Effect of stand structure and density on development of natural regeneration in two Picea abies stands in Sweden

Previous studies have indicated that the effect of overstorey density on he ight increment and mortality of seedlings and saplings is affected by the v ertical structure of the overstorey. To test this, 28 plots, including four control plots, were studied at two sites, in northern and central Sweden. Norway spruce dominated all plots. The overstorey was thinned from "above", i.e. harvesting mainly large-diameter trees and maintaining a multilayered structure, or from "below", i.e. harvesting mainly smaller trees and creat ing a shelterwood. Three different densities were created for each structur e, in the range 20-140 m(3) ha(-1) at the northern site, and 40-200 m(3) ha (-1) at the southern site. Seven years after treatment, seedling (0.1 m les s than or equal to height < 0.5 m) and sapling (0.5 m <less than or equal t o> height < 2.0 m) height increment tended to decrease with increasing over storey density at both sites for plots thinned from below, but the effect w as not statistically significant. Cutting regime had a significant effect o n seedling height increment year 5 after treatment at the northern site, an d cutting regimen and overstorey density combined had a significant effect on saplings in years 5 and 6 after treatment at the southern site. Annual m ortality rates were 2-7% for seedlings and 0-4% for saplings. Ingrowth past 10 cm in height during the 7 yrs after treatment was less than 20 seedling s per hectare and year at the northern site, and less than three at the sou thern site.