Markus Bauer , Jakob Huber, Johannes Kollmann

1. Spatio-temporal dynamics of biodiversity are a key measure when monitoring restoration success. Balanced species turnover is aimed at because it increases overall biodiversity and improves ecosystem stability and multifunctionality. For predictive restoration, it is important to analyse spatial beta diversity and to identify its drivers like site characteristics but also uncontrolled factors like spatial effects, historical factors, year effects and non-directional temporal turnover.
2. We studied dike grasslands 4–19 years after restoration at River Danube in SE Germany over five years (2017–2021, 41 plots in 12 sites). We calculated beta diversity indices to describe spatial variation and temporal turnover, including their additive components ‘replacement’ and ‘nestedness’, or ‘gains’ and ‘losses’.
3. The analysis of the spatial variation of the restored dike grasslands did not reveal homogenisation despite a significant temporal turnover, and was largely dominated by replacement-driven dissimilarity. The replacement drivers changed over time, although replacement was mainly affected by exposition and spatial factors. Historical factors were inconsistent over time, and no statistically clear drivers were found for nestedness.
4. The dike grasslands exhibited on average 37 ± 11% (mean ± SD) year-to-year turnover in species composition, with some spatio-temporal variation. Gains and losses were balanced over time, although prevalences changed over time and were most pronounced on south-exposed slopes.
5. The restored grasslands exhibited spatial variation by site characteristics but also uncontrolled spatial factors. Moreover, high non-directional temporal turnover caused by weather fluctuations, slightly varying management, and stochastic biotic dynamics influenced spatial variation. Thus, restoration targets should be defined as a desired variation of alternative states. Furthermore, the dominance of replacement should move the focus from searching the perfect fit for certain targets to a variation of the approaches to increase beta diversity.