Morphological variation of Betula pendula and betula pubescens leaves in south-eastern Europe

Abstract

Birch species (Betula pendula Roth and Betula pubescens Ehrh.) are ecologically important components of temperate and boreal forests, exhibiting considerable morphological variability across their distribution ranges. This study aimed to assess leaf morphological variation among natural Betula populations in Bosnia and Herzegovina and Slovenia and to examine its relationship with environmental and climatic factors.

Leaves were collected from 39 populations (31 in Bosnia and Herzegovina and eight in Slovenia) during 2020–2022. Ten trees per population were sampled, with 30 leaves collected from each tree. In total, 11,700 leaves were analyzed. Morphometric analyses included measured, assessed, counted, and derived leaf traits. Descriptive statistics, nested analysis of variance, population differentiation coefficients, Pearson correlations, principal component analysis (PCA), and cluster analysis (UPGMA) were applied. Relationships between morphological traits and climatic variables were evaluated using bioclimatic data from the WorldClim database.

The results revealed substantial phenotypic variability in leaf morphology among the studied populations. The first two principal components explained 73.2% of the total variation, with leaf size parameters representing the dominant axis of differentiation. Cluster and PCA analyses indicated clear regional structuring, with Slovenian populations forming a distinct group from most populations in Bosnia and Herzegovina. Qualitative traits such as leaf shape, margin, pubescence, and texture showed significant heterogeneity among populations. Leaf pubescence was confirmed in two populations previously identified as B. pubescens (Han Kram and Mokro 2). Strong correlations were observed between leaf morphological traits and climatic variables, particularly mean annual temperature, which showed a strong influence on leaf dimensions.

Overall, the findings highlight pronounced morphological diversity in Betula populations and suggest that environmental gradients, particularly temperature, play a key role in shaping leaf morphological variation in south-eastern Europe.