Greentree reservoirs (GTRs) are used in bottomland hardwood wetlands to provide habitat for wintering waterfowl. First developed in the 1930s, GTRs now exist in more than 20 States. Reservoirs are typically developed by impounding a stand of bottomland hardwoods with levee systems and water control structures. The impoundment is artificially or naturally flooded during fall and winter to provide mast (nuts and acorns), vegetation, and invertebrates for wintering waterfowl. Recent studies, however, suggest that GTRs can cause significant impacts to wetlands by altering plant-species composition.

Felsenthal National Wildlife Refuge (NWR), established in 1970, is located in south-central Arkansas. The 65,000-acre refuge is home to thousands of migrant and resident waterfowl, marsh and water birds, Neotropical migrants, and resident wildlife and has the highest density of endangered red-cockaded woodpeckers in the State. It includes the world's largest greentree reservoir. Between 1985 and 1987, the U.S. Army Corps of Engineers established permanent study plots and began gathering data on the survival and growth of trees impacted by GTR management.

The National Wetlands Research Center (NWRC) acquired the datasets from the Corps of Engineers and has maintained the study since 1990. The NWRC and Felsenthal NWR are using the results of this GTR study to develop water management plans that minimize impacts to the wetland community and thereby provide high quality waterfowl habitat for the long term. Tree species studied in the refuge include the following:

The study plots were re-measured in 1990, 1995, and 2001. Follow-up measurements took place during the summer and fall of 2006. Datasets include soil sampling, hydrology, elevation, basal area, density, frequency, species, tree height, crown size, and hemispherical photographs. This study covers several areas of biological interest; with nearly 20 years of data, it is especially relevant to land managers.

INTRODUCTION

In the fall of 1985, a lock and dam was completed at the southern end of Felsenthal NWR, creating a permanent navigation pool with a minimum elevation of 65 feet mean sea level (MSL). The dam also provided the capability of increasing the pool elevation to 70 feet MSL to create a 21,000 acre GTR.

Water within the GTR is raised from 65 feet MSL on 1 November to 70 feet MSL by 1 January, maintained at 70 feet MSL for one to two weeks and then lowered back to 65 feet MSL by 1 March. Natural stream flow on the Ouachita River, however, routinely floods the area to elevations in excess of 75 feet MSL, especially during the winter and spring. Such flooding often causes the drawdown of the GTR to be delayed well into the spring or even summer.

The primary forest type in the GTR is overcup oak-water hickory, followed by somewhat less frequently flooded types in which nuttall oak, willow oak, and/or sweetgum predominate. Additional species include persimmon, hawthorns, deciduous holly, swamp privet, water oak and an occasional baldcypress.

METHODS

Prior to the completion of the lock and dam on the Ouachita River, the U.S. Army Corps of Engineers initiated a long-term vegetation monitoring study in 1985. In total, 80 study plots were established between 1985 and 1987 for measurement of overstory and midstory species. Each main plot contained a sapling and/or shrub subplot and five subplots for ground cover vegetation. Plot locations were situated to reflect the range of forest types, stand ages, and elevations within the GTR. Elevation categories included low (L) 65'-68', medium (M) 68'-70', and high (H) > 70'. All of the plots were remeasured in 1987.

In 1990, personnel at the National Wetlands Research Center, (now a part of the U.S. Geological Survey) assumed responsibility for the study. Fifty-four of the plots were selected for further study and were remeasured in 1990, 1995, and 2001. Another remeasurement is scheduled for 2006 contingent on availability of funds.

RESULTS AND DISCUSSION

Significant changes in forest composition have been observed at all elevations since completion of the GTR. Willow oaks and nuttall oaks, especially the smaller diameter stems at the lower elevations, are being lost with little new recruitment. Although willow oak seedlings make up a large component of the ground layer, these seedlings are not surviving to grow into the sapling or tree layer. This constitutes the loss of the most important mast producing tree species within the GTR. The forest community is shifting to more water tolerant species such as overcup oak and water hickory, which have little value for waterfowl.

High stem densities in many areas of the refuge are also contributing to mortality among the smaller diameter trees. Analysis of mortality rates among plots with low versus high stem densities, however, show flooding to be the more important contributing factor in the observed loss of trees.

WATER LEVEL MANAGEMENT

To prevent continued mortality, the GTR should not be flooded during some years in order to increase seedling survival and allow the forest to recover. Lowering the water level in the GTR to 65 feet MSL in a more timely manner (by 1 March or before the trees begin growing) would also reduce the stress associated with excessive, frequent, and prolonged flooding.

Based on the results of NWRC studies, water level management practices at Felsenthal NWR have been modified. In addition, U.S. Army Corps of Engineers permits for GTRs now specify criteria based on data obtained from the Felsenthal study.

TIMBER MANAGEMENT

If GTR flooding is reduced or eliminated during some years, then small clearcuts or thinning may promote increased productivity of the leave trees (trees remaining in the stand after thinning operations) and allow establishment of new seedlings. Useful thinning (from a waterfowl habitat perspective) will only occur in areas currently overstocked with nuttall oak, willow oak, and sweetgum stands. Areas dominated by overcup oak and water hickory have always been too wet for the desirable hard-mast species to grow; therefore, thinning these areas would not allow existing willow and nuttall oaks to become more vigorous. While thinning may reduce some stress associated with stem density, prolonged flooding each year will continue to eliminate any seedlings that become established.

WHAT TO EXPECT

Because there is little recruitment of seedlings or saplings into the midstory or overstory, there is little hope of maintaining the forest as it was prior to 1985 without modifying the GTR flooding schedule. Unless flooding is curtailed during some years, the mast producing overstory trees will eventually be lost, waterfowl habitat will decrease, and waterfowl hunting opportunities will be lessened. The most aggressive trees appear to be midstory species such as swamp privet, hawthorns, and deciduous holly.