Distribution and environmental impact of alien woody species in lowland riparian forest habitats: Case study in the protected areas of Georgia, South Caucasus

Biological invasions are globally recognised as a significant threat to native biodiversity, as they can change the structure, composition, and functioning of the native ecological system. The riparian forest is considered a habitat highly invaded by invasive plants since rivers serve as corridors for alien species through the terrestrial ecosystem or may simply harbour exotic species that cannot survive elsewhere. The riparian forest, as a high conservation value forest, is an important part of the lowland forests of Georgia. These forests are distributed along large rivers, including the Mtkvari, Alazani, Iori, Aragvi, Ksani, Algeti, and Great and Little Liakhvi rivers. Here, we present a case study that was part of the scientific project titled 'Survey of Potential Invasive Alien Woody Species (PIAWS) in the Protected Areas of Georgia'. Protected areas (PAs) in Georgia play an important role in conserving native biodiversity, including forest ecosystems of distinguished biodiversity. We focused on the lowland riparian forest (LRF) distributed along the Mtkvari and Iori rivers within two PAs – Korugi and Gardabani Managed Reserves (MRs). Six main alien woody species (AWS) were identified in the study areas: Robinia pseudoacacia, Gleditsia triacanthos, Ailanthus altissima, Acer negundo, Amorpha fruticose and Morus alba. Among these AWS, the local status of R. pseudoacacia, G. triacanthos, A. altissima, and A. fruticosa is invasive, while that of A. negundo and M. alba is naturalised. The environmental impact of AWS on LRF habitat was evaluated using the Environmental Impact Classification for Alien Taxa (EICAT) classification system. A major or moderate impact of AWS on the LRF habitat is evident, which, according to EICAT impact mechanisms, is expressed as apparent competition and physical, structural and indirect impacts. The study revealed that the impact levels of three AWS, R. pseudoacacia, G. triacanthos, and A. altissima, were classified as major or moderate ('harmful' impact), while the remaining AWS, A. negundo and A. fruticosa, were of minor or minimal concern. AWS in the invaded areas of the LRF are found as an understorey layer or, otherwise, they have already formed the main canopy and replaced native dominant woody species. Therefore, undesirable structural and succession processes in invaded LRFs are evident.