By Dr Tony Whitbread
At risk of over-simplifying hugely, there are two key natural processes that shape the world we see.
One, “succession”, is familiar to us. This seems to be what happens if you leave an area alone. Plants grow, tall plants take over from shorter ones, then scrub invades, to be taken over by small trees, which grow into large trees and eventually a forest forms. Some consider this to be the end point. It is even given a name – climax forest.
However, most of our native species do not live in dense forest. Most live on forest edges or in open habitats like grasslands. Even those that require dense forest may require other habitats at some stage in their lives. So the popular idea that Britain would naturally be covered in continual dense forest is flawed.
We can see the process if we study unmanaged, undisturbed forests. If undisturbed a mixed forest with say, oak, ash, hazel, holly and beech will gradually lose species as they become over shadowed. It will head towards beech dominance with a holly shrub layer. In practice, however, this is interrupted by natural disturbance, re-setting the clock giving, for example, ash, oak and hazel a chance. This is a small scale example, but this works on a far larger scale driving whole habitat change.
Succession towards a climax forest is only one force. In the opposite direction is the other key force – natural disturbance. The 1987 storm reminded us of this. Climax forest is only a concept, in practice it is continually knocked back by natural disturbance. In effect natural disturbance continually re-sets the clock on succession. And this is what creates diversity in nature.
Windstorms, however, are only one form of natural disturbance. And windstorms alone are not enough to explain the full diversity of nature created by disturbance. So what are the other forms of natural disturbance that might have created diverse natural habitats in a true natural situation?
These are many and various.
Flooding, erosion and accretion are examples in wet areas. Tree death from disease and fungal attack also causes gaps in forests. The action of grazing and browsing herbivores are perhaps a huge driving force in some areas – a area opened up by windstorms may be maintained as a permanent open habitat as grazers are drawn into the area. In the distant past wild forests would have been roamed by herds of wild aurochs – a wild cow (now extinct) which was at least 6” bigger than the biggest cow you can ever imagine! The effect of these would not have been minor. Beavers are well-known for the way they fell trees and open up forests in wetland areas. Wild boar virtually plough areas creating swathes of disturbed soil. Some areas might have been damaged by fire, others by fighting deer stags. And the effects of grazing and browsing animals would have been ameliorated or driven by the impact of large predators.
There has been much debate recently bout the idea of “rewilding” – the restoration of ecosystems by the reintroduction of natural processes. Some (including some ecologists who should know better) consider that this is simply a matter of finding a forest and “allowing natural succession to take its course”. This presumes that the only natural process is succession and denies the presence of natural disturbance.
We do have rare patches of, “old growth forest” – areas of forest that do indeed have very low levels of natural disturbance. The Mens, one of our largest nature reserves, is an example. But, generally speaking, denying an area its cycle of natural disturbance is not “rewilding” it is simply abandonment.
Giving nature free reign – the essence of rewilding – requires that we bring back the natural processes that are absent. And this means bringing back natural disturbance as much as allowing the progress of succession.
A fine example of this – perhaps the best example in lowland Britain is the Knepp estate in West Sussex. Visit their web site to find out more. https://knepp.co.uk/
Rewilding, giving nature free reign by restoring natural disturbance and succession, is a great ambition. It is a popular aspect of nature conservation, we should do much more of it and it could yield great gain for nature and people. It is not, however, something that can be done everywhere. Most of our landscape is a cultural landscape where ecology will be driven by management by people. The 1987 storm and from it our understanding of natural disturbance, can, however, tell us another story. Human management is just another form of disturbance. This will be the subject of my next blog.