Northern Ibero-Atlantic wetland woods, Vegetation types and within-stand structure

This study was carried out in eight stands of wetland woods located in the north-western part of the Iberian Peninsula. The number of sampling sites per stand depended on the physical heterogeneity and area of each stand. Biological data recorded per site included species composition and a list of structural parameters (relative cover in each forest layer, tree height, tree diameter at breast height, number of live and dead trunks and individuals, and morphological adaptations to flooding). Hydro-geomorphologic, geological and climatic variables were also obtained. The floristic data set was subjected to a non-hierarchical cluster analysis and a principal component analysis, which provided an overall image of vegetation grouping based on community composition that only considered perennial species (trees, shrubs, non-annual herbaceous plants and climbers). Three groups of plant communities were distinguished, with an overstorey that was dominated by Betula alba, Salix atrocinerea and Alnus glutinosa, respectively, which presented different environmental drivers. The structural features of each group that was characterised by a different overstorey species were analysed and differences were tested. The number of trunks per individual, number of dead trunks, canopy height and the presence of adaptations to flooding were the structural parameters that best separated the wetland wood groups. Environmental variables were correlated with the ordination axis to analyse community distribution trends and the hydrological regime was found to be the key environmental factor controlling both floristic and structural patterns.