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 shrubs
Vera (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 snow
In 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 toxicity
As 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.
Page 4
References
Alexander 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 6. Scenario B: Thorny Prunus spinosa scrub invading
grassland in coastal Argyll, southwest Highlands.
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.