Influence of Long-term Greentree Reservoir Impoundment on Stand Structure, Species Composition, and Hydrophytic Indicators1

Abstract

Greentree reservoir management was initiated during a period when riparian forests of the Mississippi Alluvial Valley were disappearing rapidly. Greentree reservoirs (GTRs) were intended to provide a refuge for overwintering migratory waterfowl within a landscape of decreasing habitat availability. However, GTRs frequently are flooded as much as 2.5 months longer than unimpounded bottomland forests (UBF), and previous work has shown that such unnatural hydroperiods can have substantial negative effects on such ecosystem attributes as tree assemblage composition and invertebrate production. We conducted the present study to quantify (1) frequency of canopy gaps and (2) tree species composition in GTRs, as compared with UBFs. In general, GTRs were quite similar to UBF stands, in terms of canopy density, proportion of trees in the canopy versus mid-story, degree of stress exhibited by individual trees, gap frequency, and species diversity. However, multivariate comparisons of GTRs versus unimpounded areas indicated differences in species composition. Indicator Species Analyses and examination of the dominant species showed clear differences between GTRs and UBFs, with GTR tree species being, on average, better adapted to flooded conditions, based on wetland indicator status. Greentree reservoir canopies generally were dominated by a Taxodium distichum – Acer rubrum – Quercus lyrata mix, whereas unimpounded forest canopies were characterized as Liquidambar styraciflua – mixed Quercus stands. The midstories of the groups were more similar to one another, with dominance by Acer– Carpinus caroliniana – Planera aquatica in GTRs and Liquidambar – Carpinus – Acer in unimpounded stands. The GTRs included in this study had been managed for 40 to 48 years, providing a long history of flooding which served to select for highly adapted species assemblages.