Tuesday 26 November 2013

False opposites in rewilding

A criticism of the rewilding debate at the moment is the artificial certainty that some have about what wild nature must have been like.  This is simply unknowable.  There is evidence but this can be interpreted in different ways and can give rise to apparently different models.  This is one criticism of George Monbiot’s book, and articles he has since published; he has a clear and unshakable view of the wild in Britain – dense woodland - a certainty that cannot be maintained.

The nature of a past wild state is only part any argument for a proposal for rewilding today; nevertheless, it may be helpful to go over some apparently conflicting ideas of “wilderness”. 

Ideas for pre-human wilderness apparently fall into two camps: the “closed canopy” model and the “open savannah” model.

The closed canopy model is probably what most people think of as “natural”.  Leave an area alone and it goes through the process of succession to climatic climax - bare land becomes colonised by small plants which give way to bigger plants, then scrub, then small trees, then big trees and eventually the trees form a closed canopy.  This is the “climax” vegetation which is supposed to be of a particular type for a given climatic zone – hence “climatic climax”.  It is said that once formed this climax stays basically intact with only small and temporary open patches within it.

At the other end of the spectrum is the open forest or savannah model.  The closed canopy model forgets the effect of most natural processes, especially the effect of large herbivores.  The savannah model, it is said, imagines a completely open landscape, dominated by grazed habitat, with occasional groves of trees.  A cycle is envisaged whereby you start with, say a grassland, trees become established in patches of spiny shrubs or when grazing happens to be low and then this patch develops into a grove of trees.  Large herbivores then shelter in these groves, eating regeneration so trees are not replaced so the canopy opens and grassland reforms.

Both models are probably just extremes; the reality of original wilderness probably included both concepts and a great spectrum of diversity in between.  But look at the two models a little closer and perhaps the models are not too far apart.

Once a range of natural processes are introduced into the picture, proponents of the closed canopy model generally accept that openings must have been present in the wildwood, not just small scale and transient but maybe including some that are long term and even quite large.  People talk of about 80% cover in trees, and even the wooded area would have been more diverse than we could imagine.

The “groves” in the open savannah model, however, (according to its major proponent) could well have been extremely large – say 700 hectares in size, and maybe would have joined up.  The model envisages up to 70% of the wildwood might have been in closed canopy groves.

80% trees in the closed canopy model against 70% trees in the open savannah model.  Not quite the divergence you might have imagined!

So, was the original wildwood mostly forest with occasional opening or largely savannah with joined up groves of trees – and does it matter?

In my view – no it doesn’t matter!  The point is that the original wildwood was probably far more diverse than we can imagine.  It would have included all the precursor habitats to the semi-natural habitats we know today, and probably a great deal more besides.  It only matters if people use a quasi-wilderness argument to push for the destruction of habitats that for some reason they don’t like!  We don’t know what the wildwood was like, we don’t know how it relates to current habitats and we don’t know how relevant it is to modern day nature conservation.  With all that uncertainly it seems a good bet to conserve the best of the range of ecological variation we have today.

This, however is also not an argument against rewilding.  But rewilding should be about putting the natural processes in place and heading towards an unknown end point – rather than recreating some supposition of the wild (and destroying things that don’t fit your idea).

Wednesday 20 November 2013

A present day wildwood – nearly!

Sussex Wildlife Trust’s biggest woodland nature reserve is “The Mens” a fascinating ancient forest in West Sussex.  The name comes from the Old English “Gemenes”, meaning community woodland (German speakers might recognise links with “gemeinschaft” which means community) and, many centuries ago it was used by the local community for grazing and fuel wood.  For a long time now, however, it has been left unmanaged.

When the SWT purchased it in 1974 we took the brave decision to leave it entirely to nature. “Brave” because at that time the decision divided the SWT - some saw a dilapidated, untidy place that needed to be cleared up and replanted.  Fortunately, however, a non-intervention policy was established so we now have a rare example of “old growth” woodland, designated as of international importance.

This has given us an excellent opportunity to follow natural processes at work.  So, what have we learned about woodland processes and how might this help us in the current discussion on wild land?

The 1987 storm was perhaps one of the most informative events to hit woodland ecology for a generation.  People often thought of woods as stable, undisturbed places that predictably developed to form a dominant woodland type – the so-called climatic climax.  George Peterken (one of Britain’s leading woodland ecologists), however, had been explaining the vital role of natural disturbance in woodlands.  Forest ecology was not predictable, could be very variable and therefore very diverse.  Storms are examples of unpredictable, formative events that drive woodland ecology and benefit biodiversity.  Storms are good for woods and for the last 25 years I have been trying to put over this as the antidote to the “devastating damage of the 1987 storm” stories that we still sometimes hear.

Looking at The Mens you can get a rough idea of what might happen in absence of disturbance (or at least if disturbance is very light).  The wood was heading towards dominance by beech with an under-story of holly – two heavy shade-casting and shade resilient species.  Other species were gradually reducing.  Oak, hazel, hawthorn, much of the ground flora and woodland lichens were disappearing or becoming patchy.  Other species were doing well - such as fungi, fly species and hole-nesting birds.  The wood was becoming poorer in some respects but unusual species were doing well.

Oak is a good example.  Often assumed to be a woodland tree it does not, however, regenerate in dense woodland.  This was born out in The Mens – age class studies show that there were few very old trees, quite a lot of medium-aged trees but almost no young trees.  This is the classic sign of a dying population – there are no young trees to replace the old ones as they eventually die.

The storm, I thought, provided the answer.  Major gaps blown in the canopy every few centuries would provide regeneration opportunities for all the light demanding species that make up most of our flora.  Sure enough, in the 25 years that followed there was a huge pulse of regeneration in all the canopy gaps that were blown in the wood.  Broken trees sprouted, shrubs grew, ground flora colonised and we saw a few more birds inside woods that usually stay on the edges.

But the storm was not enough!

Oak has not regenerated in the wood and the canopy gaps are quickly returning to dense beech woodland.  Storms are indeed vital to create diversity and provide habitat for an array of species but alone they cannot explain the presence of species that we know have been present throughout history.

The storm, however, is just one case of natural disturbance.  By recognising the value of storms, it also opens our minds to recognising the function of a great range of other forms of natural processes.  Alongside storms is erosion and accretion by rivers and lakes, tree fall on steep slopes, gaps formed when fungi or insects damage swathes of trees and the disturbance caused by wild grazing animals as they move through the forest.  The behaviour of grazing animals would also be affected by the presence of predators which in turn would have affected the habitats they grazed or browsed on.

Even if we leave areas as non-intervention, as we have done in The Mens, we cannot simply assume that we have created wild land.  In effect we are making decisions by default by leaving areas alone – we have restricted nature (any non-intervention area is too small), we have excluded large grazing animals (they are extinct, not present or not allowed to behave naturally), we have excluded predators, we react against natural disturbance (by clearing up storm damage or preventing erosion) – and so on.  So non-intervention is a positive management decision which creates a human artefact as much as any other management decision.

So why do we keep The Mens as a non-intervention area? 

In spite of the fact that many natural processes are restricted in The Mens, any model for “the wild” would probably have included large areas of old growth forest where natural disturbance was indeed quite light.  So The Mens represents a type of habitat that would have been common in the wildwood but which is now extremely rare.  Although the site may be poor in many woodland species, it is rich in species that are restricted to old growth situations.  Also, The Mens exists in a landscape context.  Open habitat in areas around The Mens provide complementary habitat for many species that exist within the wood, so we do not need to create extra canopy gaps to maintain the biodiversity in the overall area.

The Mens is perhaps one of the best examples we have of what a “near” natural wood might be like.  It is a fascinating area to study.  But most of all – just go there!  It’s a magic place.

Thursday 14 November 2013

The myth of the dense wildwood!

George Monbiot’s excellent book “Feral” has re-opened an interesting discussion on the idea of rewilding in the British landscape.  Included within it, however, is the myth of the dense wildwood – the idea that a dense forest once clothed the British landscape from coast to coast such that a squirrel could have covered the whole of the country without ever having to tread on the ground.

An attractive idea maybe – after all if you leave an area alone it develops, eventually, into a wood.  So, natural equals dense forest.  Or so the story goes.  But it is just a story.

The story, however, appears to be backed up by some science.  Pollen deposited in soils can be dated and identified so it should be possible to build up a picture of the species that were present at given times throughout history.  And, hey-presto, we have the answer.  Tree pollen takes over after the ice age finishes and non-tree pollen does not become common again until humans clear the forest for agriculture.  What is more, if you look at evidence from remains of beetles, which again can be identified and dated, then you get a supporting picture – woodland beetles, apparently, dominated throughout the wildwood period “proving” that ours was a land of dense trees before humans interfered.

Fundamental problems start to appear if you dig a little deeper though.  If dense forest was the natural habitat then how come about 50% of our plants and animals need open habitat, and about half of the rest need forest edges?  The traditional answer has it that these species were limited to small, transient patches where they struggled for existence only to emerge when human clearance gave them an opportunity.  A feeble answer but it seems to satisfy some, so, the closed canopy model is often unquestioningly adopted.  Britain should be dense with trees and anyone who tries to do otherwise is simple fighting nature.

Having accepted this model, conservationists then fall in to two camps.  One has it that these open habitat species are just a human artefact and we can do without them.  The other has it that humans have so influenced the natural environment that any emphasis on natural processes is miss-placed and it is human management that is the key tool for nature conservation.  The first is an argument against nature the second is an argument against natural processes. 

I disagree with both as I disagree with the model they both assume for wild nature.

If you look again at the evidence then all is not as it seems.  Indeed wild nature is far more complex, interesting and wonderful than is ever likely to be covered by some simplistic human model.

Look at the pollen evidence.  First there is a history of “fiddle factors” being put into the interpretation.  This is necessary because some plants are insect pollinated so produce little pollen; others are wind pollinated so produce vast amounts.  Understandable, but small changes here can make big differences to the interpretation.

More importantly, however, is the abundance of both hazel and oak throughout the pollen record.  The standard model ignores the fact that neither of these can regenerate under a tree canopy.  Indeed hazel does not flower, so does not produce pollen, even if it does manage to grow in a dense woodland. So we are missing something – how did all that oak and hazel manage to grow if there was very little open habitat for their seeds to germinate in?

How about those woodland beetles?  Well most of these are associated with trees, not woods, and the richest tree for beetles is oak.  Furthermore, when you look at the requirements of the individual beetle species you find that they require not just any oak, but oak that has grown in open conditions – indeed some need sunlight right down to the forest floor.  A similar picture emerges if you examine fly species that are specific to oak.  Today, old open-grown oak trees that get surrounded by dense forest loose all their specialist species and eventually die themselves.   So, in the wildwood we have a picture of common trees that need open habitat in order to regenerate, and their associated insect species that will only survive if those trees grow in the open.  A very different picture to the dense carpet of dark woodland that is constantly promoted.

The only sensible conclusion is that open habitat was present, probably quite common, in the wildwood. 

This is not to say that the wildwood was only open habitat.  Other shade tolerant species (such as lime, hornbeam, elm etc) were also common.  So dense forest would also have been there, probably abundant.  My guess is that the wildwood would have included far more complexity and diversity than we could possibly imagine today. 

How could this be possible?  It is pretty clear that the simplistic “closed canopy” model, which assumes the only significant natural process is the growth of trees, has major problems.  In particular the model ignores the role of natural disturbance; indeed some take it so far as to assume that disturbance is an unnatural, bad thing that should be avoided.  The 1987 storm, however, gave us a clue on the beneficial role of natural disturbance.  Dense tree canopies were opened up, flowers, trees and shrubs were able to regenerate and more species were attracted into a forest.  On top of that is flooding, erosion, insect damage, fungal disease, the effects of grazers and browsers and also the effects of predators on grazers and browsers.

Natural disturbance is not a bad thing that is imposed on an otherwise peaceful nature – on the contrary it is a main driving force within nature.

Any rewilding suggestions that do not consider the role of natural disturbance should not be considered rewilding at all – merely the construction of another human artifact.

There is a huge and fascinating agenda relating to rewilding; this will include the reintroduction (or mimicking) of natural processes – a range of processes not just a selected few.  Rewilding is not the same thing as neglect.

There are flaws in his argument but we owe George Monbiot a great debt of gratitude.  He has succeeded where many of us have failed – he has brought the subject of rewilding into the public consciousness and stimulated a fruitful discussion.  Something that I aim to continue in future blogs.