Scientific Investigations Report 2008-5062
Prepared in cooperation with
Florida Department of Environmental Protection and the
Northwest Florida Water Management District
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Purpose and Scope
Setting and Background
Floodplain Study Area and Forest Types
Water-level Decline in the Apalachicola River
Influence of Flooding on Tree Seedling Regeneration in Floodplain Forests
Thesis Study, 1976-1977
Apalachicola River Quality Assessment (ARQA) Study, 1979
Eichholz Study, 1978
Gholson Study, 1984
Current Study, 2004-2006
Analyses of Forest Data
Forest Type Determinations using Floodplain Species Categories
Basal Area and Density
Species Composition of Forest Types
Abundance of Tree Species throughout the Nontidal Floodplain
Forest Types and Floodplain Species Categories
Growth, Age, Mortality, and Recruitment from Thesis Data
Comparisons of Forest Type Composition using Floodplain Indices
Changes in Floodplain Indices at Replicate Plots
Size-Class Comparisons as an Indicator of Past and Future Forest Composition
Size-Class Comparisons on Other North Florida Stream Floodplains
Analyses of Hydrologic Data
History of Inundation at Forest Plots
Hydrologic Time Periods Associated with Forest Sampling Groups
Flood Duration, Depth, and Frequency by Forest Type and Reach
Changes in Hydrology and Forest Composition
River Flow and Stage
Hydrologic Conditions in Floodplain Forests
Forest Composition Change
Species Composition of Forest Types
Tree Species Abundance throughout the Nontidal Floodplain
Distribution of Species
Basal Area and Density of Trees by Forest Type and Floodplain Species Category
Growth Rates, Tree sizes, Mortality, and Recruitment
Incremental Tree-Size Groups
Mortality and Recruitment Rates
1976 Size Classes
2004 Size Classes
Size Classes on Other North Florida Streams
Drier Forests Associated with Decline in River Levels
Shorter Flood Durations
Drier Forest Composition
Ecological Effects of Altered Floodplain Forests
Forests of the Apalachicola River floodplain had shorter flood durations, were drier in composition, and had 17 percent fewer trees in 2004 than in 1976. The change to drier forest composition is expected to continue for at least 80ámore years. Floodplain drying was caused by large declines in river levels resulting from erosion of the river channel after 1954 and from decreased flows in spring and summerámonths since the 1970s. Water-level declines have been greatest at low andámedium flows, which are theámost common flows (occurring about 80 percent of the time). Water levels have remained relatively unchanged during large floods which continue to occur about three times per decade.
Aástudy conducted by the U.S. Geological Survey compared temporal changes in hydrologic conditions, forest composition, forest characteristics, and individual species of trees, as well as estimated the potential for change in composition of floodplain forests in the nontidal reach of the Apalachicola River. Theástudy was conducted with the cooperation of the Florida Department of Environmental Protection and the Northwest Florida Water Management District. Forest composition and field observations from studies conducted in 1976-1984 (termed ô1976 dataö) were used as baseline data for comparison with data from plots sampled in 2004-2006 (ô2004 dataö).
Flood durations were shorter in all periods subsequent to 1923-1976. Theáperiods of record used to calculate flood durations for forest data were subsets of the complete record available (1923-2004). At sampled plots in all forest types and reaches combined, flood durations changed an average ofámore than 70 percent toward the baseline flood duration of the next drier forest type. For all forest types, changes in flood durations toward the next drier type were greatest in the upper reach (95.9 percent) and least in the lower reach (42.0 percent).
All forests are expected to be 38.2 percent drier in species composition by 2085, the year when theámedian age of surviving 2004 subcanopy trees will reach theámedian age (99 years) of the 2004 large canopy trees. Theáchange will be greatest for forests in the upper reach (45.0 percent). Forest composition changes from pre-1954 to 2085 were calculated using Floodplain Indices from 1976 and 2004 tree-size classes and replicate plots.
Species composition in high bottomland hardwood forests is expected to continue to change, and some low bottomland hardwood forests are expected to become high bottomland hardwood forests. Organisms associated with floodplain forests will be affected by the changes in tree species, which will alter the timing of leaf-out, fruiting, and leaf-drop, the types of fruit and debris produced, and soil chemistry. Swamps will containámore bottomland hardwood species, but will also have an overall loss of tree density.
Theádensity of trees in swamps significantly decreased by 37 percent from 1976 to 2004. Of the estimated 4.3ámillion (17 percent) fewer trees that existed in the nontidal floodplain in 2004 than in 1976, 3.3ámillion trees belonged to four swamp species: popash, Ogeechee tupelo, water tupelo, and bald cypress. Water tupelo, the most important tree in the nontidal floodplain in terms of basal area and density, has declined in number of trees by nearly 20 percent since 1976. Ogeechee tupelo, the species valuable to the tupelo honey industry, has declined in number of trees by at least 44 percent.
Greater hydrologic variability in recent yearsámay be the reason swamps have had a large decrease in tree density. Drier conditions are detrimental for the growth of swamp species, and periodic large floods kill invading bottomland hardwood trees. Theáloss of canopy density in swampsámay result in the swamp floor being exposed toámore light with an increase in the amount of ground cover present, which in turn, would reduce tree replacement. Theámicroclimate of the swamp floor would become warmer due to the decrease in shade and inundation. Soils would become dehydratedámore quickly in dry periods and debris would decomposeámore quickly. Aáloss of tree density in swamps would lead to a decrease in tree and leaf litter biomass, which would have additional effects on swamp organisms. Theáloss of litter would result in a loss of substrate for benthic organisms in the floodplain and, ultimately, in the downstream waters of the river and estuary.
Darst, M.R., Light, H.M., 2008, Drier Forest Composition Associated with Hydrologic Change in the Apalachicola River Floodplain, Florida: U.S. Geological Survey Scientific Investigations Report 2008-5062, 81 p., plus 12 apps.
U.S. Geological Survey
Florida Integrated Science Center
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Tallahassee, FL 32310
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