Patterns of seed bank and vegetation diversity along a tidal freshwater river

Species richness and diversity may increase with spatial scale related to increased area and heterogeneity of habitat. Yet, in bidirectional hydrologically connected tidal ecosystems, secondary dispersal via hydrochory has the potential to homogenize seed banks, and both life history characteristics and tolerances to environmental conditions influence the composition of plant communities. How species richness, diversity, and composition of seed banks and vegetation change along environmental gradients and at different spatial scales is not well understood.

We explored the relationships of seed bank and vegetation diversity across 135 plots along a tidal freshwater river in the Delaware Estuary, USA. Species richness and diversity were partitioned across three hierarchical spatial scales: individual plots, transects perpendicular to the tidal channel, and river kilometers. Community structure was also examined as it related to distance from the tidal channel and location along the tidal river.

Species richness was 89 in the seed bank and 54 in the vegetation. Species-area relationships revealed that species richness reached a near maximum asymptote inland (20 m from channel) for the seed bank and at the edge (0 m) for the vegetation. Rare occurrences of species in the seed bank and vegetation were greatest 5 m from the channel edge. As spatial scale increased, seed bank richness increased, associated with the progressive accumulation of species. Seed bank diversity, however, was maximized within small plot areas and along the river. Diversity of the vegetation was maximized locally due to the abundance of a few common species.

These findings suggest that suites of common species contributed to high localized vegetation diversity, yet large spatial scales maximized the number and diversity of species in the seed bank and vegetation through rare encounters, as well as the complexity of the landscape.