Evaluation of Undergrowth under the Canopy of Deciduous Forests on Very Fertile Soils in the Lithuanian Hemiboreal Forest

In the hemiboreal forest zone, the first-generation natural forests of Betula spp., Populus tremula L., and Alnus incana (L.) Moench, which develop after clearcutting on very fertile forest sites, change to deciduous hardwood forests (Quercus robur L., and Fraxinus excelsior L.) due to successive processes. These processes also cause a specific response to the development of undergrowth species, which can have a decisive influence on forest regeneration. The undergrowth species in the mature Betula spp. (Betula pendula Roth. And Betula pubescens Ehrh.), Populus tremula, and Alnus incana forest stands were evaluated to provide knowledge on undergrowth species composition and development under different soil conditions, and to identify the forest stand age’s impact on undergrowth species. The evaluation was based on the standwise forest inventory data and limited to the analysis of pioneer tree species forests usually developed as first-generation natural forests after clearcutting. The study results showed that deciduous forests have rich undergrowth species diversity with a dominance of the Corylus avellana L., Padus avium L., Frangula alnus Mill., Sorbus aucuparia L., and Salix spp., which are typical undergrowth species in the hemiboreal forest zone. The dense and medium density undergrowth with the predominant Corylus avellana was rather common in the Betula spp. and Populus tremula stands; and Padus avium was more abundant in the Alnus incana stands on very fertile sites. Larger areas with dense undergrowth were obtained in the low mixed and mixed Alnus incana stands than in the pure stands, while no clear relationship between the stand mixture and undergrowth density was obtained in the Betula spp. and Populus tremula stands. The area of Corylus avellana significantly increased, while the area of Padus avium decreased in all studied forests with increasing stand age. Other dominant undergrowth species–Sorbus aucuparia, Frangula alnus, and Salix spp.–decreased with increasing age of the Betula spp. and Populus tremula stands. In the context of biodiversity, a higher number of undergrowth species was obtained in the mixed Betula spp. stands than in the pure and low mixed stands. New insights about the undergrowth species and their development patterns under the canopy of pioneer deciduous forests on very fertile soils were provided. However, these findings do not strongly suggest that an unmanaged forest regime wins over conventional forest management in mature and older deciduous forests on fertile soils as regards the biodiversity and other ecological services provided by the undergrow species.