Site conditions and definition of compositional proportion modify mixture effects in <i>Picea abies</i> – <i>Abies alba</i> stands

Abstract

For most forest types in the European Alps, little is known about mixture effects on stand productivity. The comparability of studies on mixture effects often suffers from the open methodological question of whether the results depend on the definition of compositional proportion. In this study, data from the Swiss National Forest Inventory were used to investigate how the growth of Norway spruce (Picea abies (L.) Karst.) and silver fir (Abies alba Mill.) is modified by the admixture of the other species and if the mixture effect depends on site, climate, age, or stand density. Stocking proportion (proportion by area) as well as the proportion of relative density index, stem number, basal area, stem volume, and aboveground biomass were used to define compositional proportion, and the results were compared. At low-quality sites, Norway spruce grew faster in basal area as its relative share of composition increased, but this pattern diminished as the site quality increased. At cooler sites, silver fir grew faster as its share of composition decreased, but the pattern reversed at warmer sites. Overyielding was predicted only for 16% of the 679 sites used for this study. Beneficial effects of species mixture were overestimated when species-specific stocking potentials were not considered in the definition of compositional proportion.