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Restoring Longleaf Pine Sandhill Communities with an Herbicide 1
Leslie Hay-Smith and George W. Tanner2Background
Ancient ecosystems bring to us a sense of posterity and a reminder of the balance of natural forces over time. Many forests, such as longleaf pine savannas, play important roles in the natural processes for clean air and water upon which humans depend. Because our natural forests have suffered destruction over the past century, many people have an increased awareness of this dramatic loss. Consequently, natural-resource conservationists are trying to become better stewards to protect remaining fragments of these unique forests. In many cases, however, preservation efforts are no longer possible and restoration of badly degraded longleaf pine habitats, or sandhills, is necessary. Numerous restoration techniques are now being tested for use in these ecosystems.Lost Dominance of the Longleaf Pine
The longleaf pine ecosystem once occupied a major part of the southeastern United States, including peninsular Florida. It has now been reduced to about 10 percent of its original coverage. This unique forest ecosystem is known for its dominant stands of longleaf pine ( Pinus palustris ) and pineland threeawn (Arisida stricta) , with scattered turkey oak (Quercus laevis) and bluejack oak (Quercus incana) . These forests are adapted to fire, a fact demonstrated by the thick, fire-insulating bark of pine as pineland threeawn, are stimulated by fire in the growing season to produce viable seed. In natural longleaf pine forests, intervals between burns were 3-4 years. When fire is suppressed, longleaf pine sandhills will succeed to mesic hardwood forests. Although the specific types of hardwood forests that invade are determined by the seed supply, all of these forests are characterized by higher shading, greater litter accumulation and less herbaceous ground cover than are present in longleaf pine forests.Threats to Vertebrate Species
Many vertebrate species are dependent on the longleaf pine community that dominates Florida's xeric sandhills. These include the federally endangered red-cockaded woodpecker (Picoides borealis) and the state-endangered Florida mouse (Podomys floridanus) . Other animals in danger are the gopher tortoise (Gopherus polyphemus) and Sherman's fox squirrel (Sciurus niger shermanii) , both of which are Florida-listed "species of special concern." These animals are important to the longleaf pine ecosystem because they serve important ecological functions such as seed dispersal and nest cavity construction. The gopher tortoise is considered a "keystone species" because its burrows provide refuge to many other species of vertebrates and invertebrates which, in turn, serve other important roles in the sandhills.The major reason the animals of the longleaf sandhill ecosystem are in peril is that their habitat over much of the state has been destroyed or degraded through mismanagement. One method forest managers are using to restore or rehabilitate the longleaf pine ecosystem is reintroduction of summer fires to the sandhills of Florida. However, fire is a successful restoration tool only when sufficient ground cover of grass and pine needles is present as a fuel source. In stands that have been excluded from fire for 15-30 years, lack of ground fuel greatly restricts fire's intensity and its ability to spread. Consequently, new methods for restoration must be introduced.
Forest herbicides such as hexazinone have been used to suppress the growth of midstory oaks while encouraging the understory growth of pineland threeawn and longleaf seedlings. Before large-scale use of hexazinone occurs, however, a thorough evaluation of this herbicide's full impact on longleaf recovery, oak suppression and overall ecosystem functioning should be documented. Careful study of hexazinone's effects will provide a better understanding of how to restore and manage these natural forest systems in the future.
Chemical Effects of Hexazinone
Hexazinone is a selective herbicide registered for use in pine production for site preparation, release and herbaceous weed control. The chemical is absorbed from soil solution by plant roots and transported to the site of action in the chloroplasts of the plant. The compound in hexazinone binds to a specific protein and inhibits other important reactions, causing the affected plant to die from oxidative stress. Hexazinone is soluble in water and may be mobile in subsurface solution. It appears to persist in forest soils for relatively short periods. However, the herbicide has been tested only superficially for possible toxic effects on other plant and animal species and for movement off the site to nearby aquatic ecosystems.Chemical activity in soils is greatly dependent on soil pH and percentage of organic matter. Because of these factors, hexazinone is more effective on soils with relatively high sand content, low pH values and low organic matter levels conditions typical of soils in the southeastern United States. Plants differ in their resistance to hexazinone. Some pine species, such as southern yellow pine and pineland threeawn, are more resistant, while oaks are more susceptible to the chemical. Given these factors, hexazinone is a good candidate for assisting in the restoration of sandhill plant communities.
A Case Study
A degraded longleaf pine sandhill community within Wekiva Springs State Park in Orange County, Florida, was the location of an experimental application of hexazinone. Aerial photographs from the 1950s showed that the area had once been characterized by an open stand of longleaf pine and scattered turkey oak. Park records indicated the site had not been burned for 40 years, because fire control and exclusion was park policy until the 1980s. As a result of these management practices, a sparse overstory of longleaf pines was scattered over the site. Herbaceous ground cover was spotty and the growth of isolated wire grass clumps was inhibited by the accumulation of turkey oak leaf litter.In this study, three different rates of liquid hexazinone (0.42, 0.84 and 1.68 kg/ha) were applied in 1991 to estimate hexazinone effects on plant community restoration. The chemical was applied with a backpack-mounted Solo spot gun in a 1m-x-1m-square grid pattern on 12 plots in Wekiva Springs State Park. Plants within permanently marked plots were measured before and one year after hexazinone application. Trees were marked with paint and their diameters were measured. The basal area of pineland threeawn was calculated by measuring right-angle diameters of clumps at the ground. The relationship between basal area and aboveground biomass (in dry weight) was determined by clipping and weighing the vegetative growth of a portion of the plants measured for basal diameter.
Oak Mortality from Application
Increasing the rate of hexazinone resulted in increased mortality of oaks ( Table 1 ).Susceptibility to the herbicide, however, also was related to tree diameter. There was a trend toward increased mortality with decreasing diameter of oaks. The largest oaks (14-16cm in diameter) survived even the highest application rate, 1.68 kg/ha. Survival of these large oaks is desirable from a management perspective, since some oaks with larger diameters are a natural part of longleaf pine sandhill communities and are known to provide both valuable food and nesting sites for many species of wildlife.
Understory Response
The response of the understory to hexazinone treatment was favorable, which again demonstrates the potential applicability of this chemical in longleaf pine sandhill restoration. Oak seedlings died and grasses became more dominant, following increasing rates of hexazinone application. Although there was a notable development of saw-palmetto on the plots treated with hexazinone, the plant was not present on the untreated plots; therefore, no comparisons could be made. Potential stimulation of saw-palmetto through hexazinone treatment could raise concerns among forest managers. Because saw-palmetto is a natural component of scrub and xeric hardwood forests, its apparent tolerance to hexazinone will not affect sandhill restoration.Longleaf Pine Response
The response of longleaf pine to hexazinone was a chief point of interest for this study. Longleaf pine seedlings dramatically increased on all plots, including those with the lowest treatment rate. On plots with the highest treatment rate, low panicums (Dicanthelium spp), foxtail grasses (Setaria spp.) and Paspalum spp. all increased or invaded the site following hexazinone treatment. An expansion in early successional stage grasses suggests that the resources on the site (e.g., minerals and nutrients) were released for use by the understory plants. Vegetative cover of forbs also spread on all the treatment plots, mainly because of an increase in several legume species. Results of this case study illustrate that herbaceous species were not eradicated after hexazinone treatment and that non-oak woody species persisted.Pineland Threeawn Response
Pineland threeawn was a focal point for this study because of its importance to the functioning of the sandhill ecosystem's natural fire regimen. Field measurements one year following herbicide treatment showed pineland threeawn foliar cover had expanded in proportion to increasing application rate, experiencing up to a fourfold increase at the 1.68 kg/ha rate ( Table 2 ).Responses in biomass were not as consistent, however, and actually indicated a loss of biomass at the lowest and highest rates. This was probably the result of direct contact between the plants and the herbicide, which could have caused top-kill of the vegetation.
Management Recommendations
The dramatic decline of longleaf pine sandhills in the southeastern coastal plain creates a sense of urgency about conserving the remaining forests. It also brings forward the importance of learning to utilize longleaf pine ecosystems in a manner that is sustainable far into the future. The decline of these once widely scattered forests also highlights the need to restore badly degraded sandhills. Because of the small extent and fragmented nature of these forests, restoration will most likely become increasingly important for the conservation of this upland ecosystem. This is particularly true as succession to hardwood forests continues in isolated fragments, where the use of frequent fires as a restoration tool has been limited owing to high population density and urbanization. Although hexazinone can be an important precursor to prescribed fires, it cannot replace them: fire is necessary for longleaf pine and pineland threeawn regeneration. The utility of hexazinone for longleaf pine sandhill restoration has been shown in the case study described above. The herbicide not only facilitated the release of longleaf pine seedlings and pineland threeawn, but also aided in the reduction of scrub oak populations without damaging other woody and herbaceous vegetation. Our results agree with those of other researchers, indicating that woody plant communities are relatively unaltered following hexazinone application at similar rates.Need for Further Study
Additional experiments must be conducted on application rates, scheduling and patterns of hexazinone application in order to refine this restoration technique. Until additional trials are completed, we suggest that forest managers use hexazinone rates ranging from 0.84 to 1.68kg/ha. To reduce top-kill of pineland threeawn, the herbicide should not be sprayed directly on live plants. Broadcast applications of granular hexazinone should be avoided because they can damage the herbaceous plant layer.Summary
In summary, the use of hexazinone rates between 0.84 and 1.68 kg/ha produced the desired result: enhanced longleaf pine sandhill restoration. We conclude that when applied discretely at the above rates, hexazinone can serve as an important vehicle for sandhill restoration. The appropriate use of this herbicide can increase longleaf pine seedling and pineland threeawn growth. At the same time, it can reduce scrub-oak seedling and mature plant populations without damaging important herbaceous plants.
Tables
Table 1.
Table 1. Mean oak (Quercus spp.) Mortality one year following hexazinone applications. Rate (kg/ha)
Diameter (cm)
Mortality (%)
0.42 2 to 6 19 6 to 10 26 10 to 14 10 14 to 18 none 0.84 2 to 6 32 6 to 10 61 10 to 14 10 14 to 18 none 1.68 2 to 6 52 6 to 10 54 10 to 14 48 14 to 18 none
Table 2. Proportional (%) change in biomass and foliar cover of pineland threeawn (Aristida stricta) one year following hexazinone treatment. HexazinoneRate (kg/ha)
Biomass(% change)
Folio Cover(% change)
Control -23 -26 0.42 -42 +121 0.84 +49 +122 1.68 -15 +403 key: + =increase - =decrease
Footnotes
1. This document is WEC131, one of a series of the Wildlife Ecology and Conservation Department, Florida Cooperative Extension Service, Institute of Food and Agricultural Sciences, University of Florida. Original publication date May, 1994. Revised September, 2002. Reviewed September, 2002. Visit the EDIS Web Site at http://edis.ifas.ufl.edu. The use of trade names in this publication is solely for the purpose of providing specific information. It is not a guarantee or warranty of the products named, and does not signify that they are approved to the exclusion of others of suitable composition. All chemicals should be used in accordance with directions on the manufacturer's label.2. Leslie Hay-Smith, former graduate research assistant, and George W. Tanner, Ph.D. professor, Department of Wildlife and Range Sciences, Cooperative Extension Service, Institute of Food and Agricultural Sciences, University of Florida, Gainesville FL 32611.
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