Annual species abundance in a tidal freshwater marsh: Germination and survival across an elevational gradient

Annual species contribute significantly to the standing biomass of tidal freshwater marshes, but they tend to be distributed unevenly along the elevation gradient, with higher surface elevations supporting greater densities of annual species. We explored the generation of this pattern by evaluating how different life history stages of annual species are related to elevation and to each other. In 2006, we counted seedlings emerging from the seed bank under optimal greenhouse conditions, as well as seedlings and mature stems of annual species in 38 plots located at different elevations in a tidal freshwater marsh near Alexandria, Virginia, USA. Annual species seedling and mature stem density increased with elevation of the marsh surface, but seed density did not change with elevation. Seeds of annual species germinated in saturated rather than flooded conditions (4.50 +/- 0.54 versus 1.66 +/- 0.26 species/greenhouse container +/- SE). Germination and survival from seedling to mature stem affected density of annual species across an elevational gradient, but the relative importance of either process differed by species. Amaranthus cannabinus was common and frequent in the seed bank (68% of all plots), and seed density increased with elevation, but seedlings and mature stems were infrequent at any elevation (21% and 5%, respectively), suggesting that germination is limiting its recruitment to standing vegetation. In contrast, Bidens laevis was uncommon in the seed bank (18% of all plots), but was frequently observed as seedlings and mature stems (55% and 34%, respectively). We therefore conclude that B. laevis recruitment was limited by its ephemeral seed bank. Impatiens capensis density did not appear to be limited by germination or survival to maturity as density of the species was high at all life history stages. Rather, its strong positive relationship with elevation at all life stages shows that I. capensis is limited by water inundation, particularly in spring when seeds require oxygen to germinate. Overall, we show that annual species increase in density at higher elevations in a tidal freshwater marsh, but that recruitment of each species may be limited by different intrinsic and extrinsic processes.