Scientific Investigations Report 2008-5062

2008

Prepared in cooperation with
Florida Department of Environmental Protection and the
Northwest Florida Water Management District


Drier Forest Composition Associated with Hydrologic Change in the Apalachicola River Floodplain, Florida

Melanie R. Darst and Helen M. Light



COVER OF REPORT

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CONTENTS
Abstract
Introduction
      Purpose and Scope
      Acknowledgments
      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
Methods
      Forest Sampling
            Thesis Study, 1976-1977
            Apalachicola River Quality Assessment (ARQA) Study, 1979
                  Cruise Transects
                  Intensive Plots
            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
      Hydrologic Change
            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
                  Growth Rates
                  Incremental Tree-Size Groups
                  Mortality and Recruitment Rates
            Floodplain Indices
                  Replicate Plots
                  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
Summary
Selected References
Glossary
Appendixes

Abstract

      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.


Suggested Citation:

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.

For additional information, contact:

U.S. Geological Survey
Florida Integrated Science Center
Suite 100
2010 Levy Avenue
Tallahassee, FL 32310
850-942-9500

mdarst@usgs.gov



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