Following tree removal from an archaeological site, some form of management will be required to prevent invasion of unwanted weed species that would normally be suppressed under a closed tree canopy. Frequently, the preferred management option on archaeological sites is grazing. However, overgrazing can cause surface soil erosion, and stock control requires careful management. On many sites within woodland clearings, grazing is often undesirable due to risk of damage to adjacent trees or simply impractical for other reasons.
As well as possible detrimental effects from root growth of invasive vegetation, it can also mask archaeological features and put them at risk from accidental damage such as the movement of vehicles during site management. Colonizing species vary from one site to another depending upon factors such as geographic location, soil type, altitude, exposure and neighbouring species. Smaller woody shrubs also take advantage of open land, and gorse (Ulex europaeus), broom (Cytisus scoparius) or rhododendron (Rhododendron ponticum) may be a problem locally.
Non-woody species such as bracken (Pteridium aquilinum) can also be very invasive. With its main rhizome penetrating at least 0.4 m into the soil (fine roots deeper) and with a lateral growth rate of 1 to 2 m a year, the impact of bracken should not be underestimated. Bracken is also less palatable to sheep than many other plants and so grazing may not be a successful means of control. Where it has become established, current control methods require a very intensive cutting regime or extensive use of herbicide, a process that many landowners prefer to avoid. Additionally, mechanised weed control following tree clearance, such as mowing, may be hampered by the presence of tree stumps.
To maintain an archaeological site in open grassland, vegetation management will usually be required and possibly further measures to avoid erosion or to control burrowing animals. Nevertheless, the opening of areas through purposeful deforestation in the interests of both the archaeological evidence and their landscape setting has been successful in many places.
All important archaeological earthworks will need an active management to prevent the establishment/proliferation of unwanted vegetation types and reduce the risk of monument damage or enhance its setting. Vegetation monitoring (even in its simplest form) should be considered, as it provides a method of assessing the effectiveness of management plans. And, when combined with GIS, also allows longer-term changes in both the condition of the monument and its environment to be examined.
To develop suitable methods, vegetation monitoring is occurring on a sample of monuments in southern England at three levels of intensity:
- Intensive survey
This examines a monument section (typically bank and ditch section) using a 10 m2 survey grid divided into 100 quadrats. Each plant species is recorded as a percentage of the ground cover within each quadrat. Over successive surveys, this technique will allow very small vegetation changes to be resolved and is particularly well suited to smaller monuments such as barrows.
- Walk-over survey
The main plant communities are recorded as areas drawn onto a map of the earthworks. This is unlikely to show gradual or minor changes but will enable differences to be mapped following active management or rapid vegetation succession. This is more appropriate for larger monuments.
- Photographic record
This is the simplest method and involves taking photographs of the monument from a selection of viewpoints. Subsequent photographs of the same views allow changes in vegetation to be monitored.