Leaf phenotypic plasticity of European ash (Fraxinus excelsior) at its northern range in Dinaric Alps

The Dinaric Alps have been recognised on numerous occasions as a biodiversity hotspot. They host a variety of species with great importance in sustainable forestry operations and nature conservation. One such species is the European ash (Fraxinus excelsior L., Oleaceae), a broad-leaved, wind-pollinated and wind-dispersed forest tree. In this paper, we aimed to determine the morphological variability of the European ash populations of the northern Dinaric Alps. For this purpose, leaf samples from 10 individuals in seven populations were collected. Morphometric analysis of intra- and interpopulation variability was conducted using 19 morphological leaf traits. We determined great variability of trees within, and small variability among, populations. The variables that best discriminated studied populations were those relating to leaflet shape. Based on these variables, populations were grouped into two clusters. The first cluster encompassed individuals with acute leaflets, found in drier and rockier habitats, whereas the second cluster, defined by more rounded leaflets, was found in mesophilous and nutrient-rich habitats. However, this research revealed no influence of geographical or bioclimatic distances on morphological variability, which indicates that the rockiness and soil are most likely two predominant factors in shaping the phenotypic plasticity of European ash populations. These results are of great significance in the planning of future forest breeding programs, as populations from drier habitats are likely to persist and spread due to their adaptation to water scarcity, which will become more pronounced in the future.