Expansion Rates of Phragmites australis Patches in a Partially Restored Maine Salt Marsh

摘要

Non-native Phragmites australis poses an increasing threat to salt marsh eco-systems in the eastern United States. Multiple factors affect P. australis spread, including reduced salinity and nitrogen runoff. Methods for measuring expansion, such as field surveys or geographic information system (GIS) mapping, often require tradeoffs between precision and scope. We combined field work with high-precision global positioning system (GPS) technology and GIS to investigate expansion of 16 P. australis patches at Drakes Island Marsh in Wells, Maine. Our objectives were to: 1) calculate change in patch area, 2) determine whether expansion varied with respect to site-specific variables, and 3) determine whether expansion varied with respect to landscape variables. Significant patch expansion occurred in 1 year, with an 11 % increase in overall coverage. Expansion was negatively related to pore water salinity, with salinity accounting for 25 % of the variation in expansion. Expansion showed an unexpected directionality warranting further investigation, with patches expanding northeastward more so than in other compass directions. Our methodology allowed for measurement of multiple marsh patches and key variables yet captured small-scale changes over only one growing season. This methodology could potentially be a useful tool for studying expansion rates of invasive species in other salt marsh systems.