Tuesday 7 January 2020

Trouble with trees (1 of 4)

The creation of forests and woods can be a major contribution to restoring nature and can draw carbon out of the atmosphere, so helping fight climate change.  Indeed, with a UK average tree cover of just 13%, it would not be unreasonable to double this. 

Done badly, however, tree planting and tree regeneration can cause major ecological damage.  We must make sure that a frantic rush to plant trees does not repeat the errors of the past.

Enormous ecological damage was done in the mid-20th century through tree planting.  Vast areas of the Flow Country in NE Scotland were drained and planted with non-native conifer trees devastating the local ecology and (by causing the drying of peat) was a major emitter of carbon dioxide.  Many other upland areas saw similar devastation.  In Sussex too some of our most valuable habitats were lost to tree planting.  For example, over a period of about 100 years we lost roughly 80% of our heathland, nearly half of that to tree planting.  In the 1990s it was feared that many of our heathland species would disappear altogether as a result.  Fortunately, however, conservation management projects managed to reverse this trend.  Management – mainly tree removal not planting – averted an ecological disaster.

Similarly, loss of chalk grassland to tree planting and the spread of scrub is second only to loss to arable cultivation.  Chalk grassland can have about 40 species of sensitive plant per sq m; this reduces to a small number of common species if scrub invades or trees are planted.  It is likely that we have also lost hay meadows – one of our most threatened habitats – to tree planting. 

Large areas of the most diverse habitats in England have therefore been lost to trees.  These habitats were often as good as trees in locking up carbon and so fighting climate change.  We now have only small areas of these habitats remaining, tree planting on these would be unforgivable.

The desire for trees can come about from a misunderstanding about the natural ecology of Britain.  There is a presumption that a dense canopy of trees is the natural state for our country.  This is not true.  We’ve known it was nonsense since I was in college in the 1970’s.  And it is nonsense still promulgated by people who should know better.  So, let’s try to put this myth to bed!

If dense tree cover was the natural state for Britain, then most of our native species would logically require dense woodland.  They do not.  More than half of our species require open habitats or forest edges; very few require continuous dense trees.  Even species we associate with dense forest often require open habitats at some stage – oak and hazel for instance regenerate better in the open.  If the natural state of Britain was dense trees then most of our native species would never have colonised Britain, indeed they could never have evolved, in the first place.  The natural state of our landscape is one of great diversity not a continuous monotonous tree cover.  This diversity of original natural habitats is now maintained as “semi-natural” habitats through centuries of management. 

We may think of tree growth as natural, but tree growth is not the only natural process.  Importantly, there are processes of natural disturbance that limit or hold back tree growth (think of beavers!).  It is this balance of tree growth against natural disturbance that creates diversity.  Too much of one or the other and nature suffers.  This is the real value of management – it replaces natural disturbance with disturbance caused by management, putting back the diversity that can be lost through a lack of natural disturbance.

Fortunately, the situation is better today than it was in the mid-20th century.  Organisations involved in tree planting (such as Forestry Commission and Woodland Trust) are very aware of the potential problems.  These will plan planting properly, delivering benefit and avoiding problems.  Anyone wishing to plant trees should show similar care.  There are also better alternatives to planting. Natural regeneration and rewilding are more likely to deliver diversity, are cheaper, fit local ecology better and require less aftercare. 

I do worry, however, that a destructively naive view of nature and a rush to get trees in the ground will sweep all before it.  We risk repeating the errors of the past and once again cause great ecological damage.  Simple solutions to complex problems are always wrong.  We should move the conversation from “tree planting” to “natural regeneration” then to “rewilding” and to “natural climate solutions”.  Tree planting may have a role, but only as part of a diverse nature recovery network, not as an unquestioning paradigm.


Bob Thorp said...

Very pleased you've written this, chimes with much of my own fears for quota driven tree planting. We should seek optimal solutions for multiple objectives.

Brian Banks said...

Well said Tony.

Brian Banks

Bankvole said...

A very timely note of caution to a knee jerk reaction to alleviating climate change.
Nature is remarkable and best left to 'her' own devices in lots of instances.

Unknown said...

Totally get this. I'm interested in urban tree planting though, particularly managed with vegetation understorey or scrub. Thinking that this can provide habitat and continuity of existing wildlife corridors through grass deserts. Any pointers for advice?

Unknown said...

Very interesting and as you said simple solutions dont fit complex ever shifting natural processes. I'm a forest but also trained and a practicing ing ecologist involved with both sectors as I believe firmly that one should complement the other. I'm involved with a large ecological restoration project, I dont like the term rewilding. Much of what we are currently focusing on is fro Vera hypothesis, however it seems not to ring true in many instances, I have seen excellent examples of oak regenerating freely under dense canopy and surviving in all layers from floor to canopy. This year in certain areas voles are in a boom cycle eating their way happily through trees, planted and regenerated yet where are our predators, this would effect regeneration of species and have a subsequent knock on effect. Our mast in the uk for oak in our present climatic conditions is sparodic and sparse compared to other European countries, we are missing boar, plus many predators and two thirds of our deer species are non native so again a big effect. Many rewilding projects are finding that their sites scrub up, it will be interesting to see what the effect of this is.
final thought, were only approximately 10 to 15 generations of ancient oaks forward from the last ice age so are we not still in the early successional stages? Perhaps this goes someway to explaining mental vegetation as it moves infront of the more shade tolerant species. All food for thought I hope