The transformation of conifer forests in Britain — regeneration, gap size and silvicultural systems

Abstract

There are about 1.5 Mha of conifer high forest in Great Britain composed almost entirely of even-aged plantations of non-native species established since 1900. Recent changes in forest policy require managers to introduce alternative silvicultural systems to clear-felling into windfirm conifer plantations to provide greater structural diversity and so enhance aesthetic, conservation and environmental benefits. There are around 500–750 kha of British forests established on sites which are sufficiently windfirm to be affected by this requirement but only 10–20 kha that are being managed under an appropriate silvicultural system. Expanding the area in the latter category will confront managers and researchers with major ecological, silvicultural and practical challenges. The process of transformation from uniform to irregular stands is best achieved using natural regeneration, except where other species or genotypes are desired. The start of transformation must await adequate seed production which will probably not occur before 30 years of age for the four major conifer species; Sitka spruce ( Picea sitchensis (Bong.) Carr.), Scots pine ( Pinus sylvestris L.), Corsican pine ( Pinus nigra var. maritima (Ait.) Melville), and Douglas fir ( Pseudotsuga menziesii (Mirb.) Franco) as well as a number of minor species. Once this age has been reached, manipulation of the canopy can create suitable microclimates for seedling germination and growth. Suitable conditions depend upon interacting ecological factors, the most important of which is the light climate at ground level. Because of the relative shade intolerance of the major tree species and the lower irradiance and greater cloudiness in Britain, appropriate gap sizes for regeneration may be quite large. Minimum gap sizes are usefully defined by the ratio of gap diameter to height of surrounding trees ( d/ h). Proposed d/ h ratios range from >2.0 for Scots and Corsican pines, 1–2 for Sitka spruce and Douglas fir to 1.0 or less for more shade-tolerant species (e.g. Picea abies, some Abies spp., Tsuga heterophylla). Unless greater use is made of these shade-tolerant species, it appears that silvicultural systems such as single stem selection will be inappropriate and more emphasis should be given to group selection and irregular shelterwood systems. Given the limited experience of irregular systems in Britain, it is probable that trials of transformation will be confined initially to windfirm areas of high amenity, conservation or heritage value.