Local to latitudinal variation in flowering phenology in the salt marsh foundation species Spartina alterniflora

Background/Question/Methods: Salt marshes are valuable coastal ecosystems, and yet we lack a mechanistic understanding of how these communities will respond to climate change. On the U.S. Atlantic and Gulf coasts, the salt marsh foundation species Spartina alterniflora (smooth cordgrass) dominates low marsh elevations and some high-elevation pannes. Yet colonization of new sites by this species and the processes controlling its reproduction are poorly understood. S. alterniflora reproduces both sexually and clonally, but here we focus on flowering phenology. With observations and experimental manipulations, we quantified Spartina flowering patterns from local to latitudinal scales. Biweekly surveys of stem and flower properties were conducted in four sub-habitats at three New England marshes from June to October 2011, a reciprocal transplant experiment was also conducted between creekbank and panne habitats to test the hypothesis that local-scale abiotic stress influences flowering, and the relationship of phenology with above and belowground biomass allocation was tested. We also surveyed flowering patterns from Massachusetts to South Carolina on a coarser temporal scale, and conducted a common garden experiment to place our local scale investigations in context.

Results/Conclusions: Pannes have higher salinity and soil temperatures than creek and ditchbanks, with intermediate habitats exhibiting intermediate edaphic conditions. Flower densities were higher in all four sub-habitats at the northernmost site than at southern sites. Within-marsh patterns differed among sites: at the northernmost site, densities were similar between all habitats except ditchbank (which was significantly lower), while at the southernmost site (Prudence Island), flower density in pannes was higher than all other habitats. Transplantation of Spartina between creekbank and panne habitats caused shifts in flowering timing and density, and creek-source plants grown in pannes failed to flower at all. We also found latitudinal trends in flowering, with northern sites flowering earlier than southern; this flowering order was retained in a common garden. Based on these results, we propose that local-scale phenology is altered by habitat variation in edaphic stress, and hypothesize that future, climate-mediated shifts in marsh sediment properties will influence Spartina’s reproductive output. Understanding variation at different scales in life history traits, and links between reproduction and growth, will provide a better basis for predicting the response of this foundation species to an altered climate.