The vascular flora, natural communities, and natural community system on the Isles of Shoals in New Hampshire and Maine are described based on a combination of historical records dating back to 1614 and a comprehensive survey conducted in 2006. A total of 430 native or naturalized vascular plant taxa from 242 genera and 71 families have been documented on these nine maritime islands. The families best represented in the flora were Asteraceae (58 taxa), Poaceae (49 taxa), and Rosaceae (35 taxa); the largest genera were Carex (9 taxa), Juncus (8 taxa), Persicaria (7 taxa), and Solidago (7 taxa). Forty-two percent of the 430 plant taxa were non-native. Proportions among life forms were 86% herbs, 12% shrubs, and 2% trees. Plant richness for extant taxa totaled 333 and varied from 53 on White Island to 246 on Appledore. Fifteen plant taxa are rare in either New Hampshire or Maine. Fourteen natural communities, two human-disturbed cover types, and one natural community system were described from the nine islands. Eight of the 14 natural communities are extremely rare in New Hampshire; none of the communities are rare in Maine. Appledore, over twice the size of the next largest island, supported all 14 communities and both human-disturbed cover types. Seaveys, the second smallest island, supported just three communities. One natural community system was described, the maritime rocky shore, which characterizes all of the islands in the Isles of Shoals. Although not rare in Maine, this system in New Hampshire only occurs on these islands. Island size was significantly related to vascular plant richness and native plant richness but was somewhat less related to the richness of non-native plants.

The floristic diversity and vegetation communities of Leonard Marsh and Harpers Meadow, two peatland complexes of recent origin within the Lake Umbagog National Wildlife Refuge in Coos County, New Hampshire, were investigated. The combined vascular floras of Leonard Marsh and Harpers Meadow consisted of 178 species, 111 of which occurred in both peatlands. The flora included two state-listed species, Eriophorum angustifolium subsp. angustifolium (endangered) and Listera cordata (threatened). Abundance and frequency data from 617 quadrats along 35 transects were analyzed by two-way indicator species analysis (TWINSPAN) and resulted in the recognition of nine vegetation cover types (CT): Thuja occidentalis–Alnus incana CT, the Picea mariana–Nemopanthus mucronatus CT, the Nemopanthus mucronatus–Alnus incana CT, the Rhododendron canadense–Nemopanthus mucronatus CT, the Rhododendron canadense–Chamaedaphne calyculata CT, the Chamaedaphne calyculata–Carex oligosperma CT, the Scheuchzeria palustris–Carex oligosperma CT, the Carex lasiocarpa–Chamaedaphne calyculata CT, and the Dulichium arundinaceum–Potentilla palustris CT. A tenth cover type consisting of aquatic vegetation, the Brasenia schreberi–Potamogeton spp. CT, was recognized based on qualitative field observations. Despite the recent formation of these peatland complexes following the construction of the Errol Dam in 1853, the vegetation composition of Leonard Marsh and Harpers Meadow shares broad similarities with peatlands in New England, the northcentral United States, and adjacent Canada. Leonard Marsh and Harpers Meadow are best classified as oligotrophic to weakly minerotrophic level peatland complexes occupying broad and shallow outwash plains. Historical records, botanical evidence, and patterns in basin topography suggest that terrestrialization and paludification have been critical processes in the post-settlement development of these two large peatland complexes.