Woody Species Composition, Stand Structure and Growth Dynamics of the Habitat 91E0*—Alluvial Forest—in Nestos, Greece

Tree species composition, structure and regeneration status were assessed in the priority habitat 91E0* (Alluvial forests with Alnus glutinosa (L.) Gaertn and Fraxinus excelsior L.) in Nestos area, north-eastern Greece, to quantify conservation status and dynamics of the alluvial forest and properly plan restoration actions. Measurements were conducted in 15 plots distributed randomly in the area of the habitat (c. 300 ha). A total of 2091 individuals were recorded, representing 17 species and 13 families. In Group A (trees having DBH ≥ 2.5 cm), tree densities varied from 14 to 572 stems ha−1 and average basal area was 6.3 m2 ha−1, whereas the juvenile populations (Group B, trees having DBH < 2.5 cm and height > 20 cm) were more diverted presenting higher and more variable densities (28.3–2263.5 trees ha−1) and tree heights (0.21–9.75 m). The index of regeneration (IR) estimated for the juveniles was highly fluctuated among plots (0.17–31.75 cm m−2). According to our results, Alnus glutinosa dominates in the alluvial forest, establishing, however, more resilient communities with Salix alba L., Populus alba L. and A. glutinosa regeneration was adequate (IR 14.09 and 9.94 cm m−2, respectively), whereas Fraxinus angustifolia Vahl was very limited (IR = 0.17 cm m−2). Alarmingly, the regeneration of the alien species Amorpha fruticosa L. and Acer negundo L. were found to be the most extended (31.75 and 21.12 cm m−2, respectively). Results underline the urgent need for conservation actions and draw attention to the threats on the alluvial forest.