Salt marsh vegetation change at Cape Cod National Seashore between 2003 and 2018

Smith, Stephen

Seven salt marshes within Cape Cod National Seashore (CACO) were surveyed for their plant communities in 2018, constituting the most recent survey within the framework of CACO’s long-term ecological monitoring program. The main objective of this report is to describe structural and taxonomic change in salt marsh plant communities that have occurred between the first (2003) and most recent (2018) surveys. The analyses include key marsh attributes of vegetation composition, structure, and soils. Distinct changes in CACO salt marshes have occurred over the last ~15 years. For instance, the dominant low and high marsh species (Spartina alterniflora and S. patens, respectively) have both decreased, with losses exhibiting large spatial variation among and within marsh sites. These were mostly due to sea level rise (SLR), crab herbivory, and/or lateral wave erosion. The ratio of S. patens: S. alterniflora and the abundance of transitional species (marsh border and terrestrial species) also declined, leading to reduction in species richness. Soils analyses showed that the two Atlantic-side marshes (Nauset and Pleasant Bay) both have the highest densities of organic carbon in their substrates. They also make up the largest portion of the 18,000 metric tons (MT) of carbon estimated to be stored in CACO marshes. Although the results of regression analyses were not statistically significant, the declining patterns in these species were reasonably well-defined, considering they consist of only four data points (representing the four survey years). Presently, the statistical power to detect trends remains relatively low but this will quickly improve with subsequent surveys. Notwithstanding, the data suggest that ground-level monitoring of CACO salt marsh vegetation is tracking the kinds of plant community shifts that are expected to occur in response to SLR. Moreover, this report provides further evidence that long periods of monitoring are necessary to elucidate trends in this resource—particularly for species in transitional and upland areas that cannot be identified by remote sensing. Finally, this report suggests that salt marsh migration is currently not keeping up with losses given that reductions in the cover of S. alterniflora and S. patens were generally observed across the plot network.

Salt marsh vegetation change at Cape Cod National Seashore between 2003 and 2018

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