Effects of planted European beech on the understory in Scots pine forests of Lithuania

Understanding how the planting of non-native species impacts native vegetation is of most importance for forest management, as introduced species may alter environmental conditions with respect to soil composition, light intensity, and species composition. Here, we compared the stand structure, understory vegetation and site properties of a natural Scots pine (Pinus sylvestris L.) stand with Norway spruce (Picea abies (L.) Karst.) and European beech (Fagus sylvatica L.) as the second tree layer. We recorded saplings and species and their abundance in the understory vegetation, the thicknesses of organic (O) and humus (A) soil layers, soil pH and light indexes in nine Scots pine stands with a second tree layer of beech and in nine control pine stands with a second tree layer of spruce. An ordination method was used to analyze all the data together and ANOVA to determine whether there were differences between stands. We found that species diversity in pine stands with planted beech as the second layer was lower than in pine stands with spruce as the second layer. In pine stands with beech as the second tree layer, shrub, herb and moss cover was significantly lower, and the soil humus layer and organic soil layer were thicker and thinner, respectively. Stand parameters such as mean volume and mean annual increment of the second tree layer were significantly higher in pine stands with planted beech as the second tree layer than in pine stand with spruce as the second tree layer. The mean volume and the mean annual increment of the first tree layer dominated by Scots pine did not differ significantly between stands with planted beech and those with natural spruce. Scots pine stands with a beech second layer had negative effects on understory species richness and abundance. This effect was most likely due to the lower light transmittance and poor physical properties of the forest-floor litter in the Scots pine stands with planted beech.