Monitoring Over 22 Years Documents Invasion of Rare Riverscour Plant Community on the Delaware River

ABSTRACT A rare riverscour community along the Delaware River's New Jersey shoreline that supports numerous state-listed rare plant species was monitored three times between 1998 and 2020, revealing numerous significant trends in plant species richness and abundance. Cover of nonnative species and nonnative invasive species increased annually by 3.8% and 3.7%, respectively, while nonnative and nonnative invasive species richness increased annually by 1.1% and 1.3%. Cover of native species was stable; however, native species richness declined annually by 0.42%. Cover of trees and shrubs increased significantly by nearly 6% annually, while graminoid cover declined by 1.5% annually and herbaceous cover showed no significant trend. Shifts in plant species' importance values documented the establishment and spread of several invasive shrub and herbaceous species, as well as the extensive spread of the invasive reed canarygrass (Phalaris arundinacea). Native grasses and sedges characteristic of the riverscour community, as well as over half of the 147 native herbaceous species identified, declined in importance value over the sampling period. Future declines in winter ice scour due to projected warmer, shorter winters underscore the importance of managing woody invasive plants at this site. Reed canarygrass and herbaceous invasive species also pose a challenge to the management and preservation of this rare community.