by Stephen1 » Thu Jun 14, 2012 10:56 am
My view on this is one of absolute agreement with RichardKing.
Conservation "Management" is a very complex area - and this is particuarly true of woodland in the UK (if you're interested then ask me why, otherwise this post will get too long!)
Biodiversity has come to be a kind of proxy for the conservation value of an area of land. The higher the biodiversity the better.
This is very unfortunate thinking, and has come about because we live in a day and age when everything has to be reduced to easily understood soundbites. The concept of maximising biodiversity as unquestionably being the best course of action is only appropriate at the landscape scale. Where the piece of land being managed is relatively small other less easily measured parameters may be paramount.
If you have an area of woodland that has been undisturbed for a long tim,e then this may be described as being on a trajectory towards "old growth" habitat. A very rare and very valuable habitat in the UK - with very specific species associated with it. Old growth habitat is characterised by big old trees, lots of deadwood (on the floor and standing) and lots of shade. It has fewer of the 'showey' species associated with coppice, and possibly less species than areas of woodland that have been well managed as coppice for a long time. But does that make it less valuable?
Coppice mimics what happens when a large gap (wind,disease etc.) was made in the original 'wildwood' - you get a flush of light-adapted species. For most of the last couple of millenia in the UK our coppice woodlands have sat in an agricultural landscape and so grassland species also disperse into the light flooded gaps that coppicing creates - it's a kind of unstable hybrid of two habitats and this is reflected in the apparent high species count. It undoubtedly has high biodiversity and you can see the results of your efforts quickly.
The big worry for the areas of shady 'older' growth woodland in the UK is that new managers come in and decide to increase the biodiveristy by letting the light in. The idea being to get the best of both worlds - they keep some old trees with all the more cryptic species associated with old growth, and get the 'Gap phase' species associated with light.
Unfortunately there is a problem with that. Most of this management goes on at a small scale. (I notice that most woodlands sold by woodlands.co.uk are around 5 acres or so, and I believe lots of folk here have bought through them?) It ignores a concept every bit as important as biodiversity and that's bioresilience. If I fell half the old trees I half the populations of lichens, invertebrates etc. associated with them, I also half the populations of 'boring' hard-to-identify bryophytes adapted to deep shade on the woodland floor. These species are very slow to disperse and increase their populations - and even when coppicing stops take a very long time to recover. By coppicing such areas I immediately increase the apparent biodiversity, but I do it at the expense of the bioresilience of the species adapted to the shadey old growth woodland habitat.
Imagine a 5 acre wood bought as a conservation project. It's been "neglected" since felling took place during the last war - a very common situation for farm woods in the UK. The new owners decide to increase the biodiversity by starting a program of coppicing. They start a ten year cycle of about half an acre a year but to get thing's going they to "open up" all the rides, create glades throughout the woodand including one of about half an acre which they clear and sow with a wildflower meadow mix, and as the jewel in the crown of their conservation project fell another half acre to create a half acre pond. Also because the hazel is in so much shade and isn't looking "Healthy" they decide to thin out half the standard oaks straight away. Biodiversity, in terms of number of species present, dramatically increases. At the end of ten years to the human eye the woodland is alive - butterflies, carpets of wildflowers- wonderful!
But all those species that needed undisturbed, shadey older growth are suffering; the bulin snails, the slugs, many non-obvious fungi, bryophytes, arthropods etc. their popuations are drastically reduced and many become non-viable. Over time these species are lost from the wood - but people don't notice because they are hard to spot and identify and also because we are culturally unaware of them.
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(humans spend time in woods that humans spend time in! Circular I know but we tend as a culture to know the species associated with woodland that is managed by humans - so we value primroses, bluebells etc. at artifically high numbers because that is what our culture expects from woodland - we expect woodland to be full of 'healthy' trees standing vertically with plenty of light and lots of spring fowers - these are the things we value, talk about, recognise, know the the names for and want)
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The 'old growth' species that start to get lost from the wood do so because the bioresilience of these species is reduced; their populations are reduced through lack of habitat (i.e.old trees, shadey floors (let the light in and they are out competed by 'gap phase species') Not only are their population numbers reduced but so too is the genetic diversity of their populations. Meaning they are less able to cope with specific problems that may happen i.e. unusual weather patterns or disease etc. Most significantly even if the coppicing program is abandoned and the wood is 'neglected' again those 'older growth' species that remain will have very restricted genetic diversity, and so are at greater risk of being lost at sometime in the future - perhaps even centuries down the line.
The reducution in 'older growth' species, and the reduction in the bioresilience of the 'older growth' species that remain is permanent.
The new owners are pleased with their work 'rejuventaing' the wood - and that's what they've done- they've made it young again.
These days woodland ecologists no longer subscribe to the myth of a climax stable state woodland, they realise it changes over time and may shift from dominance by one type of tree to another, but it does remain woodland without human intervention (ice-ages not withstanding!) and the evolutionary strategies of many of its denizens reflect that stability. Of course there were also species originaly associated with the gaps created by wind, tree death etc. These species have come to be associated with man's activities in woodland coppicing, felling etc. and they have become over-important to us culturally at the expense of those species that require deep old shadey stability.
When people talk about increasing biodiversity and in the next breath mention improving the health of the hazel coppice by letting light in -without wanting to cause offence I suggest that they are confused. If hazel has lots of light and thus is able to make lots of energy it can produce lots of chemicals that fight of fungi and reduce insect predation on it's leaves. For example hazel is prone to honey fungus in the shade but not when grown in the light. But that honey fungus, those insects feeding on it's leaves, the dead hazel stems full of beetle grubs and the birds eating the insects that the hazel can't fight off because it doesn't have enough energy to make the phytochemicals that stop the caterpillars/aphids etc. feeding on its leaves - that IS the biodiversity. Humanwise "heathy" hazel growing in lots of light supports much less biodiversity.
The 'rejuventaing' action of human choice isn't always ideal for woodland, it brings forth the flowers and butterflies, but lets have some respect for the Old who like to stay hidden away. Don't confuse increasing the conservation value of an area of woodland with merely increasing the biodiveristy (in terms of species count) by creating what actually amounts to a mosaic of different habitats on a scale too small to also benefit the bioresilience of existing species of value in your patch. A patchwork of different habitat types is what is needed at the landscape scale, and biodiveristy needs to be measured at a landscape scale - small areas need careful consideration in terms of what is 'best' to do and the pursuit of "biodiversity" increases can often be a redherring at this scale.
Last edited by
Stephen1 on Thu Jun 14, 2012 11:50 am, edited 5 times in total.
