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.Birks HJB, Birks HH. 2004. The rise and fall of forests. Science 305:5683, 484-5.Fenton JHC. 2008. A postulated natural origin for the open landscape of upland Scotland. Plant Ecology & Diversity 1:1, 115-127.Klinger LF. 1996. Coupling of soils and vegetation in peatland succession. Arctic and Alpine Research 28, 380-387Ratcliffe DA. 1977. A Nature Conservation Review Vol.1. Cambridge: Cambridge University Press.Veldman JW, Overbeck GE, Negreiros D, Mahy G, Le Stradic S, Fernandes GW, Durigan G, Buisson E, Putz FE, Bond WJ. 2015. Tyranny of trees in grassy biomes. Science 347:6221, 484-485.Vera FWM. 2000. Grazing Ecology and Forest History. Wallingford: CABI Publishing.
WOODLAND OR OPEN GROUND? Scenarios for the persistence of woodland in the presence of grazing
Figure 13. New native woodlands in the name of ecological restoration are being planted all over the Highlands, here a new Pinus sylvestris wood near Gairloch in Wester Ross planted on unwooded moorland.
Recent conservation effort in the HighlandsA point which is often missed is that, come 1750, 95% of the Highland landscape would have been open moorland, so that, taking a wide strategic overview, any woodland loss subsequent to this becomes largely irrelevant. In practice, though, because woodland is rare in the landscape, and because rarity is a criterium indicating nature conservation importance (Ratcliffe 1977), a significant amount of conservation effort has been focussed on the woodland habitat: whether encouraging tree regeneration or on creating new woodland of indigenous species on open moorland. (Figure 13). This strong focus on one rare and declining habitat type has tended to skew the nature conservation effort: no value is ascribed to the common open ground habitats – with the exception of peatland.However, if you stand back and view the Highlands from an international perspective, then that which is locally common (temperate moorland) is seen to be globally rare, and that which is locally rare (temperate woodland) is globally common. Indeed, many of the open moorland habitats are recognised as being of international importance under the EU Habitats Directive.It should also be noted that reintroducing the wolves, although introducing more balance to the ecosystems (Figure 2a), is unlikely to cause a significant increase in woodland cover: postglacial woodland expansion and decline occurred during the 10,000 year period when wolves were present in the landscape.ConclusionsThe Highlands are different from most of Europe in that the human population over most of history has been low, with some areas, particularly away from the coast and the main valleys, uninhabited; and that indigenous large mammalian herbivores (the red deer) have been present throughout the postglacial period. This makes the Scottish Highlands unique in that the continuing processes of postglacial plant succession can be observed in a location of minimal human impact, particularly away from the coasts where population has tended to be more sparse.The analysis here shows how the open nature of the landscape can be explained through natural processes, and demonstrates the successional process of natural regression of forests in this, the oligocratic phase of an interglacial, from a postglacial maximum as described by Birks and Birks (2004). Related to this is the fact that much of the level and gently sloping ground of the Highlands is peat covered, which supports the view of Klinger (1996) that peat bogs can be the end point of succession in many parts of the world.Hence the mainstream view that the Highlands are a once-forested landscape, the forest destroyed by anthropogenic activity, needs revision, as it does for other naturally open landscapes in the world. Veldman et al. (2015) state:“The WRI [World Resources’ Institute] erroneously assumes that nonforest areas where climate could theoretically permit forest development are ‘deforested’, an assumption rooted in outdated ideas about potential vegetation and the roles of fire and herbivores in natural systems.”The open landscape of the Highlands is caused by the discrepancy between the herbivore carrying capacity and the lower density required to ensure woodland survival, combined with a lack any natural mechanism to prevent young trees from browsing which is found in other areas of Europe, such as the presence of thorny shrubs or persistent winter snow cover. The dominance of acidic, waterlogged soils, resulting in sub-optimal soils for tree regeneration, is also a major contributing factor, resulting in both a lower probability of seeding establishment and a higher probability of seedlings and saplings being eaten.It should be noted, though, that across the Highland landscape as a whole all three models of woodland dynamics (Fenton 2008) can be observed: ‘woodland as climax’ in the few locations with better soils naturally protected from grazing by topography; ‘the cyclical model‘ as proposed be Vera (2000) on the better soils and more favourable climate of coastal Argyll in the southwest Highlands; and ‘natural decline’, the dominant model over most of the landscape.The trophic level model as applied here, combined with an understanding of the ecological mechanisms which deter browsing, can provide a sufficient prediction of whether a given natural landscape will be wooded or not; although where the availability of palatable biomass is highly seasonal, the trophic level model is likely to over-estimate the expected herbivore population.It should be noted that the ecology would be expected to be different in ecosystems which have never had indigenous mammalian herbivores, such as Iceland: in these situations the probability of native wood and/or scrub being a natural component is increased (although in Iceland introduced livestock have since destroyed most of the woodland).The hypothesis presented here is that, in temperate ecosystems able to support trees, high forest is not always the pre-determined climax vegetation. An alternative model of an ever-changing mosaic of vegetation instead of a stable Clementsian climax, as proposed by Vera (2000) is supported by recent evidence for lowland Britain (Alexander et al. 2018), but will not apply to most of the Scottish Highlands where the end result of succession appears to be various types of resilient moorland plant communities. The current conservation practice of converting these areas to woodlands composed of indigenous trees therefore reduces the naturalness and biodiversity value of the Highlands.The current natural vegetation of the Highlands (Figure 14), comprising a range of open moorland plant communities, appears to have become dominant owing to its greater resilience than woodland over time. This suggests that with continuing climate change it will remain dominant. With climate change causing a reduction in consistent winter snow cover in parts of Europe, this could increase the potential for over-wintering herbivores and hence lead to woodland loss. However, it could also encourage the spread of temperate thorny shrubs, allowing woodland expansion . Hence there is possibility of both woodland loss and woodland expansion in different regions if natural processes are allowed to determine the direction of ecological change. This paper, which has gone back to first principles, will hopefully result in a wider recognition of the role of herbivores in the landscape – and of the fact that open landscapes can occur naturally in locations climatically suitable for trees.
All photographs James Fenton
Figure 14. The Scottish Highlands: perhaps one of the most natural vegetation patterns remaining in Europe?