Throughout the southeastern United States, upland sites that were once dominated by longleaf pine (Pinus palustris Mill.) have been widely converted to faster growing species such as loblolly pine (P. taeda L.). Consequently, existing populations of the federally endangered red-cockaded woodpecker (RCW; Picoides borealis) are currently occupying mature loblolly pine stands. Reports of declining loblolly pine health in some locations raised concerns about the longevity of existing RCW habitat and underscored the need to convert upland forests back to longleaf pine. Forest managers needed protocols to restore longleaf pine on sites where canopy pines are retained. Further, because protocol suitability is likely to vary among site types based on productivity and the structure and composition of the canopy and ground layer vegetation, protocol development on a range of site conditions was necessary. The need for such protocols was deemed critical at Fort Benning, Georgia, where as many as 70% of the active RCW cavities were found in loblolly pine trees.
Because longleaf pine seedling growth tends to decrease as canopy cover increases, the conversion of loblolly pine stands to longleaf was expected to require a balance between canopy removal to increase the growth of planted longleaf pine seedlings and canopy retention for RCW habitat and other ecosystem services (e.g., fuel inputs from needlefall). Retaining the trees likely to live the longest would secure the most RCW habitat value through time; thus, a model for predicting tree longevity would be a valuable management tool. Additionally, an increased understanding of environmental factors associated with reduced loblolly pine health was needed to inform RCW management decisions at Fort Benning.
The objectives of this project were to: (1) develop stand level silvicultural protocols for restoring longleaf pine forests while retaining a canopy component on Fort Benning in the sandhills of Georgia and at Marine Corps Base Camp Lejeune on the coast of North Carolina; (2) model stand vulnerability to declining loblolly pine vigor on Fort Benning where loblolly pine health was a management concern; and (3) develop a model to forecast individual loblolly pine tree mortality in stands showing reduced canopy health at Fort Benning.
The research was conducted at Fort Benning and Camp Lejeune, sites that differ in ecological characteristics including topography, soils, weather patterns, and native flora. Such differences may influence the success of restoration protocols, and thereby inform the development of management guidance across a range of site conditions. The project team installed a field experiment using a randomized split plot design replicated on eight blocks at each location. Longleaf pine seedlings were planted in loblolly stands treated with seven canopy treatments (main plot treatments), including three different sized gaps, uniform thinning to three different residual basal areas (0, 4.5, and 9 m2/ha), and an uncut check (basal area ≥ 14 m2/ha). In each plot, additional split-plot treatments included chemical vegetation release, chemical release plus fertilizer, establishment of native grasses, and an untreated check. Response variables included the survival and growth of seedlings and planted native grasses, light, soil nitrogen, soil moisture, foliar nutrients, and seedling water potential. Results of the field experiment were used to project the development of suitable RCW habitat under experimental scenarios. The project team developed tree mortality and stand vulnerability models applicable to loblolly forests at Fort Benning. The models were based on existing inventory data and on data collected from a stratified sample of all upland loblolly stands at Fort Benning. In 90 plots in mature loblolly stands, site and stand characteristics were measured and tree health was evaluated by assessing crown condition. Standard ANOVA, regression, and non-parametric methods were used for most analyses, and dendrochronology methods plus logistic regression and time series analysis were used to forecast tree mortality.
Overall, this project found a low occurrence of trees in poor health (2.9%). Tree health was positively correlated with site productivity, whereas poor tree health was more common on coarse textured soils than on fine textured soils. Individual tree mortality could be predicted from the mean basal area increment from the past three years and the basal area growth relative to tree size using dendrochronological methods. Canopy removal increased light availability for longleaf pine seedlings and ground layer vegetation, and both responded with increased growth. Ground layer vegetation provided important competition for longleaf pine seedlings, especially at Camp Lejeune, where woody vegetation dominated the ground layer. At both installations, the continuity of prescribed fires applied after longleaf restoration treatments was reduced with canopy removal (reduced needle inputs from canopy pines), suggesting that attention must be given to reaching prescribed fire objectives in areas with canopy gaps or reduced canopy density. Determining appropriate silvicultural treatments for restoring longleaf pine to loblolly stands requires an understanding of the initial stand conditions relative to RCW habitat requirements and ground layer species and composition. To increase longleaf seedling growth, a reduction of canopy basal area to below 7 m2/ha (if allowable) is recommended. In stands where canopy reduction is restricted, small canopy gaps (0.1 ha) can be used to create patches of suitable conditions for longleaf seedlings. It was found that fertilizer had few benefits for longleaf pine restoration, but chemical control that targets woody vegetation will increase longleaf seedling growth and enhance the ground layer community in stands with encroachment of woody species.
Results of this project can be used to guide longleaf pine restoration in loblolly pine stands at Fort Benning and Camp Lejeune, and experimental plots remain to demonstrate the effects of management alternatives for longleaf restoration and the quality and production of RCW habitat. The results can be used in planning and landscape models to predict consequences of stand management choices. The combined results at two locations provide a framework for restoring longleaf pine on other Department of Defense installations, though details may differ. Applying study results will improve the likelihood of southeastern installations fulfilling endangered species management and recovery goals, thereby minimizing potential conflicts with the training mission. Contributions to science include a better understanding of the gap regeneration processes and of individual tree mortality in the southern pine ecosystems.