No abstract

In a field experiment in southern Sweden we investigated the single and combined effects of shelterwood, soil scarification, and feeding barriers on pine weevil (Hylobius abietis L.) damage to Norway spruce (Picea abies (L.) Karst.) seedlings. The seedlings were planted on four different sites, each consisting of two plots, one clearcut and one with shelterwood (98–148 stems/ha). These plots were each subdivided into two parts, one scarified and one left untreated. The seedlings were protected with insecticide or feeding barriers or left as unprotected controls. Three years after planting the mortality due to pine weevil damage was high (88%) for unprotected seedlings on clearcuts with no scarification. The damage-reducing effects of shelterwood, scarification, and physical protection were additive. Using all three methods in combination the mortality due to pine weevil feeding was less than 7% after 3 years. The order of effectiveness of the individual measures for reducing damage by pine weevils was insecticide, scarification, physical protection, and shelterwood. Scarification reduced pine weevil damage equally well on clearcuts as in shelterwoods. However, the effects of scarification were more long lasting in shelterwoods.

This paper reviews regeneration research during the past 20 years. The focus is mainly on planting of Norway spruce and Scots pine. Research on root morphology and nursery practices has played an important role in the introduction of containerized seedlings in northern Europe. In recent years, mini-seedlings have been tested. Conventionally, seedlings are planted in spring and early autumn but recent research indicates that Norway spruce seedlings can be planted from spring until the end of September. Soil temperature strongly influences seedling establishment in the harsh northern Scandinavian climate and a good way to increase soil temperature is to plant in elevated planting spots, which can be achieved by mounding. Soil scarification also reduces competition from field vegetation and damage by pine weevils. In southern Finland, Sweden and Norway, pine weevils are by far the most serious causes of damage to both Norway spruce and Scots pine. Therefore, designing regeneration treatments for decreasing pine weevil damage without using insecticides is one of the more important current research challenges. Examples of possible future trends in forest regeneration research are an emphasis on more basic research, use of fast growing tree species, multidisciplinary approaches and site-specific regeneration regimes.

A field experiment was established between 1989 and 1993 to study the effects of (i) the age of clearcuts on damage by pine weevils (Hylobiusabietis L.) and (11) competing vegetation on the survival and growth of planted Norway spruce (Piceaabies (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 drough. From the middle of May until the beginning of July 1992, only negligible precipitation was recorded on all four sites. 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−1 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).

A field experiment was established between 1989 and 1993 to study the effects of competing vegetation on growth of planted Norway spruce (Picea abies (L.) Karst.) seedlings. Effects of clearcut age, scarification (mounding), herbicide treatment, and seedling stock type were investigated 5 years after planting. On fresh clearcuts, amounts of vegetation were negligible, whereas 2.1-3.7 Mg·ha-1 was found on 4-year-old and older clearcuts. Soil temperatures were about 10% higher in mounds than in undisturbed ground, while herbicide and clearcut age only marginally affected soil temperatures. Seedlings planted on old clearcuts showed significant reductions in growth due to interference from vegetation. Five years after planting, the reduction in growth corresponded to about 1 year's growth. Most of the interaction between seedlings and vegetation occurred during the first 2 years after planting. Thus, scarification was just as effective as repeated herbicide treatments in reducing competition from vegetation. Differences in periods of drought between years could largely explain variation in leading shoot length. However, leading shoot length was affected in the same way irrespective of vegetation control treatments. Five years after planting, the relative differences in diameter between bare-root and containerized seedlings were the same as at the time of planting.