suitability of this landscape for regeneration of the native tree species declined as the interglacial progressed, a topic discussed in more detail in Fenton (2008).Svenning J-C (2002) suggests that ‘the ability of large herbivores to open up the vegetation would probably … be stronger on poor soils’, but he does not state why. Figure 4(B) provides an explanation, indicating how, as the soils deteriorated in Scotland, the role of herbivores in opening up the landscape increased. In the early interglacial phases conditions are optimal for tree regeneration and the probability of at least some saplings surviving browsing and going on to develop high forest is high. However, as conditions later become sub-optimal, fewer seedlings are able to establish and the saplings take significantly longer to grow beyond the reach of herbivores: hence the probability of their being browsed becomes high with very few, if any, being able to go on to become mature trees. Figure 5 shows a location where trees can only regenerate in a localised area with optimal conditions, here a well-drained stream-side at low altitude.The concept of climax, developed by Clements (1916), assumes that in a given location the vegetation goes through seral stages until it reaches a steady-state – the climax plant community. Clements saw this stable state as being brought about by the ability of the plants involved to retain dominance over time. However in the presence of browsing animals, over long time scales conditions can slowly change such that trees lose their ability to retain dominance. In other words, the mesocratic woodland is itself but a seral stage to a more stable climax of open-ground plant communities. Whether the moorland communities themselves remain as the climax vegetation depends on the long term trajectory of climate change or, in the case of peatland, the long-term dynamics of the peat itself.In summary, the climate and soils were more favourable for tree growth in the mesocratic phase of the interglacial so that the probability of some trees surviving to above browsing height was higher then. In the current conditions, the probability on most of the now impoverished soils is low so that woodland needs some protection from browsing to remain common. Protection mechanisms are now discussed.B) Protection by thorny shrubsVera (2000) shows how high forest can persist in the landscape of lowland Europe as one component of a mosaic of grassland, scrub and woodland. Species of thorny scrub colonise open ground and thereafter protect seedlings and saplings from browsing. The saplings go on to develop high forest, which shade out the shrubs allowing large herbivores in which prevent in situ regeneration. When the trees eventually die, the cycle starts over again. Over long time scales this results in an ever-changing mosaic of vegetation types rather than continuous high forest. Alexander et al. (2018) have reviewed the evidence for this in lowland Britain and concluded: ‘the post-glacial landscape exhibited a diverse mosaic of vegetation types, with open country very prominent.’ In other words, in lowland England (which might be representative of lowland temperate Europe as a whole), the lowland landscape would not consist of continuous climax forest.The Vera model is predicated on thorny shrubs being present, which in Britain would comprise primarily Rubus fruticosus, Prunus spinosa and Crataegus monogyna. However these are all species characteristic of mesotrophic soils and do not occur in most locations across the Scottish Highlands because, for reasons of soils and climate, the area is beyond their ecological range. The exception is coastal locations in the southwest Highlands where the climate is milder and the soils richer; in this area observation suggest that the commonest habitat for woodland regeneration is within a Rubus fruticosus/Pteridium aquilinum matrix, and, less commonly, in Prunus spinosa. In these locations, the Vera model can be seen to be operating as illustrated in Figure 6. However, over most of the Highlands the Vera model will not apply, resulting in a greater probability of an open landscape.C) Protection by winter snowIn the parts of the world where winters are characterised by long periods of snow cover, then this snow protects any young trees from browsing as well as limiting the possible over-wintering herbivore population (Figure 7). Young trees are at their most vulnerable from browsing during winter and early spring when there is little other palatable food available than nutritious buds. By the time of the snow melt, there is sufficient growth of other vegetation to dilute the impact of tree browsing. Note that damage to young trees by small rodents such as voles and lemmings (Lemmus lemmus) will not be affected by snow cover.This winter snow cover explains the presence of woodland and scrub, often as the dominant community, in boreal, arctic/sub-arctic and alpine/sub-alpine locations across Europe in the absence of thorny shrubs. However upland Scotland has a montane climate, characterised by variable winter snow cover and hills of low enough altitude (maximum 1345 m) for the indigenous herbivores (red deer) to easily migrate from the valleys to summits and back in a day as the weather permits. Hence the high altitude vegetation at the putative climatic tree line can be grazed at any time of the year, particularly because the montane grasslands on the steep hill slopes can provide better grazing than the acidic often peat-covered low ground and so draw the herbivores upwards.This would explain the conclusion of Poore (1997) in relation to the Highlands that ‘there is little evidence that there was extensive scrub on the mountains within the current climatic period.’ Any small stands of existing sub-alpine/arctic in a few favoured locations are probably relicts from a colder period.D) Tree morphology and toxicityAs discussed in scenario B above, thorny shrubs can resist browsing owing to their morphology and can colonise in the presence of herbivores. In contrast, the indigenous high forest trees of the Highlands (Quercus, Betula, Pinus, Fraxinus) are all browsable by herbivores to a level which can prevent regeneration.However non-indigenous trees, particularly Sitka spruce (Picea sitchensis), can often be seen colonising the Highland landscape because its spiky leaves result in minimal browsing and also because it is perhaps more suited to the prevailing soil conditions than indigenous species (Figure 8). Similarly the tall introduced shrub Rhododendron ponticum is colonising large tranches of the Highlands because of its toxicity to herbivores. It is therefore perhaps a matter of the chance of ecological history whether a given landscape will be wooded or unwooded dependent on the characteristics of the trees or large shrubs which naturally colonised the area. This might explain the difference in characteristics of the wooded Pacific northwest coast of North America and the unwooded Scottish Highlands.
ReferencesAlexander K, Allen M, Butler J, Green T, Woods R. 2018. Britain’s natural landscapes – promoting improved understanding of the nature of post-glacial vegetation of lowland Britain. British Wildlife 25:5, 330-338.Clements FE. 1916. Plant Succession. Washington: Carnegie Institute.Fenton JHC. 2008. A postulated natural origin for the open landscape of upland Scotland. Plant Ecology & Diversity 1:1, 115-127.Poore MED. 1997. In: The Ecology and Restoration of Montane and Subalpine Scrub Habitats in Scotland. Scottish Natural Heritage Review 83, 115–116.Svenning J-C. 2002. A review of natural vegetation openness in north-western Europe. Biological Conservation 104,133-148.
WOODLAND OR OPEN GROUND? Scenarios for the persistence of woodland in the presence of grazing
Figure 5. A landscape in the northwest Highlands, with woodland localised on optimum, better-drained and mineral-flushed sites.
Figure 7. Scenario C: Above Lillehammer, Norway. Winter snow cover in boreal climates both protects trees from grazing and prevents high over-wintering herbivore numbers.
Figure 8. Scenario D:Picea sitchensis colonising moorland in the central Highlands because its spiky needles are relatively resistant to grazing and establishment conditions are suitable for this species. Hence it is better able to regenerate and grow in the Highlands than native tree species.