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?

No comments: