Scots pine (Pinus sylvestris L.) and European beech (Fagus sylvatica L.) dominate many of the European forest stands. Also, mixtures of European beech and Scots pine more or less occur over all European countries, but have been scarcely investigated. The area occupied by each species is of high relevance, especially for growth evaluation and comparison of different species in mixed and monospecific stands. Thus, we studied different methods to describe species proportions and their definition as proportion by area. 25 triplets consisting of mixed and monospecific stands were established across Europe ranging from Lithuania to Spain in northern to southern direction and from Bulgaria to Belgium in eastern to western direction. On stand level, the conclusive method for estimating the species proportion as a fraction of the stand area relates the observed density (tree number or basal area) to its potential. This stand-level estimation makes use of the potential from comparable neighboring monospecific stands or from maximum density lines derived from other data, e.g. forest inventories or permanent observations plots. At tree level, the fraction of the stand area occupied by a species can be derived from the Forest Res (2017) 136:171-183 DOI 10.1007/s10342-016-1017 proportions of their crown projection area or of their leaf area. The estimates of the potentials obtained from neighboring monospecific stands, especially in older stands, were poorer than those from the maximum density line depending on the Martonne aridity index. Therefore, the stand-level method in combination with the Martonne aridity index for potential densities can be highly recommended. The species' proportions estimated with this method are best approximated by the proportions of the species' leaf areas. In forest practice, the most commonly applied method is an ocular estimation of the proportions by crown projection area. Even though the proportions of pine were calculated here by measuring crown projection areas in the field, we found this method to underestimate the proportion by 25% compared to the stand-level approach.Eur J
& Key message This data set provides unique empirical data from triplets of Scots pine (Pinus sylvestris L.) and European beech (Fagus sylvatica L.) across Europe. Dendrometric variables are provided for 32 triplets, 96 plots, 7555 trees and 4695 core samples. These data contribute to our understanding of mixed stand dynamics. Dataset access at http://dx.doi.org/10. 5061/dryad.8v04m. Associated metadata available at https://metadata-afs.nancy.inra.fr/geonetwork/apps/ ge orche str a/?uuid=b3e0 98 ca-e 68 1-49 10 -909 9-0e25d3b4cd52&hl=eng.
Context Over the last decades, there has been an increasing interest in mixed species forests because of their expected positive impacts on ecosystem services (including productivity), ecosystem stability and risk management. However, the mechanisms at play in species interaction and their dependency on spatial and temporal variations of environmental conditions are still insufficiently understood.Methods To assess the impact of species mixture on tree response to water availability, we constructed 22-year time series of wood carbon isotope compositions from a network of sites covering a large gradient of environmental conditions throughout Europe. Each site included a mixed Fagus sylvatica L. / Pinus sylvestris L. stand and one monospecific stand of each species, with all the stands at a given site in similar environmental conditions.Results A positive species-mixture effect for both species was found on dry sites. On moderately wet sites, the results were contrasted, with pine showing a negative effect and beech a positive one. The contrasted results can be explained by the differences in how each species manages the trade-off between carbon acquisition and water loss, which are highlighted in pure plots. No species-mixture effect was found on extremely dry or extremely wet sites. There were no differences in reactions to drought between pure and mixed stands.
ConclusionMixing species did not improve trees response to a drought event but influenced their average isotopic composition according to the species-specific functional traits and average site conditions. The pattern of mixing effect along the gradient of water availability was not linear but showed threshold points.
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