercus petraea) has regrown from a single coppice stool, and birch (B, Betula spp.) established from windblown seed in regeneration gaps is here outcompeting young Douglas fir. HM, creeping soft-grass Holcus mollis and other low-growing herbs; RF, bramble Rubus fruticosus; OA, wood-sorrel Oxalis acetosella; PA, bracken Pteridium aquilinum; DFM, male-fern Dryopteris filix-mas; TS, wood sage Teucrium scorodonia; EA, wood spurge Euphorbia amygdaloides. (Drawn by Peter R. Hobson. From Packham et al., 1992. Functional Ecology of Woodlands and Forests. Chapman and Hall, Fig. 10.2. With kind permission of Springer Science and Business Media.)

Figure 10.13 Mixed conifer-broadleaved area of the Wyre Forest, England. Douglas fir Pseudotsuga menziesii (unlabelled) planted by the Forestry Commission is regenerating naturally from seed, though suffering losses from windthrow in some places. Sessile oak (QM, Quercus petraea) has regrown from a single coppice stool, and birch (B, Betula spp.) established from windblown seed in regeneration gaps is here outcompeting young Douglas fir. HM, creeping soft-grass Holcus mollis and other low-growing herbs; RF, bramble Rubus fruticosus; OA, wood-sorrel Oxalis acetosella; PA, bracken Pteridium aquilinum; DFM, male-fern Dryopteris filix-mas; TS, wood sage Teucrium scorodonia; EA, wood spurge Euphorbia amygdaloides. (Drawn by Peter R. Hobson. From Packham et al., 1992. Functional Ecology of Woodlands and Forests. Chapman and Hall, Fig. 10.2. With kind permission of Springer Science and Business Media.)

using forest produce, while peeled bark was used for tanning (Hobson, 1995b). The forest is also interesting in that part of it has been managed by private enterprise, while that depicted in Figs 10.10 and 10.13 is in areas taken over by the Forestry Commission whose first purchases here were made in 1925/6.

The area of the Wyre Forest coincides closely with an outlier of Middle Coal Measure of Upper Carboniferous age and has had a complex tectonic history with much folding and faulting. Rocks exposed at the surface vary from flaggy sandstones to clays, silts and shales; the soils to which they have given rise show major differences in nutrient content, pH and ability to retain moisture. All these features greatly influence the biodiversity of the forest, some of whose rarest plants were associated with flushes or acidic bogs. Until the 1930s bog asphodel Narthecium ossifragum, the insectivorous round-leaved sundew Drosera rotundifolia, marsh lousewort Pedicularis palustris and bogbean Menyanthes trifoliata occurred in their hundreds in a bog which is now part

Figure 10.14 Larva of terrestrial caddis Enoicyla pusilla on pine needles in Chaddesley Woods, Worcestershire, UK. (Drawn by David J. L. Harding. From Packham et al, 1992. Functional Ecology of Woodlands and Forests, page 268. Chapman and Hall. With kind permission of Springer Science and Business Media.)

Figure 10.14 Larva of terrestrial caddis Enoicyla pusilla on pine needles in Chaddesley Woods, Worcestershire, UK. (Drawn by David J. L. Harding. From Packham et al, 1992. Functional Ecology of Woodlands and Forests, page 268. Chapman and Hall. With kind permission of Springer Science and Business Media.)

of a golf course and of which only a mown fragment remains. Losses of vascular plants in particular can be traced through botanical records kept over the centuries, but although much has gone very much remains and the forest and its orchids are still carefully conserved. The forest contains many fascinating insects including the terrestrial caddis Enoicyla pusilla, which has also been studied (Harding, 1995) nearby at Chaddesley Woods National Nature Reserve near Kidderminster and Shrawley Wood south of Stourport (Fig. 10.14).

Fruiting bodies of fungi abound in autumn when some of the edible species including the parasol Macrolepiota procera, shaggy parasol M. rhacodes, lawyer's wig Coprinus comatus, wood blewit Lepista nuda, wood hedgehog Hydnum repandum and penny bun Boletus edulis are frequently collected and cooked, particularly by those with East European connections. As mentioned previously, under the right conditions almost all woody plants appear able to exist in mutually advantageous symbiotic relationships with a suitable species of fungus. Larches associate in this way with both Suillus grevillei and S. luteus. Such mycorrhizas, which are very important to the trees of the forest, are often more sensitive to local factors including pH than the trees. In consequence the same species of tree may be associated with different species of mycorrhizal fungi in different habitats. The forest trees also have a much less helpful relationship with parasitic fungi; although the forest contained relatively few elms virtually every one was affected by the epidemics of Dutch elm disease. Honey fungus Armillaria mellea is present throughout Wyre Forest, but as in so many mixed UK woodlands, it appears to be primarily saprotrophic. This species causes greater damage in simplified situations with much smaller populations of other fungi (see Section 5.4.2).

One of the most encouraging aspects of recent management developments in Wyre has been the way the many different interests have been reconciled so that, as Ian Hickman put it at the symposium, cross-purpose has become multi-purpose. The Forestry Act 1967 allowed the Forestry Commission to build car parks, forest walks and visitor centres on its land and this bore fruit with the creation of the Callow Hill Visitor Centre with its forest walks at the southern boundary of Wyre Forest.

Stages in the preparation of a charcoal burn. (Drawn by Peter R. Hobson.)

Was this article helpful?

0 0
Worm Farming

Worm Farming

Do You Want To Learn More About Green Living That Can Save You Money? Discover How To Create A Worm Farm From Scratch! Recycling has caught on with a more people as the years go by. Well, now theres another way to recycle that may seem unconventional at first, but it can save you money down the road.

Get My Free Ebook


Post a comment