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 c haracterised by basal area, on seedling density and growth of advanced rege neration in two irregularly thinned stands of Sitka spruce (Picea sitchensi s (Bong.) Carr.) and to investigate whether any relationships found were af fected by the method in which basal area was determined. Surveys were carri ed out in two contrasting Sitka spruce plantations in which the age and hei ght growth (total and current year's) of advance regeneration was measured and basal area of the crop trees was determined using different methods, in cluding point sampling with a range of basal area factors, The density of y oung (up to 4-year-old) regeneration was found to be positively correlated with overstorey basal area, with the strongest significant relationship, al beit weak (r(2) = 0.18, P < 0.01) found when basal area was determined usin g point sampling with a basal area factor of 7.5 (metric). Growth of natura l regeneration, as determined by total height, leader length and leader/lat eral ratio, was found to be negatively correlated with overstorey basal are a. The strength of these relationships varied according to how basal area w as deterridned and the significance of this is discussed. In the stand with older regeneration the basal area of the overstorey above those plots wher e natural regeneration was in check was significantly (P < 0.001) higher th an where natural regeneration was growing well. It would appear that in ord er to encourage growth of the advance regeneration, the stand should be kep t at a basal area of 30 m(2) ha(-1) or less. This is less than the value (3 8 m(2) ha(-1)) for a fully stocked stand [Edwards, P.N., Christie, J.M., 19 81. Yield models for forest management. Forestry Commission Booklet 48, HMS O]. <(c)> 2001 Elsevier Science B.V. All rights reserved.