Monday, 10 February 2014

Biodiversity offsets

8 January 2014

Recent announcements about the coalition Government's proposals for biodiversity offsetting raise some very serious questions about the degree to which policy is based on sound science. In 2005 a team of English Nature specialists investigated the issue of offsetting from the perspective of how long it might take replacement habitat to reach a condition commensurate with established semi-natural habitats. The evidence was disturbing because it highlighted a series of fundamental problems:
  • Established habitats are very largely the product of management that existed prior to deep ploughing and modern herbicides, pesticides and fertilisers. These modern practices mean that it is not possible to look at arable reversion from the same perspective as times when agriculture was less intensive.
  • All of the evidence pointed to very long time-frames for the creation of habitat that is any way analogous to existing semi-natural habitats. Apart from some wetlands, the timescales run into many decades or hundreds of years, and ancient habitats look to be extremely unlikely to be replicated. For example the plants and animals inhabiting a secondary woodland younger than 400 years are rarely, if ever, as rich as those in ancient woodlands.
  • The most critical factors involve a combination of soil chemistry, mycology, lithology and hydrology, and we know precious little about the role of bacteria in determining conditions. Soils that have been recently ploughed and fertilised with petrochemical-based fertilisers bear little resemblance to the chemical and biological conditions that existed prior to WW2, let alone those of the 17th Century, which marks the point at which woodlands start to be considered to be 'ancient'.
A paper published by this team in 2006 provides the foundation for the scientific debate about the likely efficacy of biodiversity offsets. Yet this, and associated evidence, seems to have been ignored, if pronouncements concerning ancient woodlands are genuinely the Government position. Suggestions that the replacement of an ancient woodland with 100 trees for each ancient tree lost is environmentally sound need to be tested scientifically. Is the Government thinking of the environment in a visual or an ecological sense? I think it must be assumed that this is a landscape rather than an ecological judgement because I cannot see how the Government's conservation advisers could conclude that there would be anything other than biodiversity loss from the destruction of ancient woodlands.

I have spent a substantial amount of time studying the insects of the British Isles and have visited a great many sites, from Shetland to Cornwall and Kent, both ancient and modern. In many cases one barely gets past the entrance before it is apparent that the site is going to be of interest or is going to be pretty mundane. One gets the feeling from the ground flora, the canopy structure (in woodlands) and the overall layout of the site. The insects are pretty telling too. Long-established sites invariably prove to be more interesting and there are good reasons for this.

At one time the countryside was far less regimented and there were uses for virtually everything. Coppiced trees provided small timber for tools, fencing or other utensils, and for firewood. Standard trees provided structural timber and planking, whilst certain trees were deliberately pollarded to provide timbers of particular shapes for construction or shipbuilding. Grassland might have been rotated to lie fallow to improve soil productivity; elsewhere it would have provided grazing or hay. The vocabulary of the countryside reflects this diversity of use with terms such as coppice (copse), meadow and pasture forming place names. This mixed environment in which habitat was closely juxtaposed allowed plants and animals to disperse relatively readily. Today, such habitat is isolated in little pockets that we call nature reserves or protected sites.

Site protection has arisen because a part of society actually values wild places. This is particularly so for ancient woodlands, which form a central part of the countryside, for their landscape, cultural and wildlife value. The landscape attributes are obvious, but the wildlife is rarely recognised. Much of the important wildlife (in terms of rarity and specialness) is small and relatively inconspicuous. Few visitors to the countryside other than the most ardent specialists would delight in the sight of a Duke of Burgundy Fritillary Butterfly or perhaps a Musk Beetle. It therefore follows that relatively few would miss their passing. But, many older people lament the loss of fields full of orchids or comment that butterflies seem to be much scarcer these days. In the case of my passion, numerous associates comment on the decline in insect activity at hogweed. Only today one of these friends commented that the hogweed fauna of Shropshire seemed to have declined substantially in recent years. One wonders why?

Once the basic landscape matrix has been lost, wildlife cannot move as freely. If a special site is destroyed, the special plants and animals don't just move - they die! Once they are gone, the overall numbers available to set seed, disperse spores or lay eggs to create a new generation also decline. This is why there has been an ongoing effort to arrest biodiversity loss. The first target date was 2000. That date was ambitious and the target was not achieved. A new target was set for 2010 and it too was missed. The current target is 2020 and yet in the midst of this decline, the Government determines that further ancient sites should be destroyed and replaced with pastiche. After all, nobody will notice providing the countryside has trees to create visual spectacles! If one waits for 100 years for the site to attain the characteristics needed by the plants and animals displaced when the ancient woodland was cleared, where will the colonists come from? They disappeared when the wood was bulldozed!

This saddens me greatly because we must brace ourselves for a further round of accelerated wildlife loss. There are already many parts of the country that are to all intents and purposes ecological deserts. The fens of south Lincolnshire immediately spring to mind, but there are others such as the Vale of York, the southern uplands of Scotland, much of Radnorshire and many parts of the uplands of northern England where the landscape may be spectacular but is dominated by sheepwalks! And then there are the urban habitats where the gardens are turned into concrete and wooden shrines to the motor car or the barbecue!

9 January 2014

Looking at the pronouncements on possible ratios of replacement trees to original trees, I wonder if the Government has seriously considered the areas of land required to offset loss of ancient woodland? Using some basic calculations, it seems to me that they are going to have to require some major areas of the countryside to be allocated to tree planting if they are serious about a 100:1 replacement ratio.

I have just done some 'back of a fag packet calculations (thankfully I gave up 10 years ago, so I have to use scrap paper but I doubt that makes much difference to the science!) and the following figures come up:

1. In traditional coppice with standards, tree density is likely to be anywhere between 30 and 100 crowns per hectare. That, I suspect refers solely to the 'standards' and thus the question must be whether additional provision needs to be made for the coppice stools?

2. Working on a density of 65 crowns per hectare as a median, that suggests that the Government is proposing a compensation ratio of 6.5 ha for every 1 lost to development. This figure is based on a planting density of 1,000 trees per hectare in the replacement woodland, giving a ratio that far exceeds any so far for displacing migratory waterfowl.

3. If the pastiche is to replicate ancient woodland, then there also needs to be management to create the mix of coppice stools and standards. What needs to be borne in mind from an ecological perspective is that the coppice stools form a substantial part of the ecological value. This is largely underground and comprises the decaying timber that supports many fungi and the animals whose larvae feed on the fungi and bacterial soup that results.

4. It follows that the offset will require intensive management, unless of course we are simply looking at replacing an ancient woodland with a poor quality plantation. If so, there is even more to go on in terms of ecological loss because the relative importance of plantations and semi-natural habitats ought to be relatively easily demonstrated.

There is plenty of sound ecology to challenge the proposals, but perhaps the more important issue is economics? Is it really worth trashing an ancient woodland if the developer has the cost of buying anything between 3 and 10 ha to replace each hectare lost, and then has to pay for intensive management for sufficient time to create anything approaching pastiche? Someone better than me can do the economics, but it seems to me that the likely cost of creating the offset will at least mirror the cost of the ancient woodland and may indeed greatly outstrip it because woodland is relatively cheap and even low grade arable is phenomenally expensive, without adding on the cost of long-term silviculture.

No comments:

Post a Comment