Neophytes in Pannonian hardwood floodplain forests – History, present situation and trends

Invasions of neophyte plant species are considered as one of the major threats to the diversity of natural ecosystems including floodplain forests. Pressure from human activities has increased rapidly during last few decades, therefore we suppose that the penetration of neophytes into forest habitats is greater nowadays than it was in the past. The aims of our study were: to find out if there is a significant increase in the number of neophyte species and their cover in the hardwood floodplain forests in the Pannonian region over time; to compare the occurrence of neophytes in Slovak and Hungarian datasets in different periods; and to explain the relationship between ecological factors and species richness of native vegetation, and the level of invasion of hardwood floodplain forests.The study is based on the dataset of 365 relevés ordered within the suballiance Ulmenion (riparian mixed oak–elm–ash forests along great rivers) in the Slovak national database of phytocoenological relevés and 216 relevés ordered within the Ulmenion in the Hungarian national database. The dataset was divided into decades, and the number and cover of neophytes in each period was compared by Kruskal–Wallis nonparametric ANOVA, with multiple comparison of mean ranks for all groups. To analyze the influence of ecological factors we used the number of native species in the plot, Shannon–Wiener diversity index and information about vegetation structure – cover of tree, shrub and herb layers, together with Ellenberg’s indicator values for ecological factors, namely light, temperature, moisture, soil reaction and nutrients. The influence of time and other factors on the number of neophyte species was tested using generalised linear models (GLM).Along the time gradient, there is a significant increase in the number and cover of neophytes between analyzed periods in the Pannonia. Plots from Slovakia had higher number and cover of neophytes in comparison with plots from Hungary from the same periods. Eight ecological factors were involved in the minimal adequate GLM – with six of them being significant and two being close to the significant level. In a broader analysis, where more ecological factors were included, time emerged as one of the most significant factors positively affecting the number of neophytes. The most important ecological factor is the amount of nutrients. Other significant factors are light, soil reaction, cover of herb layer and moisture.