Fine-Scale Spatiotemporal Variability of Plant Species Composition in Old-Field Succession—The Cycling Heterogeneity Hypothesis Revisited

Spatial heterogeneity of species distributions modulates local interactions and dynamics, playing a key role in the development of diversity and ecosystem functioning during secondary succession. Here, we tested the cycling heterogeneity hypothesis, which predicts fluctuating spatial beta diversity, i.e., alternating periods of high and low heterogeneity during succession, driven by the changes in the abundance of dominant species. We analyzed long-term monitoring data collected annually along 52 m long permanent transects over 15 years in abandoned fields. Recovery of grassland vegetation was fast due to the humus-rich chernozem soil, the rich species pool, and the fast colonization of native grassland species from adjacent natural meadow steppe. Heterogeneity was quantified by spatial beta diversity as the mean pairwise dissimilarity among sampling units. Incidence-based (Jaccard) and abundance-based (Bray–Curtis) indices were used. We found large temporal fluctuations in spatial heterogeneity, with amplitudes reaching 80–100% of the total beta diversity range across the successional gradient. Two major beta diversity peaks were identified: maximum heterogeneity occurred during transitions between successional phases, whereas periods of minimum heterogeneity coincided with the sequential dominance of a few particular species. Bromus sterilis and Festuca valesiaca were the most important species driving heterogeneity. Similar patterns were recorded at two monitoring sites. Changing the sampling unit size computationally, varying the dissimilarity indices, or excluding dominant species had little influence on the results. Using null models, we removed the effects of species richness and abundance and found an increasing degree of spatial dependence as succession progressed. However, the corresponding beta deviations also showed non-linear, fluctuating patterns. Our results support the cycling heterogeneity hypothesis in secondary grassland succession. Increasing understanding of heterogeneity patterns provides new opportunities to optimize the temporal and spatial design of grassland restoration measures.