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The growth effects of mixtures are generally assumed to be a result of canopy structure and crown plasticity. Thus, the distribution of leaf area at tree and stand level helps to explain these mixing effects. Therefore, we investigated the leaf area distribution in 12 stands with a continuum of proportions of European larch (Larix decidua Mill.) and Norway spruce (Picea abies (L.) Karst.). The stands were between 40 and 170 years old and located in the northern part of the Eastern Intermediate Alps in Austria at elevations between 900 and 1300 m a. s. l. A total of 200 sample trees were felled and the leaf area distribution within their crowns was evaluated. Fitting beta distributions to the individual empirical leaf area distributions, the parameters of the beta distributions were shown to depend on the leaf area of the individual trees and, for spruce, on the proportion of spruce in the stands. With the equations determined, the leaf area distribution of all trees in the stand, and thus its distribution in the stands, was calculated by species and in 2 m height classes. For the individual trees, we found that the leaf area distribution of larch is more symmetric, and its peak is located higher in the crown than it is the case for spruce. Furthermore, the leaf area distribution of both species becomes more peaked and skewed when the leaf area of the trees increases. The mixture only influences the leaf area distribution of spruce in such a way that the higher the spruce proportion of the stand, the higher the leaf area is located within the crown. At the stand level, a strong relationship was found between the proportion of spruce and the distance between the peaks of the leaf area distributions of larch and spruce.

An increasing number of investigations into mixed forest stands shows clear interactions between complementarity and stand and site characteristics. One of the least-investigated mixture types are mixed stands of Norway spruce and European larch. We investigated pure and mixed stands of these species in the northern part of the eastern intermediate Alps in Austria, at altitudes between approximately 880 and 1330 m above sea level. In these stands, 12 plots sized between 0.25 ha and 1.6 ha, with varying ages and proportions of Norway spruce, were established. All trees were measured for their coordinates, diameter at breast height, tree height, crown height, and crown projection area. The trees were cored at breast height, and from about 200 felled sample trees, equations for leaf area and for the five-year volume increment were developed. Growth efficiency (volume increment of a species per its fraction of the stand area) exhibited a clear interaction with age: in young mixed stands, spruce as well as larch grew less than the reference from the pure stands, while in the older stands especially spruce grew much better in the mixed stands. When the Clark Evans index was entered into the growth efficiency equations, it could be seen that the spatial distribution of the trees (i) explained more variance than the species proportion and (ii) showed an additional influence of stand density on the complementarity of the species.

An increasing amount of research is focusing on comparing productivity in monospecific versus mixed stands, although it is difficult to reach a general consensus as mixing effects differ both in sign (over-yielding or under-yielding) and magnitude depending on species composition as well as on site and stand conditions. While long-term experimental plots provide the best option for disentangling the mixing effects, these datasets are not available for all the existing mixtures nor do they cover large gradients of site factors. The objective of this study was to evaluate the effects and uncertainties of tree species mixing on the productivity of Scots pine–European beech stands along the gradient of site conditions in Europe, using models developed from National and Regional Forest Inventory data. We found a positive effect of pine on beech basal area growth, which was slightly greater for the more humid sites. In contrast, beech negatively affected pine basal area growth, although the effects switched to positive in the more humid sites. However, the uncertainty analysis revealed that the effect on pine at mid- and more humid sites was not-significant. Our results agree with studies developed from a European transect of temporal triplets in the same pine–beech mixtures, confirming the suitability of these datasets and methodology for evaluating mixing effects at large scale., This research was partially funded by the COST Action FP1206 through the Short Term Scientific Mission COST-STSM-ECOST-STSM-FP1206-091115-068270. This article is based on work from COST Action FP1206 (EuMIXFOR), supported by COST (European Cooperation in Science and Technology). The authors would like to thank the Spanish Ministry of Economy and Competitiveness for funding the research project AGL2014-51964-C2-2-R, which supported part of this study, and to the project CARE4C (# GA 778322). The authors are also grateful to Bayerische Staatsforsten, BaySF for providing the forest inventory data for Bavaria/Germany and to all those who make the National Forest Inventory possible in the different countries, and to those providing the data used for this study. We also thank the German Science Foundation, DFG, for funding the project PR 292/15-1 “Structure and Dynamics of mixed-species stands of Scots pine and European beech in comparison with mono-specific stands”.