Alien species invasion and diversity of riparian forest according to environmental gradients and disturbance regime

摘要

Forty two vegetation samples (0.1 ha nested field plots), taken fro m lowland riparian forest, located on the right floodplain of Tundzha river (Southeastern Bulgaria), were analyzed. Coarsest picture of vegetation pattern was outlined, using classification and ordination techniques. Detailed analysis of alien and native species distribution was carried out by correlation and regression methods. Described variables were tested for difference with parametric and nonparametric tests. Classification resulted in four plant co mmunity types, dominated by different native and exotic trees, shrubs and herbaceous species. Ordination showed that main environmental gradient responsible for the vegetation pattern explanation was habitat moisture. Correlation and regression analyses revealed that most abundant alien and invasive species, such as Acer negundo and Amorpha fruticosa, preferred the mo st humid part of the moisture gradient, but other alien species had complex response surfaces. Logging is considered as unreliable predictor of alien species distribution at 0.1 ha scale, but as a promoter for xerophylous alien herbs establishment at the 1 m2scale on the studied territory. Flooding disturbance certainly has played essential role in this context too. Species diversity demonstrated patterns in accordance with Intermediate Disturbance Hypothesis. We recommend using different sampling scales with greater area covered