Influence of Shelterwood Density on Survival and Height Increment of Picea abies Advance Growth
A shelterwood experiment with eight different shelterwood densities was established in southern Sweden in 1989. Advance-growth seedlings of Norway spruce (Picea abies (L.) Karst.) were the focus of this study, in which damage, mortality and height growth were registered. Mortality was generally high, especially on clear-cuts and in low-density shelterwoods (≤ 80 stems ha-1). Seedlings with a small initial size showed higher mortality compared with larger ones ( > 50 cm). Mortality was caused by "release effects" (25%), pine weevils (Hylobius abietis L., 28%), and undetermined factors (47%). Mortality was attributed to "release effects" in cases where seedlings wilted in the first spring following cutting, and no other damaging agent could be detected. This damage was most frequent among seedlings shorter than 20 cm on clear-cuts or in shelterwoods of low density. The most severe damage by pine weevils was found in the same plots and was especially pronounced in seedlings 20-50 cm in height. Although frost frequently damaged seedlings (treatments ≤ 80 stems ha-1), no seedling mortality was ascribed directly to frost. The mean annual height growth for 1989-96 was greatest for seedlings that were largest at the start of the experiment. For all seedling sizes, growth was highest at densities of 80-160 stems ha-1, whereas the optimum density for survival was about 160 stems ha-1. Height, top-shoot length and top-shoot diameter the year before release cutting showed significant positive correlations with both survival and height growth for 1989-96.