Plant species richness or soil fertility: which affects more the productivity of Scots pine in Central Europe?

It has been highlighted that forest productivity is related both to species richness and to soil fertility; however, thus far it has not been investigated which of these agents is more important for the productivity. The goal of this study was to examine this problem with regard to Scots pine (Pinus sylvestris L.) stands in Central Europe. The study was conducted in 129 plots located in SW Poland. The productivity of even-aged pine stands was estimated based on site index. Plant species richness was investigated regarding the total richness as well as the richness referring to particular forest strata (overstorey, understorey, herb layer). Soil fertility was studied regarding the nutrients’ stocks, the contents of fine-textural classes, pH, the TOC content and the values of Soil Trophic Index. The importance of the variables for the site index was examined using Spearman correlations and the stepwise regression. Productivity of the studied stands was predominantly correlated stronger to species richness than to soil properties being related to its fertility. The higher importance for the productivity of soil variables than of species richness was exclusively found for the poorest plots being represented by Podzols. This study highlights the high importance of species richness occurring in particular forest strata for the forest stand productivity, which could involve consequences for forest economy as well as for CO2 sequestration. Thus, the research delivers strong argument for the conversion of pine mono-stands occurring in Central Europe for mixed forests.