Dune vegetation monitoring: analysis of 2011 survey data and changes in plant communities since 2005

Smith, Stephen

In 2011, monitoring was conducted to follow up on a 2005 survey of dune vegetation habitat and for continued development of a long-term vegetation monitoring protocol for this ecosystem. In 2005, thirty vegetation sampling sites were established in what was then dune grassland habitat throughout Cape Cod National Seashore (CACO), arranged along seven transects running perpendicular to the coastline from Chatham to Provincetown. Each site consisted of two 10×10-m plots. In addition to distance from coastline, sites were characterized by integrating GIS technology to assess landscape changes and succession. In 2011, forty-seven vascular plant species were recorded, three of which were exotics. This is the same number of species recorded in 2005, although some changes were observed. Specifically, twelve species not recorded in 2005 appeared in the 2011 survey and nine species present in 2005 were not documented in 2011. Exotic taxa found in 2011 were Artemisia stelleriana (dusty miller), Bromus tectorum (cheatgrass), and Rosa rugosa (salt spray rose); all of which were present in 2005. One formerly state-listed species, Corema conradii (broom crowberry [Special Concern]), was found at a single site (also noted in 2005 at the same site). By 2011, seven bluff edge plots had converted to bare beach due to bluff erosion. In the other 53 plots, species richness was highly variable among plots, ranging between zero and 13, with a mean value of 4.6 (±0.4 SE). As in 2005, species richness generally increased with increasing distance from the coast. Notwithstanding the seven bluff edge plots that converted to bare beach due to bluff erosion, there were very few statistically significant changes in dune vegetation between 2005 and 2011 within the permanent monitoring sites. In addition, two sites appear to be converting from grassland to salt marsh as a result of changes in the tidal prism of adjacent estuaries, although they are still mostly dune habitat. Aerial photography showed that succession is occurring at a rapid rate in many areas of the dune grassland habitat, although this process has slowed or reversed in areas closest to the bluff edge. In fact, considerable erosion of the bluff edge on both the Cape Cod Bay and Atlantic sides of CACO was evident from aerial imagery. This process results in a substantial narrowing of dune grassland habitat. Analysis of climatological data revealed winter season rainfall is increasing while summers are becoming dryer. In addition, while nitrogen and sulfur deposition are decreasing, they are still at levels that can substantially alter successional processes by disrupting competitive processes, which results in species replacements. It is clear from comparisons with 2005 data that these plant communities are largely influenced by physical factors, such as erosion and changes in coastal geomorphology; however, other factors, such as constituents in atmospheric wet deposition (e.g., precipitation and nitrogen in precipitation) and temperature are likely to be having an effect over longer time scales. Future plant community surveys, as well as additions of sites to the monitoring network for even better spatial coverage (based on species-area analysis), should be effective in tracking these kinds of changes over time.

Dune vegetation monitoring: analysis of 2011 survey data and changes in plant communities since 2005

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