Marginal Areas And Conservation

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When nature reserves are examined in detail it is often apparent that the greatest areas of species richness are localized. In many cases the regions with greater biodiversity are at margins or ecotones where one community merges with another. In the case illustrated above (Fig. 12.2) at the Tentsmuir Nature Reserve, the immediately visible loss from erosion was the disappearance of 10 m high dunes and replacing them with a deeply cut bay reaching back to where the shoreline had been almost a century ago. The physical loss of the dune did not deprive the reserve of any particularly rare species. However, when the erosion reached the eco-tone zone between the flood-line alder community (Fig. 12.6) and the adjacent dune slack it removed a large part of the former territory of the coralroot orchid (Corallorrhiza trifida; Fig. 12.7). This was a regrettable loss as Tentsmuir once hosted one of the largest populations of this rare orchid in the British Isles.

The above example illustrates that often it is not any one community in a reserve that is important for maintaining biodiversity but the boundaries and margins between them are.

Ecotones are not only rich in species but also in hybrids between species which gives these areas a special significance in facilitating both plant adaptation and migration (see Chapter 4). It has frequently been pointed out that it is not just individual plants that migrate as seeds but genes also do through pollen dispersal. A time of climatic change is also a time for migration. Ectones, with the niches that they provide for hybrids, will also facilitate gene migration and therefore have a vital conservation role for the movement of plant populations. On a mountainside, high-altitude populations of dwarf birch will hybridize with the common birch that grows at a lower elevation. During a period of climatic warming the F1 hybrid generation will survive and may backcross with the lower altitude common birch, resulting eventually in the lowland birch replacing the mountain birch (see Chapter 4). Surrogate motherhood by the mountain birch has facilitated by gene migration the movement of common birch to higher altitudes (Fig. 12.8). Marginal situations where the parent species are less viable than the hybrid are ideal areas for preserving the mobility of plant communities, especially in a modern world where intensive land use confines many plant species to isolated wilderness habitats in a matrix of managed land.

12.5.1 Regeneration and the role of margins

Many forest trees have difficulty in regenerating under the shade of the parent trees. Fruiting of hazel (Corylus avellana) as discussed in Section 11.4 is much reduced when hazel becomes an understorey tree. Young saplings therefore tend to arise either in clearings or at the margins of the forest. A romantic and controversial hypothesis has even suggested (Vera, 2000) that light-demanding trees and thorny shrubs in temperate plant communities may reflect adaptations to now-extinct large grazers, such as aurochs and tarpans (see Section 2.4.1). The hypothesis suggests that grazing induces a shifting mosaic of grassland, shrub thickets and woodland, and that thorny scrub margins of woodlands might be favoured places for tree regeneration. The hypothesis goes further and claims that the biodiversity of forest trees declines in the absence of large grazers.

Saltmarsh Conservation GrazingSaltmarsh Conservation Grazing

Fig. 12.5 Salt marsh at the head of Long Tongue, Sutherland (Northern Scotland). (Upper) Photograph taken in 1949. (Lower) Photograph taken in 2002. Note that even the small islands at the head of the major spits have not visibly altered. (Upper photograph is reproduced from the Valentine collection with permission from the Archives and Muniments of the University of St Andrews.)

Tentsmuir Nature Reserve

Fig. 12.6 Eroding margins on the National Nature Reserve at Tentsmuir due to record high tide levels. Photograph taken in October 2006 showing where erosion has removed an extensive area of dune and slack to breech the flood-line alder association. The vanished margin of the alder stand from the foreground to the middle distance was a former site of the coralroot orchid

(Corallorrhiza trifida).

Fig. 12.6 Eroding margins on the National Nature Reserve at Tentsmuir due to record high tide levels. Photograph taken in October 2006 showing where erosion has removed an extensive area of dune and slack to breech the flood-line alder association. The vanished margin of the alder stand from the foreground to the middle distance was a former site of the coralroot orchid

(Corallorrhiza trifida).

The validity of this assertion has been vigorously challenged, as in many parts of the world, from New Zealand to Scotland, grazing by animals such as deer has greatly reduced forest cover and diversity. However, for Scotland at least it has to be remembered that the present red deer population is probably greater than at any time in the past. Deer grazing would have been less severe in times when native wolves controlled the population. There is therefore, despite the obvious criticisms of the Vera hypothesis, some truth in the assertion that thorny thickets around woodlands could protect young saplings from large herbivores and that forest clearings would allow vigorous regeneration. Managers of the Nothofagus forests of Patagonia favour wild pigs for regeneration as their rooting of the soil encourages germination and seedling establishment. Winter grazing by sheep increases heathland biodiversity through the opening up of the turf by the impact of their trotters on wet soils. It has even been shown to be particularly helpful in maintaining populations of the rare endemic Scottish primrose (Primula scotica; Section 2.2.3). Grazing is essential in herbaceous communities to preserve diversity so there is no a-priori reason that it cannot also aid the regeneration of forest species.

Marginal areas, whether they are forest or any other type of vegetation due to their fluctuating

Fig. 12.7 The coralroot orchid (Corallorrhiza trifida) — a saprophytic herb that inhabits shaded damp Salix and alder carr as well as freshwater dune slacks with Salix repens. (Photo T. Cunningham.)
Priori Forest
Fig. 12.8 Birchwood near the treeline in Sutherland (northern Scotland). The dominant form ofbirch is Betula intermedia (B. pubescens X B. nana), a variable form where the rounded shape and sweet smell of the small leaves (see inset) indicates hybridization with dwarf birch.
Esferas Ecologicas
Fig. 12.9 Oak (Quercus petraea) established in pine (Pinns sylvestris) forest in Glen Affric, Scotland. The mild winters of recent years are already favouring the spread of oak in many Scottish glens.

environment, provide opportunities for regeneration and also modification of the species composition. In clearings in pine forests in Scotland, oak establishment (Quercus petraea) is now taking place as a result of warmer winters (Fig. 12.9). These marginal areas are even raising the possibility that the native pine forests of the Scottish Highlands are now experiencing very marginal conditions for their survival and may be gradually replaced by oak if the present climatic warming trend continues. The importance of margins in facilitating vegetation succession in response to climatic change should not be underestimated.

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