These conclusions suggest that the indigenous herbivores of the Highlands are likely to keep the landscape open. It is a situation we are unfamiliar with in Europe as a whole because we have made most such species extinct or manage them to very low population levels. For example, the environmental historian Kaplan says that removal of large animal species by humans has had effects on the landscape that are apparent almost everywhere: “A lot of land would be semi-open, kept partly open by these big herds of grazers and browsers and predators. It is important to keep in mind that landscape is also shaped by animals. These giant herds of bison would be trampling down little trees and keeping the landscape open (Kaplan 2013).” Perhaps within Europe it is only the Scottish Highlands where free- ranging herds of indigenous large herbivores have been present throughout the postglacial period that we can observe their role in natural ecosystems. Hence the Scottish hills are not treeless through ‘overgrazing’ which is the current opinion of many conservation organisations: the grazing level of 4-8 deer km -2 necessary to maintain woodland in the landscape is a low grazing level according to the trophic level model. To quote Milner et al. (2002) in reference to the Highlands: “Overgrazing is a controversial term ... Its precise definition is dependent on management objectives ... Overgrazing is not generally applied to natural ecosystems, even under heavy grazing pressure, because wild herbivores are regulated by their food supply during the unfavourable season ... For example, there is no evidence of habitat degradation on St Kilda or the North Block of Rum where herbivore populations are naturally regulated.” 2) What is the expected natural distribution of woodland at this stage of the postglacial cycle? In northwest Europe during interglacial periods there is recognised plant succession from an initial protocratic phase following ice retreat, through a mesocratic to an oligocratic as illustrated in Figure 3 (Birks & Birks 2004). This succession has taken place in previous interglacials in the absence of humans. The middle mesocratic phase is optimal for woodland with the highest percentage of the landscape covered with trees, a percentage which thereafter declines. The succession is brought about through long-term changes in both soil and climate, with the availability of soil nutrients, particularly phosphorus and nitrogen, reducing over time through leaching. In the past, the successional changes finished with a return to glacial conditions (the cryocratic phase), although anthropogenic global warming means that this is unlikely to be the case in the future. Scenarios of woodland survival For woodland to persist in the light of these long-term changes, conditions have to be permanently favourable for the survival of seedlings and saplings. Given below is a list of conditions which would enable woodland to persist in the European landscape (i.e. young trees can survive browsing) assuming indigenous large herbivores are present (and/or free-ranging domestic analogues) in accordance with the theory of trophic levels discussed above. Looked at logically from first principles, woodland will remain or become common in the landscape if: A. Optimal soils/climate allows numerous seedlings to germinate and grow so fast that the probability of some surviving browsing is high: e.g. mesocratic phase (and also likely to be the case in the tropics). B. Young trees are protected from browsing by thorny shrubs: temperate forest – the Vera hypothesis (Vera 2000). C. Young trees are protected by snow in winter and early spring, the period when seedlings and saplings are most vulnerable: e.g. boreal & sub-alpine forests, sub-alpine & sub-arctic scrub. D. Tree morphology or toxicity reduces browsing: e.g. spiky needles of spruce, Rhododendron ponticum. E. The anthropogenic activities of removal or reduction of grazing: e.g. compartmentalised broadleaved woodland (browsing animals significantly reduced or excluded). Woodland may remain localised in the landscape: F. Where young trees are protected from grazing by natural features such as cliffs, boulder fields, gullies, or, occasionally, dense vegetation such as tall Calluna vulgaris. G. Episodically, owing to temporary reduction in grazing through extreme weather or disease events. H. Locations commonly visited by predators. Each of these eight scenarios is now discussed in more detail. Woodland dominant or common in the landscape A) Optimal soils/climate In conditions favourable for tree growth with abundant production of viable seeds and seedlings, then there is a greater probability of woodland persisting in the landscape (Figure 4(A)). It should be noted, though, that optimal conditions for trees are also likely to result in optimal conditions for the associated herbivores so it may not always be the case that optimal conditions for tree growth result in closed high forest without the help of the other factors (as discussed below). But with optimal conditions, there will be a higher probability of at least some woods surviving in the landscape whatever the grazing level. As the current interglacial progressed in the wet climate of the Scottish Highlands, with its generally base-poor bedrock, continual leaching resulted in soils too acid for earthworms causing the formation of unmixed, stratified soils, often underlain by an impermeable ironpan. The presence of a thick humus layer together with the ironpan makes the soils liable to waterlogging and has resultedin the large-scale development of blanket peat on land with a gentle gradient. This characterises the natural succession from the mesocratic to the oligocratic phase (Figure 3). Thompson (2004) states that such ‘a combination of very low soil nutrient availability and high soil moisture provides very unfavourable conditions for colonisation of birch (Betula), rowan (Sorbus aucuparia) and Scots pine (Pinus sylvestris)’. Hence the
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References Birks HJB, Birks HH. 2004. The rise and fall of forests. Science 305:5683, 484-5. Kaplan JO. 2013 November 16. New Scientist. p.37. Milner J, Alexander J, Griffin C. 2002. A Highland Deer Herd and its Habitat. London: Red Lion House. Thompson R. 2004. Predicting site suitability for natural colonisation: upland birchwoods and native pinewoods in northern Scotland. Edinburgh: Forestry Commission Information Note. 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
Red deer grazing on Scottish moorland
Figure 3. The interglacial cycle in northern latitudes. The mesocratic phase is optimal for woodland, which thereafter declines in the oligocratic. Anthropogenic warming may mean that there is no return to a cryocratic phase. From Birks and Birks (2004) Reprinted with permission from AAAS.
Figure 4. Diagrammatic examples of seedling and sapling establishment and growth, showing why tree regeneration is more susceptible to browsing in the oligocratic phase of interglacials, and hence the probability of woodland surviving in the landscape decreases. Growth rates are indicative only. (A) In the mesocratic phase optimum soils and climate results in numerous seedlings becoming established which subsequently grow fast and have a higher probability of surviving browsing. (B) In the oligocratic phase conditions are less suitable for seedling establishment and any that do establish have a higher probability of being browsed owing to their slow growth rate.