Ground vegetation composition and heterogeneity in pure Norway spruce and mixed Norway spruce-birch stands

We examine how the ground vegetation is affected when a deciduous tree species is introduced into spruce stands. The abundance and composition of ground vegetation in pure Norway spruce (Picea abies (L.) Karst.) and mixed Norway spruce - birch (Betula pendula Roth and Betula pubescens Ehrh.) stands were compared at four experimental sites in southern and central Sweden. The cover and heterogeneity of bryophytes were lower in mixed stands than in more thinned pure spruce stands. Differences in the field layer between mixed and pure stands were in general small, but the cover of field layer vegetation decreased with increasing stand basal area. When adjusting for differences in stand basal area, the cover of the total field layer and the cover of herbs were higher in mixed stands than in pure spruce stands. Plant species composition differed between mixed and pure stands (as revealed by canonical correspondence analysis). The abundance of vascular plant species was higher in mixed stands whereas the abundance of bryophyte species was lower. This change in species composition occurred along a gradient of increasing litter pH, litter base saturation, and humus base saturation. We propose that the presence of birch leaf litter was likely to be the most important factor causing differences in vegetation composition.