Wednesday 25 October 2017

30 years after the 1987 storm, part 4 – a taste of rewilding

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 – an 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 about 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.

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.

Friday 20 October 2017

30 years after the 1987 storm, part 3 – a ripple of disturbance

There may be a belief that nature is best when it is undisturbed.  Leave nature alone, prevent disturbance, keep it calm and peaceful and it will thrive.  This is not true!

High levels of disturbance may create one sort of habitat – a weed community - at the expense of others.  So too much disturbance is a bad thing, and this is probably no surprise. 

However, it is sometimes still not appreciated that too little disturbance is also damaging.  Woods become dark and monotonous.  They have a limited range of conditions within them and so support a limited range of species.    Woods kept as undisturbed, dense, shaded places are not “natural”, they are probably best seen as unnaturally undisturbed!  Without realising it we may have removed and prevented (or just cleared up) the agents of natural disturbance that create diversity in nature.  Take away disturbance and you are not left with nature, you are left with abandonment.

As part of a greater matrix, “old-growth” undisturbed forest is a rare and valuable thing.  It will contain species that are rare elsewhere, often species that are slow to colonise and prone to local extinction. Even this, however will have its own dynamic of natural disturbance and some species in old-growth areas rely on disturbed patches nearby for some stage in their life cycle.  Insects are a good example – some may need old-growth for part of their life cycle, but also need nectar sources from the flowering shrubs in disturbed, open patches at other times.

Ecologists now recognise that an intermediate level of disturbance in a patchwork better explains the presence of our native species than provided either by heavy disturbance or no disturbance.

One thing has become clear over the 30 years that follow the storm, however.  When I surveyed woods after 1987 I thought that the storm provided the answer – this is how forests are kept diverse and this explains the presence of our native species.  On reflection, however, 30 years on we realise that this is not the case.  The canopy gaps formed 30 years ago have now disappeared, they have become part of the forest and, whereas the diversity created can still be seen, these gaps have now become part of the forest canopy.  One storm every 300 years is not enough!  The storm gave us great insight, but it is only one form of natural disturbance.  If we wish to understand nature then we need to look more broadly at all the different forms of natural disturbance.

So what can we learn from this?  If our forests are unnaturally undisturbed and so poorer as a result, can we bring back natural disturbance, or can we manage forests in a way that has the same effect as natural disturbance?

Wednesday 18 October 2017

30 years after the 1987 storm, part 2 – what happened?

The night of 15th October 1987 saw storm force winds around 100 mph hitting the south eastern corner of Britain – from Hampshire to Suffolk, some 14 counties in all.  This followed a time of heavy rainfall, soils were water-logged and, being a mild autumn, the trees were still in leaf.  So the full force of the storm hit at a time when trees were actually quite vulnerable.

In practice, however, the wind came with such force that enormous amounts of disturbance were inevitable.  Some 15 million trees (probably an under-estimate) had been windthrown (uprooted) or wind-snapped (broken at the stem).  In addition and uncountable number of trees had experienced major loss of branches from the crown.

The effects were dramatic.  Huge areas had blown flat and, even more frequently, holes of various sizes had been blown into woods.  Swathes of damage and canopy gaps had appeared everywhere.

Strangely, trees seem much bigger when they are laying down than when standing up!  The amount of wood just lying around was enormous

In terms of living memory, this sort of event is so rare that it was considered a freak of nature.  No one could remember anything like this.  But trees live for a long time – forests even longer.  There have been similar events in the past - the great storm of 1703 being even more devastating.  Storms like this may have a return time of around 200 to 300 years.  On the scale of the life span of individual trees, and in terms of the age of whole forests, this was not an unusual event.

So – what were the real effects of the storm on our woods and forests?

A dispassionate examination of what actually happened to the ecology of forests gives a very different picture to the tales of destruction so beloved of the media reports.  The areas blown flat and the canopy gaps created, generated great diversity in forests that had often become dense and over shadowed.  Light was able to get to the forest floor, often for the first time in decades.  In the years that followed we saw a burst of regrowth of the ground flora.  Species flourished where they had previously been overshadowed, in some places species appeared (heather for instance) that had not been seen in a wood for many decades.  Insect life flourished and birds were drawn into the newly created patches. 

As the years went by we saw shrubs regenerating in gaps – species that would not have stood a chance under dense woodland.  As light got into the forest shrubs were able to flower, attracting nectar-feeding insects – and insect eating birds.  Trees sprouted from broken limbs or crushed root plates and spread to fill in the gaps that had formed.  In bigger gaps there was a flush of regenerating tree seedlings – a diversity of species often far greater than was represented in the earlier woodland canopy.

Changes to structure also had unforeseen effects.  Damaged trees supported more fungi and wood boring insects, hole nesting birds had more of a chance to nest and piles of decomposing brushwood provided nesting, roosting and foraging sites for birds and small mammals.  Windblown trees left upturned root plates, regeneration sites for plants and shrubs, and water filled hollows used by wetland plants and amphibians.  We had a case of kingfisher nesting in an upturned root plate here at Woods Mill.

One worry after the 1987 storm was the human need to do something – an impetus to clear up the mess and get things back to the way they should be.  Whilst management might have been entirely justified in special places (tree collections and arboreta for instance) tidying up afterwards caused far more damage to our woods than did the storm.

There may have been great mortality of wildlife on the night of the storm.  But the living space created by the storms beneficial disturbance created enormous opportunity for all manner of wildlife which was able to thrive as a result.

When it comes to wildlife, nature and forests – storms are great!

30 years after the 1987 storm, part 1 – a personal reflection.

1987 was an interesting year for me.  I had just moved to Sussex and was carrying out ancient woodland inventories on behalf of the then Nature Conservancy Council (now Natural England).  And then, overnight on the 15th October, everything changed.

In fact I managed to sleep through it.  Perhaps the most significant ecological event of the century, and I was asleep!  I was living in South Chailey at the time. I awoke to a strangely quiet day - no traffic, no electricity and not many people around.  I ended up walking to work, gradually realising that this was a really significant event.  The whole of the south eastern corner of Britain had been hit by the strongest storm that this part of the country had seen for over 300 years.

The thing that stays in my mind is the smell of crushed wood.  Freshly cut wood has a certain smell, but it is usually limited to saw mills or timber yards; you don’t usually notice it hanging across an entire landscape. Trees were down, roads were blocked, buildings severely damaged and (often forgotten now) areas were flooded because of the rain that preceded the storm.

Enormous amounts of damage had been done, and quite a large number of human tragedies as well.  But, alongside this, the 1987 storm was a fundamental ecological event that deserved proper study. 

I was lucky; I was in the right place at the right time.  I happened to be working for some of the best woodland ecologists in the country – namely George Peterken and Keith Kirby.  They saw the opportunity and decided to take me away from my normal job and engage me in work looking at the ecological effects of the storm.  A short term contract, and later I was able to do similar work for the Wildlife Trusts.

And so it was that I was able to go around some of the most interesting forests and woods in the south east to research what had actually happened and assess what ecological story this might be telling us.

Ecologists had for a long time been looking at how natural disturbance creates diversity in nature.  The old idea of nature being stable and unchanging, that disturbance was a bad thing to be avoided, had been dispelled a long time ago (even if it still remains in popular myth).  Here we had an example of natural disturbance on a huge scale.  This was a rare chance to see something of how nature works in practice.

The storm was not “damage” inflicted upon nature.  It should not even be seen as separate from nature at all.  The storm was nature.  It was an inherent, even a required part of nature – a natural process that drives the way whole ecosystems work.  Indeed this could be just one example of the process of natural disturbance that drives nature.  If we understand that then maybe we can gain a better insight on how to manage nature, how to encourage our natural world to look after itself, maybe even gain a better understanding of our relationship with nature.

30 years later this remains the case.  There is a big story to tell here – and nobody is telling it.  This is what I aim to do in my next few blogs.