Spatial tree interactions in European mixed forests

Tree species diversity is known to promote key forest ecosystem functions, such as primary production, stand growth stability, resistance to disturbances and soil nutrient cycling. Tree and species interactions afect forest functions by means of the efects of diferent tree-tree interaction types. Using the explorative research platform of FunDivEUROPE project, we used an integrated approach to analyse tree and species interactions in 209 forest plots along gradients of tree species richness and composition in six European sites (Spain, Italy, Germany, Romania, Poland, Finland), using both spatially and no spatially explicit diversity measures. Mingling (i.e proportion of the neighbours that difer from a reference tree in terms of tree species), dominance ( i.e., proportion of the four nearest neighbour trees that are smaller than a reference tree), diameter diferentiation ( i.e., measure of the diferences between the dimensions of a reference tree and the four neighbours) and mean directional index (measure of spatial arrangement of trees in the stand) were calculated for each tree in the plot taking into account species, stem diameter and reciprocal distance. Traditional diversity indices (species richness, Shannon and Simpson indices) were also calculated. The interplay between diversity and functioning was explored measuring the photosynthetic eficiency of some target trees. Mixed forests in Central Europe showed higher values of taxonomic and structural diversity than boreal and Mediterranean forests, except stands with 2 species, in which trees were more species-diferentiated in Italy. The dominance index was higher in northern and central sites, while Mediterranean forests showed a more randomly spatial arrangement of trees within the plots. At plot level, in all sites, the diameter diferentiation was positively correlated with Shannon and Simpson indices. The photosynthetic eficiency of target trees was afected by species diversity at diferent scales (neighbor trees, stand level). Our results show that integrating spatial and non spatial measures we can gain a deeper understanding of tree and species interactions. Tree spatial distribution and their atributes influences the resource use availability of individual trees, stand structure and their ecological stability and should be included in the quantitative information of forest ecosystems in view of conservation or management issues.