Johansson, Maj-Britt (Swed. Univ. of Agric. Sciences, Uppsala, Swedish University of Agricultural Sciences (SLU),Uppsala, SE 750 07, Sweden; Phone: +46(0)18-672232; Fax: +46(0)18-673470; Email: Maj-Britt.Johansson@sml.slu.se)
M.B. Johansson *, A. Nilsson
Site data from the National Forest Inventory (NFI) and the National Survey of Forest Soils and Vegetation (NSFSV) in Sweden were used to investigate the impact of tree species on soil carbon content. From the NFI, data on tree species composition were utilized. Soil characteristics such as soil texture, soil type, moisture conditions and chemical characteristics, e.g. carbon and nitrogen content in different horizons, were obtained from the NSFSV for the same sites. The amount of soil carbon to 1 m depth was calculated for pure middle-aged stands of Scots pine (Pinus sylvestris L.), Norway spruce (Picea abies (L.) Karst.), silver birch (Betula verrucosa Ehrh.) and downy birch (Betula pubescens Ehrh.), the commonest Swedish forest trees. Only sites were selected where the site demands of all four tree species were fulfilled, viz. sandy and finer-textured soils, iron podzols, iron-humic podzols and mesic to mesic-moist soils. The soil carbon content was positively and linearly related to the effective temperature sum P less than 0.001, R2 = 0.38, n = 323). Norway spruce and birch forests had significantly more soil carbon (36 t ha-1) than Scots pine forests, but did not differ mutually. To discover whether changed species composition, i.e. a spruce/birch admixture in pine forests, would affect the soil carbon content, relationships between total soil carbon (to 1 m depth) and tree species composition were examined. The sites were selected according to the criteria above. Differences among sites in climate and fertility probably contributed to the absence of a strong relationship between the proportion of (spruce + birch) in pine forests, and soil carbon content. Tree-species composition alone explained only 11% of the variation (P less than 0.0001). When other site variables were included in a multiple regression analysis, a four-factor relationship which included (proportion spruce + birch), (temperature sum), (nitrogen concentration in the B-horizon) and (proportion spruce + birch)2, the relationship was highly significant (P less than 0.0001, n = 794) and explained 48% of the variation. Data on tree-species composition and the other site variables above were used to estimate the average soil carbon content in individual cells in a grid covering the forests of the country by means of the relationship. Maps were produced by weighted interpolation, to show the soil carbon pool in Swedish forests with the current tree-species composition. Soil carbon increased from an average of less than 80 ton C ha-1 in the north to greater than 140 ton C ha-1 in the south. These figures agree with other estimates. Maps were also produced to show soil carbon content in Swedish forests after a 10% increase in the spruce/birch fraction above the present level. Regions were identified in which a changed species composition significantly increased the soil carbon content.