a comparison of different methods to estimate species proportions by area in mixed stands

Aim of the study: This paper presents the most appropriate ways to estimate the species proportions by area in mixed stands of Norway spruce (Picea abies L. Karst.) and European beech (Fagus sylvatica L.) by comparing stand level and individual tree level approaches. It also investigates whether different ways of describing species proportions by area can result in different judgments on the over- or under-yielding of species in mixtures.

Area of the study: Three triplets of pure and mixed stands of Norway spruce and European beech in three locations in the northeast of Austria are investigated. The three locations differ considerably in slope, bedrock and soil type as well as in site index.

Material and Methods: In all 9 plots the coordinates of all trees, their dbh, height, height to the crown base and five year increment were measured. The potentially available areas of individual trees are calculated by Voronoi- diagrams and potential densities are estimated from the comparable pure stands, yield tables, and published equations for maximum basal area and Reineke’s maximum density line.

Main results: The species proportions estimated by the individual tree approach with leaf area as growth characteristic gave the best fit with the stand approach with the most appropriate, regional maximum basal area equations. By using various definitions of species proportions, in the worst case the mixing effects on individual species can be seriously over- or underestimated while the mixing effects on the total increment is only negligibly affected.

Research highlights

• Measures of species proportions by area are needed for comparing growth per hectare of a species in a mixed stand with that of the same species in a pure stand
• Species proportions at the stand level are based on estimates of the species’ potential densities, either in terms of maximum basal area or of maximum stand density index
• Species proportions at the tree level are derived from the area potentially available (APA) to the individual trees, based on the coordinates of trees in the stands, and on their growth characteristics, such as crown projection area or leaf area
• For the examples of Norway spruce - European beech stands, the species proportions derived according to the individual tree approach using leaf area as growth characteristics fits best with the stand approach using the most appropriate maximum basal area equations

Keywords: Picea abies; Fagus sylvatica; mixture proportion; growth efficiency; mixing effect.