Tree root growth on earthworks
Concern has been expressed over trees causing a load at the top of earthwork slopes resulting in the banks mechanical failure. However, woodland has also been recommended as a stable form of ground cover providing appropriate species and management are considered. Species that favour a more vertical root system, such as oak, may be beneficial in the stabilisation of earth banks whereas surface-rooting trees may minimise surface soil erosion or be used to avoid deeper archaeology.
While tree cover may preserve the form of an earth bank, consideration must also be given to the possible impacts on any artefacts, buried soil and palaeoenvironmental indicators that may exist below it. If tree cover is to be retained on an earthwork, management will be required to minimise the risk of monument damage from windthrow. Coppicing or pollarding will reduce the above-ground biomass and the risk of root upheaval. However, species respond differently to such management.
The likelihood of a species producing regrowth is given in the table below (0=never, 1=rarely, 2=likely, 3=vigorus growth):
|Species||Regrowth||Typical root architecture *||Typical root depth (m) *||Mechanical root penetration||Water requirements
* When grown in well drained sandy soils
While areas of known archaeological importance are excluded from any new planting scheme, some proposals may include land that is considered to be of archaeological potential but with no known evidence. Here tree species can be chosen to reduce the potential risk of damage to any archaeological features that may exist. The promotion of sustainable, multipurpose forestry allows species to be selected that suit the soil and environmental conditions, thus reducing the need for excessive drainage and cultivation. Species may also be chosen according to their rooting characteristics or water requirements.
Rooting depth and architecture are dependent primarily on soil conditions, but are also influenced to a lesser extent by the individual species. The table above gives some generalised rooting habits for major woodland species. The rooting characteristics are grouped into three different types:
- Taproot species, where a strong main root descends vertically from the underside of the trunk.
- Surface rooting species, where large horizontal lateral roots extend below the surface, from which smaller roots descend vertically.
- Heart rooting species, where large and small roots descend from the trunk diagonally into the soil.