Influence of overstorey basal area on density and growth of advance regeneration of Sitka spruce in variably thinned stands

The aim of this study was to investigate the effect of the overstorey, as characterised by basal area, on seedling density and growth of advanced regeneration in two irregularly thinned stands of Sitka spruce (Picea sitchensis (Bong.) Carr.) and to investigate whether any relationships found were affected by the method in which basal area was determined. Surveys were carried out in two contrasting Sitka spruce plantations in which the age and height growth (total and current year's) of advance regeneration was measured and basal area of the crop trees was determined using different methods, including point sampling with a range of basal area factors, The density of young (up to 4-year-old) regeneration was found to be positively correlated with overstorey basal area, with the strongest significant relationship, albeit weak (r(2) = 0.18, P < 0.01) found when basal area was determined using point sampling with a basal area factor of 7.5 (metric). Growth of natural regeneration, as determined by total height, leader length and leader/lateral ratio, was found to be negatively correlated with overstorey basal area. The strength of these relationships varied according to how basal area was deterridned and the significance of this is discussed. In the stand with older regeneration the basal area of the overstorey above those plots where natural regeneration was in check was significantly (P < 0.001) higher than where natural regeneration was growing well. It would appear that in order to encourage growth of the advance regeneration, the stand should be kept at a basal area of 30 m(2) ha(-1) or less. This is less than the value (38 m(2) ha(-1)) for a fully stocked stand [Edwards, P.N., Christie, J.M., 1981. Yield models for forest management. Forestry Commission Booklet 48, HMSO]. <(c)> 2001 Elsevier Science B.V. All rights reserved.