The role of waterlogging in maintaining forb pannes in northern New England salt marshes

Areas of high plant diversity, known as forb pannes, characterize many northern New England salt marshes. These pannes are physically harsh habitats where stress-tolerant forbs escape the competitively dominant clonal turfs. In this paper, we experimentally examine the hypothesis that soil waterlogging maintains forb pannes. Experimentally draining pannes led to the replacement of typical panne vegetation by Spartina patens. After four years, S. patens cover increased by greater than 300% in the drained plots, while in adjacent, unmanipulated controls, its cover remained unchanged. In a second experiment, forb panne elevation was manipulated to either increase or decrease soil drainage. After three years elevated plots were dominated by S. patens, whereas lowered plots were dominated by Spartina alterniflora, with reduced forb densities. Our results demonstrate that forb panne communities are the consequence of poor drainage and waterlogged soils that limit the success of competitively dominant clonal turfs and permit the persistence of panne forbs. These results imply that the extensive ditching of New England salt marshes over the past three centuries to increase the value of marshes for livestock grazing, development, and mosquito control has likely eradicated most of these forb panne habitats.