Effects of regeneration methods on drought damage to newly planted Norway spruce seedlings

A field experiment was established between 1989 and 1993 to study the effects of (i) the age of clearcuts on damage by pine weevils (Hylobius abietis L.) and (ii) competing vegetation on the survival and growth of planted Norway spruce (Picea abies (L.) Karst.). On each of four sites Norway spruce seedlings were planted on 0 to 4-year-old clearcuts. Effects of mounding, herbicide and mowing, removal of slash, and seedling stock type were also investigated. As many seedlings suffered severely from a drought that affected southern Sweden in 1992, this study reports a separate analysis of mortality due to drought. From the middle of May until the beginning of July 1992, only negligible precipitation was recorded on all four site. The biomass of ground vegetation was correlated with the age of the clearcut. On fresh and 1-year-old clearcuts, only negligible amounts of vegetation were found, whereas about 2 Mg-ha of ground vegetation were found on 2- and 3-year-old clearcuts. Low soil water potentials were recorded on 2- and 3-year-old clearcuts but not on fresh clearcuts. There was no effect of the slash removal treatment,on soil water potential, but there was a significant effect of the soil and vegetation treatments. Mounding and herbicide treatments increased the soil water potentials compared with untreated controls and mowed plots. Mortality by drought was higher for seedlings planted on old clearcuts than for seedlings planted on fresh ones. Mortality was reduced by mounding, but late planting increased the number of dead seedlings significantly. The mortality of containerized seedlings was considerably less than that of bare-root seedlings. On old clearcuts growth of seedlings planted on plots with no vegetation control (untreated control and mown plots) was less than that of seedlings planted on plots where transpiration by ground vegetation was reduced (herbicide and mounding)