A human life span is fleeting when compared to that of a wild forest - for the most part we can only gain a snapshot view of a forest's life. Our native woodlands have cycles that operate over centuries or even millennia, and from this perspective natural events that are apparently destructive can, paradoxically, breathe life and health into the ecosystem.
Natural disturbance can take many forms, and on a broader scale, almost any ecological interaction can be seen as a kind of disturbance: a gust of wind carrying a pine seed onto new ground, the movements of predators such as wolves causing their prey to run for safety, and the beak of a woodpecker making openings for fungi in a tree are all kinds of disturbance. The word tends to have negative connotations, so it is important to recognise that the right kinds of disturbance are not only beneficial, but actually essential to the long term health of all ecosystems.
The picture becomes much clearer when looking at how an ecosystem behaves over a long period of time. We can then see that disturbance actually allows the flow of nutrients and energy through the ecosystem and is a driving force of evolution. Below we will look more closely at larger scale, 'catastrophic' disturbance, particularly the effects wind, fire, water, insects and disease.
Catastrophic disturbance by fire has played a natural part in the ecology of Scots pine (Pinus sylvestris) across many parts of its range. Interestingly, the deeply fissured bark of Scots pine acts as a defence against fire. Fire caused by lightning strikes can burn substantial areas of this resinous tree - but the flip-side is that it helps to remove dense vegetation such as heather. This can expose mineral soils on which seedlings can become established, with less competition for light, water and nutrients.
The effects of fire would have been modified by other influences, such as wetness of the ground. In the east of Scotland, where it is drier, fire had a more pronounced impact on the structure of the forest, resulting in larger, more even-aged stands. It would have affected species composition, leading to a dominance of pine in some patches, and birch (Betula spp.) in others. By contrast, pinewoods in the west are wetter and fires would have been less frequent. These environmental influences also operated on a more local level. North-facing slopes for example are usually cooler and damper, moderating the impact of fire.
Aspen (Populus tremula) can often respond well to fire, as its root system can survive below the soil, ready to send up more suckers quickly after the above-ground vegetation has been burned, where they can grow relatively free of competition. Fire may have played a far greater role in aspen's ecology in Scotland than we are aware, as it is certainly a significant influence on large aspen stands in Scandinavia and Russia, and on the closely-related trembling aspen (Populus tremuloides) in North America.
Fire has been used by humans as a management tool. Muirburn - burning the moors to regenerate heather for grouse or sheep - illustrates how fire, when used with artificial frequency, suppresses forest regeneration. In contrast, the RSPB has used carefully controlled burning to help regenerate native pinewoods in Abernethy.
Flooding can be frequent along some rivers, to the point where specially adapted wet woodlands of willow (Salix spp) and alder (Alnus glutinosa) can develop as a result. Floods may occur after exceptionally heavy rain, snow melt or on a smaller, site-specific scale, as a result of dam-building by the European beaver (Castor fiber), a keystone species in riparian and freshwater ecosystems which was extirpated from Scotland by the 16th century.
Flooding can serve a valuable ecological function. In particular it may kill a stand of trees, providing important dead wood habitat and leaving an opening in the canopy when the water retreats. At the same time, flood events can transport nutrients, in the form of debris, to different parts of the forest.
A number of severe, but infrequent, storms over the last few decades have demonstrated the enormous power of the wind to alter the landscape. This can often seem disastrous when individual trees which have personal, cultural or economic significance are damaged. From a broader ecological perspective, however, this process is quite natural, and even essential to the health of the forest.
Because of their proximity to the western edge of the European landmass and the prevailing oceanic climate, the Scottish Highlands are particularly windy, and this makes wind the key large-scale disturbance agent in places like Glen Affric. The wind creates a whole host of niches: it can open up the canopy, and thus in the long-term leads to a diverse canopy structure, supporting more life than an even-aged stand. Wind-created glades allow light to the forest floor (which is essential for light-demanding species such as Scots pine and oak (Quercus petraea), while simultaneously providing much more shelter than the open moor.
It also helps create an invaluable part of the forest ecosystem - dead wood. Shattered branch stumps, fallen branches and fallen trees support an incredibly rich array of highly specialised life-forms. Windblown trees also create habitats for organisms such as Agabus wasastjernae, a rare predatory diving beetle which lives in the pools of water that form in the hollows left by the upturned rootplates of Scots pines. The exposed, sometimes sandy, soil remaining in the rootplate often provides the ideal medium in which solitary bees and wasps can make their burrows, and it is not unusual for birds to make their nests among these raised roots.
The top of an upturned rootplate can provide a haven for young trees to grow, beyond the reach of the mouths of red deer (Cervus elaphus). In spite of being blown down, a tree will not necessarily be killed, especially if some of its roots remain intact, and species such as aspen will send up vigorous new shoots in this situation. Aspen has also been observed to flower in response to stress such as wind-throw, and this may be a significant part of its ecology, as it tends to flower infrequently in the Highlands.
In addition to occasional catastrophic events, wind has a chronic effect. Trees in very exposed situations, or on the forest edge will often take on gnarled, stunted or windswept forms, and these sometimes act as a protective barrier to trees in the forest interior.
The action of herbivores, known as herbivory, is another example of positive disturbance in the forest. Trampling by the now extinct auroch (Bos primigenius) for example, would have broken up the sward and pushed seeds into the ground, thereby facilitating their germination and growth. Wild boar (Sus scrofa) formerly played a key role in the forest, as they still do elsewhere, in parts of mainland Europe. wild boar are omnivores, and their habit of rooting for food disturbs the soil, again creating a seed bed for trees, and reducing the dominance of bracken by breaking up its rhizomes.
Epidemics of insects can cause significant damage to trees, and as such are another agent of natural disturbance. The pine weevil (Hylobius abietis) can kill young pines by girdling the seedlings. This can be disastrous in commercial clear-cut areas which have been replanted, but these weevils rarely cause a major problem in natural forests.
Pine sawfly larvae (Neodiprion sertifer) tend to eat the old needles of Scots pine, so that an infestation may slow a tree's growth, but rarely kills it. Again, on a landscape scale, the structure of the forest is diversified.
In natural forests, monocultures are rare, and the diversity of species they contain generally makes them more resilient to pest damage. Insects are attracted to the scent of the trees they feed upon, so a target species may be 'masked' by the other species around it, thereby reducing the likelihood of insect attack.
Tree diseases may be caused by a range of organisms, such as fungi, viruses or bacteria, and are another diversifying influence in the forest. There are certain diseases which have been accidentally introduced, and others about which little is understood. Alder dieback, for example, has been observed along riverside trees in parts of Glen Affric. This fungal disease, which kills the upper parts of the tree, occurs more frequently in north-west Scotland. Rivers in this area are subject to extreme fluctuations, and this apparently causes enough stress to riverside trees that they are more susceptible to the disease. Similarly, birch dieback has recently been observed on newly planted birch, and research is now being carried out into its causes and effects.
As would be expected in an ecosystem, these events don't occur in isolation. For example, flooding may weaken a tree's natural defences, thereby allowing insects and fungi to invade. Additionally, certain types of fungi may weaken trees' root systems, and this in turn can make them more susceptible to wind-throw.
Over the millennia, disturbance puts pressure on all manner of species to evolve in ways. For example, Scots pines in the relatively cold and dry eastern Highlands are adapted to withstand the weight of heavy snow, whereas in the wetter west, they have evolved to have increased resistance to fungal infection.
Scotland's overall area of native woodland habitat has been drastically reduced and the surviving remnants are generally fragmented and isolated from each other. The fact that large-scale natural disturbances can actually pose a threat to our native woodland remnants is an indication that these fragments are far too small. In a prehistoric setting, there would have been enough forest cover to be able to withstand, and even thrive on, the effects of storm or fire. In a fragmented forest, species within small pockets of forest may be entirely wiped out, whereas in a more connected habitat, species may migrate to escape, or recolonise from surrounding areas.
Scientists are warning that human-induced climate change may lead to more extreme weather events, such as storms, floods and droughts. Since large, wild and robust ecosystems are more resilient than those that are fragmented, human-made and impoverished, the large-scale restoration of wild land should become a conservation priority. Native woodland ecosystems need to be given the space and opportunity to enable healthy responses to natural disturbance.
Sources & further reading
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