Convergence or divergence: Taxonomic, phylogenetic, and functional beta diversity during a 50-year old-field succession
Whether communities converge towards a similar structure over succession has been hotly debated for nearly a century, but have been rarely tested in natural communities over sufficiently long time scales. Clements’ deterministic view suggests successional convergence regardless of starting conditions and assembly history, while Gleason’s individualistic view emphasizes that communities should follow divergent trajectories dependent on the stochastic colonization and extinction of species and on initial conditions. We conducted an analysis on a nearly 50-year span of old-field succession across 480 permanent plots in 10 replicate fields in New Jersey, where the initial conditions of each field were directly manipulated. Using unifying dissimilarity measures through Hill numbers, we compared the taxonomic, phylogenetic and functional beta diversity among fields and plots in each year, and calculated temporal turnover rates over time of each field/plot. Together, these analyses allow us to examine whether community dissimilarity decreases over succession from taxonomic, phylogenetic and trait perspectives.
Our results reveal that the trajectory and rate of old field succession depended on the focal spatial scale and the abundance of the focal species. At the field scale, the taxonomic, phylogenetic and functional dissimilarity of common species among fields significantly decreased over time. Simultaneously, the temporal turnover rates of each field also significantly decreased over succession. The initial conditions of the fields, such as the identify of crops before abandonment and the year of abandonment, significantly influenced the structure of the fields at the early stages of succession, but their contributions decreased over time and were no longer significant after 20 years of succession. These results demonstrate that the fields were gradually converging towards a stable community structure, with common species that are closely related to and functionally similar with each other regardless of starting conditions. However, the dissimilarity of rare species in each field did not decrease over time, with no sign of convergence among fields. In contrast, we found evidence for weak divergence at the plot level for both common and rare species. Although the temporal turnover rates of each plot significantly decreased over succession, the taxonomic, phylogenetic and functional dissimilarity among plots within each field showed no significant decrease over time. Overall, our results suggest that the relative importance of stochastic and deterministic processes in mediating community assembly will be better understood when combining different scales, long-term data series, and appropriate analytical tests.