EFFECTS OF THINNING AND SOIL PROPERTIES ON ACCUMULATION OF CARBON, NITROGEN AND PHOSPHORUS IN THE FOREST FLOOR OF NORWAY SPRUCE STANDS

Area-based sampling was carried out to investigate the effect of thinning and soil properties on accumulation of forest floor carbon (C), nitrogen (N) and phosphorus (P) in Norway spruce (Picea abies (L.) Karst.) stands in Denmark. Four thinning intensities (unthinned, and about 83%, 67% and 50% of unthinned basal area) were investigated at three sites in Denmark: a calcareous, relatively nutrient rich soil with a sandy loam/loam texture and two soils with low to intermediate nutritional status and sandy loam and loamy sand textures, respectively. The effect of thinning on accumulated carbon and nitrogen was significant at two of the investigated sites. Accumulated phosphorus was significantly affected by thinning at one of these two sites and at the third site. Accumulated carbon and phosphorus were negatively linearly correlated with thinning intensity. pH tended to be highest and C/N and C/P ratios tended to be lowest in the heaviest thinned plots. It is hypothesized that the differences in accumulation may be due to a more favourable microclimate and substrate for saprophytic organisms in the most heavily thinned plots. However, the differences between sites were greater than differences between thinning intensities. The accumulation of carbon, nitrogen and phosphorus in the forest floors was much higher at the two less fertile sites with loamy sand and sandy loam than at the relatively fertile site with sandy loam/loam. Significant differences in pH and in C/N and C/P ratios at the three sites indicate that the amounts of available nutrients influence the mineralization pattern. In addition, at the site with the greatest forest floor root density, competition for nutrients and moisture between mycorrhiza-infected roots and free-living saprophytic decomposers may be co-responsible for the large amounts of accumulated carbon, nitrogen and phosphorus.