Silvicultural possibilities for increasing structural diversity in British spruce forests: the case of Kielder Forest

Abstract Kielder Forest comprises some 50 000 ha of first- and second-rotation spruce forest created during this century on surface-water gley and peaty gley soils in northern England. The original limiting factors to tree establishment and growth, high water tables and low nutrient status, were largely eliminated by appropriate silviculture using cultivation, some drainage and remedial fertilisation. The main limiting factor to tree growth is now wind disturbance compounded by shallow rooting on the gleyed soils. To avoid the risk of windthrow, stands are left unthinned and are clear-felled at 35–40 years of age. However, the deterministic nature of the windthrow hazard classification used to predict the onset of wind damage means that the possibility of retaining stands for longer rotations may have been underestimated. Recent evidence suggests that, provided stands are planted using cultivation techniques that promote a stable root architecture and are respaced at an early stage to promote stem diameter growth, it should be possible to maintain some stands for at least 75–80 years to enhance structural diversity. The spread of gaps formed by windthrow in spruce forests and the development of a 72-year-old self-thinning spruce-pine mixture exemplify these possibilities.