Relationships between canopy transmittance and stand parameters in Sitka spruce and Scots pine stands in Britain

The changing emphasis within British forestry from a clearfell/replant system focussed on timber production, to multipurpose forestry encompassing biodiversity and recreation, has resulted in a need for changes to forest management. Manipulation of the forest canopy through thinnings is a powerful tool for forest managers to modify the canopy transmittance, and therefore the below-canopy light levels. This helps to achieve specific objectives such as habitat management or seedling growth as part of transformation of an even-aged stand to a continuous cover forestry regime. In this study, hemispherical photography was used to assess canopy transmittance in a range of Sitka spruce (Picea sitchensis (Bong.) Carr) and Scots pine (Pinus sylvestris L.) stands in Britain. Species-specific relationships were developed between canopy transmittance and easily-measured stand parameters. The models that provided the best fit to the data were based on basal area and stocking for Sitka spruce and basal area alone for Scots pine. The models indicate that a Sitka spruce stand with a basal area of 30 m(2) ha(-1) should have a stocking density < 450 stems ha(-1) to favour growth of Sitka spruce seedlings. Similarly, a Scots pine stand should have a basal area < 27 m(2) ha(-1) to achieve transmittance suitable for growth of Scots pine seedlings. In conjunction with a knowledge of the light requirements of different vegetation types, these models can provide a valuable contribution to guidance on current and changing forest management practice.