It is not that the Scottish Highlands are ecologically unsuited to all trees, just to the tree species that naturally colonised Scotland in the postglacial period. If soils are modified to reflect the early postglacial conditions by draining, ploughing and addition of fertiliser (particularly phosphate) then the indigenous trees grow well in locations not exposed to the strong winds characteristic of the area, as indicated by the numerous successful plantations of native trees.E) Anthropogenic grazing reductionWhere humans have either eliminated grazing, or reduced it significantly from the expected natural level, then this can result in an increase in woodland cover, or allow woodland to persist where it otherwise might have declined or moved around the landscape (Figure 9). This has been a common situation within Europe in historical times where enclosed woodland blocks have been managed to provide a permanent supply of wood and timber; and perhaps also in large scale landscapes such as western Norway where in the past humans have made large herbivores functionally extinct.Woodland localised in the landscapeF) Geomorphological featuresIn many landscapes there are locations where the geomorphological features provide a natural barrier to large herbivores, resulting in no or reduced grazing (Figure 10). These are generally steep slopes or areas of rough topography. However it should be noted that in wet climates steep slopes may also hold trees because the better drainage and mineral flushing results in soil conditions more suitable for trees.Hence the presence of trees in such places might be a result of a combination of both reduced grazing and better soil conditions. There are also instances where dense vegetation such as tall Calluna vulgaris can provide some protection from grazing, allowing occasional trees to survive.G) Episodic eventsIt is possible to envisage situations where episodic events provide a temporary reduction in grazing allowing some woodland regeneration. Examples could include an outbreak of disease or, as in tundra ecosystems harsh winters causing heavy herbivore mortality (Ims and Fueglei 2009).H) Predation hotspotsThe trophic level model discussed above indicates that there has to be a significant herbivore population in order to support the presence of large carnivores, and that a population sufficient to allow tree regeneration would not be large enough to allow for a significant wolf population at the local level. Indeed, Warren (2002) concludes:“The evidence from Norway and America is that low numbers of large predators have little effect on deer numbers ... To have a significant impact on deer populations, a very large number of wolves would be needed, and there is unlikely to be enough space in Scotland (either ecologically, or socio-politically) for such large wolf populations.”Large carnivores have become extinct over much of Europe so that it is hard to know what their impact in practice would be on both the population levels and distribution of large herbivores. In the Highlands stalking (shooting) of red deer by humans causes the deer to move up to the higher slopes of the hills. This suggests that predation by wolves might also cause the deer to move up the hill, thus increasing the grazing level on the higher slopes and also preventing the growth of montane scrub.Although both the climate and ecology are different, there has been much debate about the impact of the reintroduction of the wolf (Canis lupus) on the elk (Cervus elaphus Canadensis, a sub-species of red deer) population of Yellowstone Park in the USA. However MacNulty et al. (2016) conclude:“Scientific consensus about the role of wolves in driving the dynamics of the northern herd has yet to emerge, despite 20 years of research by numerous federal, state, and academic investigators.”A key point is that the wolf became extinct in Scotland in the eighteenth century, which is relatively recently in ecological terms, and that the woodland declined naturally over the previous millennia when wolves were present. This, together with the trophic level model, suggests that their reintroduction is unlikely to result in large scale recolonisation of the Highlands by trees. It is possible the presence of wolves might cause a reduction in grazing pressure in locations favoured by them and so promote localised woodland regeneration; but the only way to be sure of this is reintroduce the wolf and observe the result.Application of the above scenarios to different areas of EuropeThe above analysis indicates that in northwest Europe different geographical regions might be expected to possess different percentages of woodland cover. As an example, Table 1summarises the above scenarios and applies them to three different areas of Europe.Discussion in relation to the Highlands of ScotlandThere has been a lot of work carried out on pollen analysis in the Highlands, and the results are generally in keeping with the interglacial succession process as discussed above and supporting the original views of Geikie (1867) who stated ‘It can be shown that the destruction of our ancient forests has not been primarily due to man.’; i.e. a natural regression of woodland from a postglacial maximum. Paterson (2011) gives a good summary of this research in relation to indigenous pinewood:“In core areas, woodland is subject to fragmentation from as early as c. 7500 cal BP; fragmentation is diachronous and is believed to have been earliest in the west. Human activity is sometimes implicated in woodland fragmentation but is more often cited as reinforcing the effects of a maritime climate preferentially affecting Pinus dominated woodlands … Only in Speyside is human activity thought to initiate disintegration.”Fyfe (2018) has recently reviewed the evidence from pollen analysis on the openness or otherwise of the British vegetation and concluded:“At the continental scale, western Atlantic Europe has for long been more open than other parts of the mainland. Britain and Ireland (especially western and northern regions) are particularly notable in this context, and are different from much of inland continental Europe. This conclusion is replicated irrespective of which analytical method is applied to the pollen data.”
ReferencesFyfe R. 2018. ‘Natural’ vegetation in Britain: the pollen-eye view. British Wildlife 25:5, 339-349.Geikie J. 1867. On the buried forests and peat mosses of Scotland, and the changes of climate which they indicate. Transactions of the Royal Society, Edinburgh XXIV: Part II, 363-384.Ims RA, Fuglei E. 2009. Trophic interaction cycles in tundra ecosystems and the impact of climate change. BioScience 55, 311-322.MacNulty DR, Stahler DR, Wyman CT, Ruprecht J, Smith DW. 2016. The challenge of understanding Northern Yellowstone elk dynamics after wolf reintroduction. Yellowstone Science 24:1, 25-33.Paterson D. 2011. The Holocene history of Pinus sylvestris woodland in the Mar Lodge Estate, Cairngorms, Eastern Scotland. PhD dissertation. University of Stirling.Warren CR. 2002. Managing Scotland’s Environment. Edinburgh: Edinburgh University Press.
WOODLAND OR OPEN GROUND? Scenarios for the persistence of woodland in the presence of grazing
Figure 9. Scenario E: An example of an ancient broadleaved woodland in England (Dorset) where enclosure, grazing exclusion and management has allowed it to persist in situ.
Figure 10. Scenario F: Glen Coe in the Highlands showing persistence of woodland on steep slopes, cliffs and gorges where grazing is naturally reduced or absent.
Table 1. Theoretical probability of woodland in the landscape in three different areas of Europe.