Riparian hardwood forests have become very rare in most areas across Europe, as their fertile habitats have mostly been transformed into grasslands and arable land. Furthermore, where small patches remain, these are seen to be subject to major changes in soil cover and plant composition, thanks to habitat change induced by drainage, river engineering, the construction of river embankments and forestry. A further, highly visible symptom of the degradation of riparian hardwood forest is invasion by alien species (neophytes). This article therefore draws on work to analyze Poland’s vegetation of riparian hardwood forest, by reference to some 249 phytosociological relevés from 83 sites located along river valleys (Fig. 1). The work came within Natura 2000 habitat monitoring, and specifically a research project entitled Riparian hardwood forest services in the middle Vistula river valley. The main objectives here were to point to any relationships that might pertain between the share of invasive alien species and the structure and composition of riparian hardwood forest vegetation, as well as to determine the former’s habitat requirements. The studied communities were divided into two groups: ZI – with the presence of invasive alien species, and ZN – natural communities lacking such species (as listed by Tokarska-Guzik et al., 2012). Relationships between the share of invasive species and the structure and composition of native vegetation were tested by comparing species richness (number of species: S – general, SA – trees, SB – shrubs, SC – herbs, SD – bryophytes), species diversity (H – the Shannon diversity index (Shannon and Weaver, 1949), J – the Pielou evenness index (Pielou, 1975)), habitat preferences of species by reference to Ellenberg ecological indicators (Ellenberg et al., 1992) and socio-ecological affinity (after Schmidt et al., 2011). Spearman rank correlation coefficients were used to assess relationships between the numbers of invasive alien species and the cover-shares they accounted for on the one hand, and the values of all studied parameters on the other. Mean values were compared across the ZI and ZN groups using the Mann-Whitney U-test. Statistical analyses were performed using PAST 2.17 (Hammer et al., 2001). Invasive alien species were recorded on ca. 70% of the plots studied. Small balsam was species among the 15 observed most frequently and achieving greatest abundance (Table 1, Fig. 2). Lower general richness of species in the communities where invasive alien species are present results mainly from decline in numbers of hygrophilous and shade-tolerant forest species, as well as shrubs and bryophytes (Table 2). This may be related to changes in habitat conditions that diminish competition from the existing composition of the phytocoenosis. The undergrowth of communities featuring invasive species is composed of species preferring habitats with higher light availability, with a higher soil pH and a richer trophic status, but there are few species of more moist habitats. The broad habitat range characteristic for small balsam (as regards light and soil pH) combine with its preference for drier mesophilous sites and a marked capacity to disperse providing for the expansion of the species, which in fact comes to dominate in disturbed forest communities. On a more-positive note, the analysis shows how the degradation of riparian hardwood forests could be limited, if only their natural habitat conditions can be assured.
The invasive alien species Amorpha fruticosa in Bulgaria and its potential as economically prospective source of valuable essential oil
