Mischbestände aus Kiefer (Pinus sylvestris L.) und Fichte (Picea abies (KARST.) L.): Ökologie, Ertrag und waldbauliche Behandlung

Scots pine (Pinus sylvestris L.) and Norway spruce (Picea abies (KARST.) L.) cover around one fourth of European forest land (Fig. 2). Approximately 80% of this area is covered by pine- and spruce-dominated monospecific stands. About 8.5 million hectares (20%) are covered by mixed stands of the two species together. In relation to the present occurrence of Scots pine and Norway spruce the state of knowledge concerning their association in mixture is rather low. Hence we reviewed the species-specific characteristics for an association in mixed stands (Tab. 2). We complemented this overview by an evaluation of own data from temporary experimental plots and forest inventories. This resulted in an extended knowledge base for adopted silvicultural regulations depending on the site-specific competitiveness of both species (Fig. 11, 12, 13, 15). Beyond the well-known differences in the natural distribution of Scots pine (boreal, continental; dry/wet, oligotrophic) and Norway spruce (boreal, montane; moist, mesotrophic) the beneficial mixture of both is based on their complementarity. Scots pine and Norways spruce complement each other especially in their light ecology and growth velocity (light demanding early successional species vs. shade tolerant late successional species), crown and root structure (wide crown and deep root versus slim crown and shallow root), and in their reaction on drought (isohydric, less drought sensitive vs. isohydric, more drought sensitive) (refer to Tab. 2). On sites with medium water and nutrient supply, where Scots pine and Norway spruce can coexist in the main canopy, we found the following stand characteristics: In size growth (mean height and diameter) pine is ahead of spruce in the early stand development phase; however, mean height and diameter of mature mixed stands in total equal the weighted mean of the monocultures. Mixed stands exceed monocultures in the maximum stand density, quantified by tree number (+25%), stand basal area (+10%), Stand Density Index (+10%) or standing volume (+8%). In the same way the volume growth (+5%) can be higher in mixed compared to monospecific stands (Fig. 4). The density-growth-relationship (Fig. 5) can reach a higher level and have a broader saddle than in monocultures, so that density reductions by silvicultural interferences or other disturbances can be compensated by the remaining stand over a wider range of stand densities. This may be caused by a significantly higher crown projection area in mixed compared with pure stands. The range of tree sizes (stem diameter, height, and volume) is wider and the distribution more evenly dispersed (Fig. 6). Nevertheless the share of standing tim ber volume in mixtures is more unevenly distributed at the favor of the tall trees (Fig. 8). In mixture crown width and length of spruce expand (+10%) on the expense of pine (-5%) (Fig. 9). In mixture Scots pine stems show a reduced slenderness in an early phase of stand development, whereas the h/d-ratio of spruce at first increases in mixture. These differences settle in the course of the further stand development. Drought reduces growth of spruce (-38%) much stronger and longer compared to pine (-18%). Mixture reduces drought stress of both species, so that the increment losses at the stand level are lower than in monocultures (Fig. 4). Mixing of pine (pi) and spruce (sp) can increase productivity, stability and resistance to drought events. Silvicultural options in mixed stands of Norway spruce and Scots pine depend strongly on the site-dependent competitiveness of both species. According to the competitive relation between pine and spruce three categories can be assigned: pi > sp, pi ≈ sp, pi < sp (Fig. 15). Both species complement each other when of equal relative competitive strength (pi ≈ sp), on all other sites silvicultural input of questionable intensity is required to maintain them in mixture.