Changes in presence of alien species in the ruderal vegetation of a representative ecosystem in a major city over 30 years: a case study from Bratislava

The study is focused on evaluating changes in the presence of alien species in ruderal vegetation. Two datasets comprising phytosociological relevés of ruderal communities during two time periods within the Bratislava City (in southwestern Slovakia) were analyzed. The old dataset consisted of 387 relevés recorded in the years 1975–1982, and the more recent dataset consisted of 308 relevés from the years 2011–2014. The relevés from both time periods were assigned to phytosociological classes via the same procedure – numerical classification (hierarchical clustering) using HIERCLUS software. The average values of the percentage number and percentage cover of the archaeophytes and neophytes as groups in the earlier and more recent relevés of each class were compared by the main effects ANOVA analysis in the STATISTICA 7.0 software. The proportion of each taxon of archaeophytes and neophytes in both datasets was also calculated. The invasive status, origin, and life forms of alien species in both datasets were compared. In total, both datasets comprised 120 archaeophytes and 71 neophytes. The old dataset contained more archeaophytes than the more recent dataset. On the contrary, the more recent dataset contained more neophytes than the old dataset. In the years 2011–2014, more invasive neophytes were recorded than in the past. The results revealed a statistically significant decrease in the average percentage number of archaeophytes in the classes Stellarietea mediae, Artemisietea vulgaris, and Galio-Urticetea, and a decrease in the average percentage cover of archaeophytes in the class Artemisietea vulgaris, compared to the past. On the contrary, a statistically significant increase in the average percentage number and cover of neophytes in the classes Artemisietea vulgaris and Galio-Urticetea was recorded. Some rare species of archaeophytes were only recorded 30 years ago (e.g., Kickxia elatine, Silene gallica). Furthermore, several species of neophytes (including some invasive taxa, e.g., Fallopia japonica, Helianthus tuberosus, Juncus tenuis) occur only currently in the ruderal vegetation of Bratislava.