resulting from U.S. Army Corps of Engineers projects, reduced washover sedi- 

 ment available for dune building. The depauperate section of belt G may, in 

 time, be colonised by shrubs as has occurred at belt D. It should be noted, 

 however, that belt D is backed by glacial headlands which provide an adjacent 

 seed source for woody species, wh.er.eas belt G Is flanked by open water 

 (Pleasant Bay). 



From these vegetative physiographic transects, it is apparent that the 

 rate of development of all plant communities and physiographic features on 

 North Beach is extremely rapid. Dunes have been evident from aerial photog- 

 raphy analysis on washovers deposited over salt marshes in as little as 3 

 years. Since the major dune species on North Beach can recover from substan- 

 tial overwash burial, washovers on dune communities may appear unvegetated 

 on aerial photos when, in fact, dune building is occurring very rapidly in the 

 area. The greatest Ammophila breviligulata biotaass on North Beach occurs in 

 areas that have recently overwashed (belts H, I, and J; Table 55). These 

 biomass data even exceed data collected on building foredunes (belts K, L, 

 X, and Y) . As much as 70 centimeters of sand can be added to a low dune by 

 overwash in a single year (see Sec. II). 



Most new dune development has been associated with massive washovers on 

 Old North Beach and with spit elongation on New North Beach and Nauset Spit- 

 Orleans. Dune development on massive washovers at North Beach often begins at 

 the western edge of the feature in drift lines. Often these dunes at the bay- 

 ward edge do not survive once overwash and continued sand supplies are reduced 

 or eliminated. These low dunes may deflate rapidly and become colonized by 

 high marsh vegetation, or If these dunes do not deflate ro high marsh eleva- 

 tions, Agropyron pungcns may colonize these slightly elevated sites. 



Dunes will, in a short while, build on the landward part of massive wash- 

 overs at the location of remnant dunes or drift piles. Small washover fans 

 and breaches may provide some sand for the expansion of the dune line locally, 

 but do r jt play a major role in dune-building processes and landward displace- 

 ment of the physiographic zones of a barrier system. 



Species diversity on sand dunes at North Beach increases after the initial 

 rapid building period, but does not continue to increase with time (Table 56). 

 Once a dune no longer increases in elevation, several plant species rapidly 

 invade less vigorous stands of Ammophila breviligulata. Shrubs ussy colonize 

 the area if foredune height reduces salt spray and seeds are available. If 

 the site remains stable for many years, stands of Ammophila breviligulata 

 may begin to decline in vigor, giving way to Hudeonia tomentoea, Chrysopsis 

 falaata, Artemisia caudata, Lechea maritima. (pinweed) , and Cladonia spp. 

 These species can be used as indicators of substrate stability, but may 

 reflect only very recent conditions; senescent dune zones develop rapidly. 

 Belt K, which is less than 26 years old, has a broad, stable dune area in the 

 interior of two large spit recurves with extensive stands of dead and dying 

 Ammophila breviligulata. 



Spit recurves have developed at the terminus of North Beach opposite 

 embayments in the Chatham mainland, increasing dune field and barrier width. 

 In several areas (belts H, I, and J) , these recurves have been truncated 

 on the bay side by tidal currents in Chatham Harbor, resulting in apparent 

 parallel dune ridges on the bay and ocean shorelines of the barrier. The bay 

 shoreline dunes should not be confused with washovtr-derived dunes which, if 

 isolated from the ocean dune line, are also low in elevation. 



212 



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