Direct seeding of beech (Fagus sylvatica L.) in Norway spruce (Picea abies [L.] Karst.) stands -: effects of canopy density and fine root biomass on seed germination

Direct seeding of deciduous trees below the canopy of conifer stands has proven to be a very economical method for converting pure stands into mixed stands. However, many direct seedings of beech seeds provided unsatisfactory results without a clear explanation for this lack of success. In order to identify the main factors determining seed germination under the canopy of pure Norway spruce stands, three different treatments of direct seeding were tested. These treatments were: (i) direct seeding without additional measure (control), (ii) direct seeding and liming, and (iii) direct seeding covered with beech leaf litter. The experiment consists of two stands. Direct seeding was carried out on six plots per treatment and stand. Each plot includes 81 sample units of 0.5 m(2) upon each of which 15 g of seeds were sown. The germinative capacity of the seeds was pre-determined (68%). Germination of the seeds was recorded repeatedly from May to September 1997. On the related sample units canopy density was measured and fine root biomass of the overstorey trees was estimated. There was a significant difference in seedling number between the two stands. The number of seedlings did not increase by liming (coincident with seeding). In contrast, the covering of seeds with leaf litter caused a remarkable and significant increase in the number of germinated seeds. As ANCOVA showed, models including canopy density or estimated fine root biomass of the overstorey trees explained 72 and 52% of the variation in mean seedling number per plot. As canopy density as well as estimated fine root biomass determine the water content of the soil. it can be assumed that the germination of beech seeds originating from direct seedings under the canopy of pure spruce stands depends to a great extent on soil moisture at the time when the direct seeding is carried out, as well as the weeks following seeding. For this reason, direct seedings in very dense stands are not recommended against the background of this study. (C) 2002 Elsevier Science B.V. All rights reserved.