Changes in the functional composition of a Central European urban flora over three centuries

Documents on historical floras provide unique opportunities to analyze past changes and to show trends in biodiversity. We studied the historical and recent flora of the city of Halle in Central Germany. Our earliest records date back to the year 1687; the youngest were sampled in 2008. More than 20 other floras provide information for time in-between, covering ca. 320 years in total. We checked all historical plant occurrences for plausibility. The species turnover of 22% that took place in the study period should also yield changes in the functional composition of the flora. We identified native species and archaeophytes that went extinct since 1689 and 1856, respectively, and all neophytes that were introduced since 1689 or 1856. This ‘double’ calculation minimized the influence of so-called possible pseudo-absences. Contingency tables assisted to identify trait states which were associated with extinction or introduction. Time-series analysis identified temporal trends in trait state ratio development after testing for temporal autocorrelation. Within the study period, species of bogs, nitrogen-poor habitats or plants with helomorphic leaves got extinct more often than expected by chance. Species dispersed by humans, plants preferring nitrogen-rich or warm habitats, shrubs and trees, and species with mesomorphic leaves were, amongst others, over-represented among introduced neophytes. Land-use changes such as the transformation from agriculture to urban land use or the drainage of bogs are discussed as main drivers of these developments. Additionally, climatic changes, contamination of habitats and gardeners’ preferences for specific plants are presumed to having caused floristic changes. Our study shows the vast influence humans had and still have on biodiversity by intentionally or unintentionally selecting specific functional plant types and thus changing the composition of the flora.