Structural Heterogeneity of European Beech (Fagus sylvatica L.) Stands at Its Northernmost Limits

Abstract

The expansion of European beech to northeastern regions due to climate change is anticipated, especially if assisted migration techniques are employed. Marginal populations of European beech are exposed to unfavorable growing conditions that are challenging for their survival and multifunctionality. Under such conditions, the structural complexity of stands is a critical factor that supports the sustainability of these populations. In this study, five stands of European beech in Latvia, which are currently the most northeastern stands in Europe, were investigated. In each of the stands, two sample plots (area 500 m2) were randomly established. The dimensions of trees, stem quality features, and spatial structure of the stands were assessed. The stands varied in density but were found to be productive as indicated by comparable tree dimensions to those in core populations. The studied beech stands displayed low species mingling and tended towards monospecies composition, with some structural diversification likely due to small-scale disturbances and varying stand densities, suggesting that spatial diversity was influenced by species composition and competition among trees. The analyzed European beech stands were in the maturing phase, but displayed diverse diameter and height structures, indicating that natural ecological processes were occurring, akin to those found in non-marginal regions. The stem quality of the trees was intermediate, with frequent occurrences of ramicorn, epicormic branches, and forking (41.8%, 53.5%, and 26.3%, respectively), while stem cracks were rare (4.6%). However, these features can provide crucial microhabitats for biodiversity. Therefore, European beech has the potential for diversification in forestry and ensuring sustainability at the edge of its range expansion. The main implications of this study highlight the diverse structural characteristics of the European beech stands, indicating the influence of species competition and small-scale disturbances, providing valuable insights for forest management and conservation strategies. Although, this study has a few potential limitations that should be considered, including the relatively small sample size and the absence of long-term data.