Vegetation patterns in the Saco River floodplain forest in relation to elevation and flooding

Hoffman, Christopher
Sue Gawler
Department of Environmental Studies
This paper examines the relation of vegetation pattern to fluvial dynamics in the fluvial marches and hardwood floodplain forest of the Saco River in Maine. Plant sampling occurred from July until September of 1994. TWINSPAN was used to clarify cover types of four layers – tree, sapling, shrub, and herbaceous. Environmental variables such as elevation, drainage characteristics, and landform type were examined by Canonical Correspondence Analysis (CCA) in the context of known historical changes to river morphology such as oxbow cutoffs. Long term records of discharge data were used to calculate frequency of annual and summer flooding at all sampled points. TWINSPAN groupings of site and species correspond to groupings in CCA ordinations site, species, and environmental data, particularly in the herbaceous layer. Drainage characteristics of freely draining versus impounded are the dominant environmental factors differentiating cover types in the tree and sapling layer, whereas fluvial landform categories of depression, slough, low plain, and high ridge, when combined with flood frequency data, differentiate the shrub and herbaceous layer cover types. Depressions are dominated by graminoid species, and low plain sites are characterized by a tree layer dominated by Acer saccharinum and an herb layer dominated by Onoclea sensibilis. Considerable variation in floristic composition occurs at slough sites. No high terrace features occur at the study site; high plain communities occur, instead, on levees with elevations above the bankfull stage level. A shifting of forest and marsh communities occurs in the floodplain, and channel avulsion rather than channel migration is suggested as the more important macroform process shaping the landscape. This paper illustrates the importance of classification by fluvial landform, use of long term hydrological records, and analysis of maps and aerial photographs in describing the ecology of a meandering river floodplain.

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