What are our woodlands like?

Native and non-native

Native tree species are those which established themselves in Britain after the most recent glacial period when the botanical slate was all but wiped clean approximately 11,000 years ago, and before significant human civilisation. Native woodland is descended directly from those trees which spread over the British Isles after the last glaciation. Native tree and shrub species are valuable components of the nature and landscapes of Britain, providing much of the native wildlife with its natural habitats, priority is therefore given to maintaining the native trees and shrubs in our woods. Some native tree and shrub species are commonly associated with a prodigious diversity and biomass of wildlife, whilst other less bountiful species provide habitat for specialised flora and fauna. Consequently, tree and shrub species are usually of the highest value to nature conservation when they grow in places where they would occur naturally in the absence of human disturbance or assistance through the ages.

It is, however, a far from simple task to detect which tree species were natural colonists of the British Isles and which were introduced by humans. It is likely that many species may have colonised and been introduced to Britain on many occasions. The modern fragmentation of British woodland cover and increase in tree planting has obscured the present-natural tree species distributions. It is very difficult to predict what their natural biogeography would now be. Similarly it is impossible to do more than speculate as to which of the European species of tree which failed to colonize any part of Britain naturally before the Mesolithic would subsequently have arrived without human assistance. Nor can we predict accurately what the geography of their subsequent invasion might have been.

Notwithstanding the difficulties related to identifying "nativeness", the UK Forestry Standard   recommends using plants of local provenance (provenance=the location of the stand from where the seed was collected), preferably from semi-natural parent trees, where planting is to be undertaken in semi-natural woodlands or where native woodlands are to be created by planting. The UK Woodland Assurance Scheme UK Woodland Assurance Scheme   also seeks the use of seed of local origin(i.e. the natural range from which the seed originally derived) for restocking and planting semi-natural woodlands where this is available and considered appropriate. The Forestry Commission has developed a system intended to facilitate the identification of, and trade in, locally sourced stock for the planting of native species of trees and shrubs in Britain. This system of using local stock for planting native trees and shrubs is described in a Forest Practice Note   to help woodland owners make choices of planting material in these circumstances. It contains a map of local seed zones and a linked table of species native to each of the seed zones.

Non-native trees

These can alter the species composition and character of the woods in which they occur. In some cases they may increase the potential richness of the woodland, whereas in others the overall impact on the woodland may be negative where too heavy a shade is cast, or a smothering leaf is litter affects the development of the field layer. The impact of each non-native species needs to be assessed within the site-specific context of the wood where it occurs. For instance, in limited situations - where, for example, sycamore is not invasive -  sycamore may possibly enhance the biodiversity value of a woodland:  for example, in ravine woods where the native shade-bearing elm has been lost to Dutch elm disease, sycamore can replace the elms' role within the woodland ecosystem. 

However, there are some species whose impact on native woodland is so universally negative that blanket prescriptions can be given. For instance rhododendron has such a smothering negative impact on most aspects of native woodland that the only acceptable prescription is complete eradication within and close to the site.

Even though a woodland may not have been obviously subject to direct management or exploitation, this does not mean that it is free from important human influence. Many important processes of man-made environmental change now act at a landscape scale, and even protected forest areas are subject to their influence. Predicted changes in climate will bring about noticeable changes in plant and animal distributions, with a significant movement of species northwards, and implications for our definition of nativeness.

Most woods show changes in their composition over a stand life-cycle. Species distributions are also expected to alter as a consequence of expected climate change. It is unclear precisely what these changes will be but SNH is encouraging habitat networks and increased landscape permeability, and de facto encouraging species to move between sites in future. This could include spread to areas where they do not currently occur (not currently locally native) and the loss of species (or reductions in abundance) in other regions. Our definition of nativeness therefore needs to be sufficiently robust to be applied to this dynamic situation.