Defining Illyrian Forests: The Role of Biogeographical Patterns and Species Co‐Occurrence

ABSTRACT Aim This study had four aims: (1) to group traditional mesophilous indicator species of Illyrian forests based on their distribution patterns; (2) to gain insight into the biogeographic attributes of each group by hotspot analyses; (3) to clarify the level of importance that certain species have in the floristic differentiation of Illyrian from non‐Illyrian forests; (4) to explore the possible distribution of Illyrian mesophilous forests. Location Europe, Asia and Africa. Taxa Illyrian forest understory plant species. Methods A dataset of 40 understory plant species traditionally associated with Illyrian forests was compiled from the literature. The distribution of these species was digitised in the form of polygons, following numerous sources. A 25 × 25 km grid cell system was established to convert the polygon distribution data into a matrix. This matrix was used for hierarchical clustering in order to classify species based on distribution into groups. Hotspot analysis was performed for groups using Getis‐Ord Gi* analysis. The dataset was further divided into clusters to reveal less pronounced distribution patterns. Results The classification resulted in three major groups of species: (1) a Steno‐Illyrian group, with a restricted hotspot area mostly distributed in Slovenia and Croatia; (2) a Sub‐Illyrian group, with a larger distribution area but with a significant part of the distribution located in the Dinarides; and (3) a Pseudo‐Illyrian group, which has a widespread distribution across Europe, North Africa and parts of Asia. Hotspot analysis identified the Dinarides as the main area of Illyrian understory plant diversity, with extensions into the Southern Alps and some Pannonian mountains and hills. Main Conclusions The area of the Dinarides and Southern Alps is characterised by the presence of Steno‐Illyrian species and a high‐frequency overlap with Sub‐Illyrian species. Pseudo‐Illyrian species lack biogeographic specificity and therefore cannot be used to identify Illyrian forests.