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

SOPHIE E. HALE 2 3 COLIN EDWARDS 2 3 W. L. MASON 2 3 MARTIN PRICE 0 1 2 ANDREW PEACE 2 3 0 Present address: Forest Research , Linmere, Delamere, Northwich CW8 2JD, England 1 School of the Environment and Natural Resources, College of Natural Sciences, Bangor University , Gwynedd LL57 2UW, Wales 2 Published by Oxford University Press on behalf of Institute of Chartered Foresters 2009 3 Forest Research, Northern Research Station , Roslin, Midlothian EH25 9SY, Scotland 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 belowcanopy 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. Speciesspecific 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 m2 ha1 should have a stocking density <450 stems ha1 to favour growth of Sitka spruce seedlings. Similarly, a Scots pine stand should have a basal area <27 m2 ha1 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. Summary Introduction The objectives of forest management in Britain are changing from predominantly timber production to include a wide range of other functions, such as biodiversity and recreation. Accompanying these developments is a change in emphasis from clearfelling/replanting to continuous cover forestry (CCF) (Anon, 2004). CCF is an approach to forest management whereby the overstorey is retained, at one or more levels, during the establishment phase (Mason et al., 1999). It incorporates a wide range of silvicultural systems, such as shelterwoods, and group or single-tree selection. There is a presumption towards natural regeneration, although underplanting may sometimes be preferable. The understorey light environment is one of the primary factors influencing the growth of vegetation beneath a forest canopy (Lieffers et al., 1999). The proportion of incident radiation that is transmitted through the canopy (canopy transmittance) depends on the size, distribution and density of the tree crowns (Brunner, 1998; Jennings et al., 1999). Manipulation of the stand is therefore a powerful tool for forest managers to modify the light environment to help to achieve management objectives (Messier, 1996), by controlling the growth of ground vegetation and seedlings. However, practical guidance is needed, specific to the species and conditions relevant to British forestry, to enable managers to estimate the light regime beneath a stand. This will enable them to make informed decisions on how to manipulate a stand to obtain the desired light levels, in support of a number of issues, such as transformation of even-aged plantations to continuous cover management and habitat management within native woodlands. Many complex, distance-dependent singletree models have been developed for predicting canopy transmittance and understorey light (e.g. Wang and Jarvis, 1990; Brunner, 1998), but the intensive data requirements of these models (e.g. individual tree positions and crown dimensions) mean that they are of more use as research tools rather than management tools for forestry. For practical purposes, there is therefore a requirement for models, for the key species in Britain, that link canopy transmittance to stand parameters that are routinely measured by foresters. Basal area is a routinely easily-assessed stand parameter derived from measurements of tree diameters, and several studies have investigated relationships between basal area and transmittance in both conifer and broadleaf forests (e.g. Kuusipalo, 1985; Mitchell and Popovich, 1997; Comeau, 2001; Hale, 2001; Hale, 2003; Sonohat et al., 2004). The theoretical basis of using basal area as a surrogate for canopy transmittance is simplistic, as it essentially ignores site-to-site and age- or size-related variation in the relationship between diameter, sapwood area and leaf area (Gower et al., 1987; Mencuccini and Bonosi, 2001; McDowell et al., 2002; Mencuccini et al., 2005). It also ignores the influence of the spatial distribution of trees crowns and the gaps between them; for example, two stands with the same basal area will have very different canopy transmittance if one is closed canopy and the other has just been thinned (Garca, 1990). Despite these limitations, relationships between decreasing transmittance and increasing basal area have been found in relatively open stands (Palik et al., 1997; Comeau, 2001; Mitchell, 2001; Balandier et al., 2002; Comeau and Heinemann, 2003; Hale, 2003). In closed canopies, however, the relationship does not hold, with low transmittance irrespective of the basal area (Mitchell and Popovich, 1997; Comeau, 2001; Hale, 2001; Parker et al., 2002). Recent stand management is likely to have a critical influence on the relationship between stand parameters and canopy transmittance. Some authors have considered that it is not the actual basal area that is important, but the proportion removed from a closed stand (Jenkins and Chambers, 1989; Hale, 2003) and the time since this intervention. Stand age and stocking density (stems ha1 ) are other parameters that have been considered in conjunction with the basal area (Comeau and Heinemann, 2003; Sonohat et al., 2004) as predictors of understorey light regime. This study assesses the canopy transmittance of stands of Sitka spruce (Picea sitchensis (Bong.) Carr) and Scots pine (Pinus sylvestris L.), which are the dominant conifers in British forestry (Smith and Gilbert, 2003). It builds on work presented in Hale (2001) and Hale (2003). In Sitka spruce stands, the primary management objective requiring information on canopy transmittance is the transformation of uniform stands to unevenstructured stands, by ensuring seedling growth beneath the canopy (Mason et al., 2004). In many cases, there will be a trade-off between light requirements for seedling success and the growth of competing vegetation (Messier, 1996). Transformation will also be a management objective in some (...truncated)