Date of Award

8-2005

Degree Type

Dissertation

Degree Name

Doctor of Philosophy

Major

Natural Resources

Major Professor

Sammy L. King, David Buehler

Committee Members

Carol Harden, Jake Weltzin, Jennifer Franklin, Ray Albright

Abstract

Hydrologic and sedimentation processes are critical in determining floodplain site conditions and the distribution of bottomland hardwood (BLH) forest communities (Hodges 1997). Channelization of streams associated with BLH wetlands has occurred extensively throughout the southeastern United States, altering the hydrologic and sedimentation processes that sustain these systems. In western Tennessee, channelization and past land-use practices have resulted in drastic geomorphic and hydrologic changes including excessive sand deposition on floodplains, and in extreme cases, the formation of valley plugs and shoals.

Our understanding of the processes associated with valley plugs and shoals, their rates and variability, and their impacts on BLH forest succession is limited but required for conservation and restoration efforts to be successful. The objectives of this study were: (1) to quantify the deposition rates and determine the temporal and spatial patterns of overbank sedimentation associated with different geomorphic features including valley plugs, shoals, and unchannelized systems (natural meandering channels), (2) to determine differences in surface and sub-surface hydrology associated with the three geomorphic features, (3) to experimentally determine the effects of hydroperiod and deposition rate and texture on germination and early growth of three BLH tree species, and (4) to quantify differences in floodplain forest communities as a result of altered hydrologic and sedimentation processes associated with the three geomorphic features.

This study was conducted in the Hatchie River watershed, located in western Tennessee, from 2001 to 2005. The Hatchie River is the longest unchannelized stretch of river in the Lower Mississippi Alluvial Valley; however, extensive channelization of the Hatchie River tributaries has occurred. Channelization, the geology of the region, and past land-use practices have resulted in the formation of valley plugs and shoals within many of the altered tributaries (Diehl 2000).

Field studies conducted at three unchannelized sites, two shoal sites, and four valley plug sites indicated that overbank sedimentation was dramatically influenced by geomorphic features. At valley plug sites, deposition rates x ̅ [sample mean] = 5.46 ± 0.44 cm/yr) were 10 times greater than at unchannelized (x ̅ [sample mean] = 0.46 ± 0.05 cm/yr) and shoal sites (x ̅ [sample mean] = 0.57 ± 0.24 cm/yr). At valley plug sites, sediment deposition contained significantly more coarse sands than at shoal and unchannelized sites and a larger extent of the floodplain was affected by high deposition rates. Sedimentation rates at both valley plug and shoal sites were variable because of other factors such as channel recovery processes and anthropogenic disturbances. Dendrogeomorphic analysis indicated that there had been a significant increase in deposition rates at valley plug sites since 1970, corresponding to the time period of channelization of most western Tennessee streams (Hupp and Bazemore 1993).

Field studies conducted at three unchannelized sites, two shoal sites, and three valley plug sites also indicated that both surface and sub-surface hydrology were affected by channelization and subsequent formation of valley plugs and shoals. Contrary to previous research, surface flooding at valley plug sites was less than at unchannelized and shoal sites. This result demonstrated the variability in hydrologic responses to valley plug formation. Water tables were also lower at valley plug and shoal sites, possibly as a result of channel bed lowering during channelization. However, even though water tables were lower at valley plug sites, root systems of trees at these sites were inundated for extended periods of time (x ̅ [sample mean] = 32.75 ± 12.76 days) during the growing season.

A greenhouse experiment was conducted to determine the effects of hydroperiod and sedimentation rates and textures on germination and growth of red maple (Acer rubrum), swamp chestnut oak (Quercus michauxii), and overcup oak (Q. lyrata). Poor germination of red maple prevented reliable testing of its germination or growth response to the treatments. Germination of swamp chestnut oak and overcup oak were most affected by hydroperiod, with sediment rate and texture being a secondary factor. The most important effect of the 8 cm deep sediment was reduced overall height, which may reduce a seedling’s ability to compete for resources.

Floodplain forest species composition and structure and the environmental factors important in structuring plant communities were investigated at three unchannelized sites, two shoal sites, and three valley plug sites. Hydrologic and sedimentation conditions associated with channelized streams and valley plug formation were the main processes influencing site conditions, including soil characteristics, resulting in extensive changes to floodplain forest communities. At valley plug sites, typical BLH forest associations of oak species (Quercus spp.) and baldcypress/water tupelo (Taxodium distichum/Nyssa aquatica) are being replaced by several disturbance tolerant species including boxelder (Acer negundo), black willow (Salix nigra), and red maple (Acer rubrum). This study, however, has demonstrated that there is considerable temporal spatial variability in hydrologic and sedimentation processes associated with valley plugs. Thus, there is an interrelated temporal and spatial variability in forest response. The lack of predictability of abiotic processes associated with valley plugs makes the future composition of these forests uncertain.

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