Longleaf Pine Plantations Research Articles

Tradeoffs between timber and wildlife habitat quality increase with density in longleaf pine (Pinus palustris) plantations

Longleaf pine (Pinus palustris), a historically abundant tree species in the southeastern United States, is often planted to restore the ecologically and culturally important longleaf pine ecosystem that once covered vast acreages in the southeastern United States. Government cost-share programs that support establishment of these plantations place restrictions on planting rates to promote wildlife habitat, as greater tree planting density may reduce canopy openness and herbaceous plant cover that are critical components of habitat for priority species, including gopher tortoise (Gopherus polyphemus) and Bachman’s sparrow (Peucaea aestivalis). However, there is expressed concern among some forest managers that more open grown trees in the plantations will be of inferior timber quality with more and larger horizontal branches and associated knots. We examined how density affects dynamics and tradeoffs among understory vegetation structure and composition, longleaf pine stem form (branch density and straightness), and longleaf pine survival by sampling 73 plantations of various ages (5–25 years) and planting rates (653–2445 trees per hectare (TPH)/264–990 trees per acre (TPA)) throughout the southeastern United States. We documented a relationship between planting rate and longleaf pine stand density at time of sampling (r = 0.69, p = 0.0001) and relationships between stand density and habitat and timber quality metrics. Greater stand density resulted in lower tree diameters but greater stand basal area than lower stand density. Higher planting rates led to lower branch density and lower straightness grades than lower planting rates. Canopy openness decreased with greater stand density, and bare ground cover and herbaceous cover decreased as density and stand age at time of sampling increased. Based on our results, we suggest that lower maximum planting rates are appropriate when wildlife habitat is a program objective because lower rates result in fewer tradeoffs, as reducing planting rates slows degradation of wildlife habitat when compared with higher maximum planting rates that have only mixed benefits on timber quality.

Experimental throughfall reduction has little effect on shoot and needle developmental patterns or leaf area dynamics in a young longleaf pine (Pinus palustris Mill.) plantation

Reduced precipitation and, consequently, low soil moisture are known to limit tree growth and function by affecting shoot and foliage development, as well as canopy-scale leaf area dynamics and litterfall. Longleaf pine (Pinus palustris Mill.) is considered one of the most drought-resistant forest species in the southeastern U.S. and could serve as a pathway to increase drought resistance of forests in the region. Still, reduced precipitation and low soil moisture impacts on longleaf canopy phenology and growth are not well understood. Over three years (2017–2019), we determined the effects of a 40% throughfall reduction (TR40) (relative to ambient throughfall treatment, TR0) on shoot and foliage phenology and growth in a young (12–14 yr. old) longleaf pine plantation. Each year, we repeatedly measured shoot and needle lengths on primary and secondary branch axes of multiple trees in each treatment plot. We fit growth curves for each tree and branch axis to estimate shoot and needle growth rate, growth start and cessation dates, growth duration (growth start - growth cessation), and final shoot and needle lengths. At the plot level, we documented temporal patterns of leaf area index (LAI) and litterfall to determine whether branch-scale phenological and growth responses to reduced water availability corresponded with temporal changes in LAI and litterfall. We observed significant and consistent differences in shoot and needle elongation patterns between primary and secondary branches. Timing of needle development varied among years and was generally later each successive year. However, shoot elongation patterns were relatively consistent across years. Although soil moisture was lower under throughfall reduction, shoot and needle growth patterns were not affected. LAI and litterfall patterns were also not affected by throughfall reduction. Our results indicate that reductions in rainfall amount (without changes in rainfall frequency or timing) may have little impact on shoot and needle phenology, canopy development, and litter production in established longleaf plantations.

Re-Assembly of the Longleaf Pine Ecosystem: Effects of Groundcover Seeding on Understory Community, Fire Behavior and Soil Properties

Planting native groundcover is often recommended to restore the understory of longleaf pine stands in the southeastern United States, but the effectiveness of such restoration activities remains poorly evaluated. We conducted a study in 25-year-old longleaf pine plantation stands in Georgia, USA, to examine the effects of seeding native groundcover on understory characteristics, fire behavior and soil properties. In 2015, four stands were seeded with five warm-season C4 grasses and a legume and four served as controls. In Fall 2020, we sampled the understory and analyzed soils collected from these stands, and in Spring 2021, fire behavior was evaluated. A total of 120 species were recorded in the understory across the stands, with the seeded species average foliar cover of 15%. There were no significant differences in species richness and Shannon diversity index of the seeded and control stands but understory species composition changed significantly. Soil properties and fire behavior during the prescribed fire also did not differ significantly between treatments, however, mean flame residence time was higher in seeded stands (108 s). Agricultural legacies of elevated soil P and old-field indicator species were prominent across stands. Overall, seeding had a minor effect on longleaf pine ecological characteristics in five years.

Factors affecting the spatial density of longleaf pine plantations under the Conservation Reserve Program in Georgia, United States

Several initiatives are promoting longleaf pine (Pinus palustris) restoration in the Southern United States for conserving biodiversity and mitigating climate change. Under the Conservation Reserve Program (CRP), landowners plant longleaf pine on farmlands and pasturelands in exchange for annual payments and other financial incentives. While the introduction of this program was an important step towards longleaf pine restoration, there exists a necessity to understand how the decisions of landowners to enroll in CRP are manifested over space. An understanding about the role of a range of factors on the spatial density of longleaf pine plantations enrolled under CRP would help in prioritizing locations for longleaf pine plantations, thereby increasing the efficacy of conservation-related resources. We evaluate the effects of socioeconomic, topographic, and distance variables on the spatial density of longleaf plantations enrolled under CRP using clustered point process model in Georgia, a state with 47% of the total acreage under CRP-supported longleaf plantations. Only 30% of the current longleaf pine locations under CRP were within the Significant Geographic Area. About 86% of current longleaf plantations under CRP are present on former croplands and pasturelands. The spatial density of longleaf pine plantations is negatively associated with the distance from cropland and pastureland and positively associated with land capability classes, and distance from the sawmills. Longleaf pine restoration efforts should focus on those croplands or pasturelands which are closer to existing CRP-supported plantations, located on lower soil capability classes, and are far from wood consuming mills. Our study would support current programs that are supporting longleaf pine restoration in the Southern United States.

Intra-Annual Variation in Soil C, N and Nutrients Pools after Prescribed Fire in a Mississippi Longleaf Pine (Pinus palustris Mill.) Plantation

Prescribed fire is an essential tool that is widely used for longleaf pine (Pinus palustris Mill.) stand management; periodic burning serves to reduce competition from woody shrubs and fire-intolerant trees and enhance herbaceous diversity. Low-intensity, prescribed burning is thought to have minimal long-term impact on soil chemistry in southern pine forests, although few studies report the intra-annual variation in soil chemistry after burning. We monitored changes in C, N, oxidation resistant C (CR), pH and elemental nutrients in the forest floor and soil (0–5, 5–10 cm depths) before and after burning (1, 3, 6, 12 months) in a mature longleaf pine plantation at the Harrison Experimental Forest, near Saucier, Mississippi. Prescribed fire consumed much of the forest floor (11.3 Mg ha−1; −69%), increased soil pH and caused a pulse of C, N and elemental nutrients to flow to the near surface soils. In the initial one to three months post-burn coinciding with the start of the growing season, retention of nutrients by soil peaked. Most of the N (93%), Ca (88%), K (96%) and Mg (101%), roughly half of the P (48%) and Mn (52%) and 25% of the C lost from the forest floor were detected in the soil and apparently not lost to volatilization. By month 12, soil C and N pools were not different at depths of 0–5 cm but declined significantly below pre-burn levels at depths of 5–10 cm, C −36% (p < 0.0001), N −26% (p = 0.003), contrary to other examples in southern pine ecosystems. In the upper 5 cm of soil, only Cu (−49%) remained significantly lower than pre-burn contents by month 12, at depths of 5–10 cm, Cu (−76%), Fe (−22%), K (−51%), Mg (−57%), Mn (−82%) and P (−52%) remain lower at month 12 than pre-burn contents. Burning did not increase soil CR content, conversely significant declines in CR occurred. It appears that recovery of soil C and N pools post-burn will require more time on this site than other southern pine forests.

Site preparation for longleaf pine restoration on hydric sites: Stand development through 15 years after planting

Longleaf pine (Pinus palustris Mill.) restoration is an important land management goal throughout the southeastern U.S. On hydric sites within the Atlantic Coastal Plain, restoration may involve site preparation prior to planting to overcome challenges to seedling establishment, such as abundant competition and poor soil drainage. Investment in site preparation assumes that treatments will result in long-term benefits to stand development, yet lasting impacts of site preparation on longleaf pine are not well understood. We sampled longleaf pine plantations in Onslow County, North Carolina through three years and at 15 years after site preparation and planting. The eight study treatments we tested include an untreated control, six combinations of two vegetation control treatments (chopping or herbicide) with three soil manipulation treatments (mounding, bedding, or flat-planting [no treatment]), and a chopping-herbicide-bedding treatment. Our findings indicate that site preparation significantly improved survival and growth of longleaf pine through 15 years. Herbicide resulted in greater growth, higher survival, and earlier grass stage emergence than chopping. Similarly, soil manipulation treatments resulted in improved stand establishment outcomes relative to flat-planting (no treatment). Effects of site preparation treatments on diameter growth were observed early and maintained through the end of the study period, while effects on survival were not observed within the first three years. Differences in stand height among treatments were more strongly driven by growth rates following grass stage emergence than timing of emergence. Our results demonstrate that site preparation improves longleaf pine stand establishment on hydric sites, although the intensity of site preparation treatments recommended for restoration depends on management objectives.

Long-term effects of thinning and woody control on longleaf pine plantation development, understory abundance, and tree damage from an ice storm

Longleaf pine (Pinus palustris) plantations have been established across the southeastern United States as the first step in restoring the declining savanna ecosystem, yet their long-term stability and development to open canopied woodlands may depend on early density and vegetation management treatments. Commercial thinning (CT) was applied in 2012 (year 18) to a long-term study of longleaf pine community restoration at the Savannah River Site near Aiken, SC to increase abundance of herb species. Although precommercial thinning (PCT) and woody vegetation control increased herb species cover during the first five years of the study, measurements from years 17 to 22 indicated a continuing decline in their cover since year 9 despite application of CT in year 18. Herb species richness, however, increased from 21 to 43 species per 100 m2 area during the last six years of the study, likely a result of disturbances associated with CT and an ice storm in year 20. Application of PCT in 1994, when the plantations were 8–11 years old, reduced their susceptibility to stem bending and breakage from the ice storm, which occurred 20 years later in 2014. Measured 5 months before the storm, pine height: dbh ratio (HD) averaged 78 and 90 for trees growing with and without PCT, respectively, and stem breakage (% of trees) from the ice storm increased linearly (r2 = 0.53) with HD. Pine mortality from the combined effects of glaze ice and wind differed with (28%) versus without (45%) PCT, indicating that long-term stability of longleaf pine plantations depends on early density management.

Economic viability of longleaf pine management in the Southeastern United States

The lack of economic information for management of longleaf pine, a forest type that once dominated the landscape in the southeastern United States, can be a major barrier to landowners to planting this species. This study compares the economic performance of even-aged longleaf pine with loblolly pine. We assume that a longleaf pine stand produces timber, water yield, wildlife habitats and pinestraw raking, while a loblolly pine stand is managed exclusively for timber production. For both species, future timber prices are uncertain and harvest decisions will be made following an optimal adaptive harvest strategy. Our findings show that investing in longleaf pine plantations is not generally an economically attractive option compared to loblolly pine for landowners. On average, the land expectation value for loblolly pine is $4610 ha−1 higher than the land expectation value of longleaf pine. Stronger markets for water yield ($0.04–$0.073 k-liter−1) can favor the competitiveness of longleaf over loblolly pine. In the absence of increased payments for water production, landowners require financial incentives between $235–$642 ha−1 over 15 years, to switch from planting loblolly to longleaf pine. When water payments are included ($0.03–$0.0 k-liter−1), incentives between $173–320 ha−1 are required to plant longleaf instead of loblolly pine.

A Comparison of Spider (Arachnida: Araneae) Diversity from Adjacent Mesic and Xeric Habitats Within the Pocosin Nature Preserve, Pike County, Alabama

Knowledge of Alabama's Araneae fauna is limited, with estimates ranging from 580 to 1000 species within the state. Until now, the most recent surveys were conducted in the 1940s and focused on 3 families. Studies elsewhere indicate that spider diversity is correlated to habitat and plant diversity. Alabama's diversity of ecoregions and flora should therefore support a highly diverse spider fauna. This study focused on spiders inhabiting a xeric upland and a mesic ravine area in Pike County, AL. The xeric area is a fire-maintained Pinus palustris (Longleaf Pine) plantation dominated by herbaceous vegetation. The ravine is relatively undisturbed and is dominated by hardwoods with little herbaceous growth. We employed a variety of techniques to collect a total of 1224 spiders from October 2015 until May 2016. Mature individuals represented 82 species in 24 families. About 16% of these species are new records for Alabama. Over half of the spiders collected were from the xeric area, and 61.0% of all specimens were male. Abundant prey and a varied understory in the xeric area appears to support a more diverse spider fauna compared to that found in the ravine.

Initial Mortality Rates and Extent of Damage to Loblolly and Longleaf Pine Plantations Affected by an Ice Storm in South Carolina

Initial Mortality Rates and Extent of Damage to Loblolly and Longleaf Pine Plantations Affected by an Ice Storm in South Carolina

Ectomycorrhizal fungal mycelia turnover in a longleaf pine forest.

Elucidation of the patterns and controls of carbon (C) flow and nitrogen (N) cycling in forests has been hindered by a poor understanding of ectomycorrhizal fungal mycelia (EFM) dynamics. In this study, EFM standing biomass (based on soil ergosterol concentrations), production (based on ergosterol accrual in ingrowth cores), and turnover rate (the quotient of annual production and average standing biomass estimates) were assessed in a 25-yr-old longleaf pine (Pinus palustris) plantation where C flow was manipulated by foliar scorching and N fertilization for 5yr before study initiation. In the controls, EFM standing biomass was 30±7gm(-2) , production was 279±63gm(-2) yr(-1) , and turnover rate was 10±3 times yr(-1) . The scorched×fertilized treatment had significantly higher EFM standing biomass (38±8gm(-2) ), significantly lower production (205±28gm(-2) yr(-1) ), and a trend of decreased turnover rate (6±1 times yr(-1) ). The EFM turnover estimates, which are among the first reported for natural systems, indicate that EFM are a dynamic component of ecosystems, and that conventional assessments have probably underestimated the role of EFM in C flow and nutrient cycling.

Modeling the effects of forest management on in situ and ex situ longleaf pine forest carbon stocks

Assessment of forest carbon storage dynamics requires a variety of techniques including simulation models. We developed a hybrid model to assess the effects of silvicultural management systems on carbon (C) budgets in longleaf pine (Pinus palustris Mill.) plantations in the southeastern U.S. To simulate in situ C pools, the model integrates a growth and yield model with species-specific allometric and biometric equations and explicitly accounts for the impacts of both thinning and prescribed fire. To estimate the ex situ C pool, the model used the outputs of merchantable products from the growth and yield model with current values of forest product conversion efficiencies and forest product decay rates. The model also accounts for C emissions due to transportation and silvicultural activities. Site productivity (site quality) was the major factor controlling stand C density followed by rotation length. Thinning reduced C sequestration, as the slow growth rate of longleaf pine reduced the potential of C sequestration in forest products. Prescribed burning reduced average C stock by about 16–19%, with the majority of the reduction in the forest floor. In a comparison of longleaf pine C dynamics with slash pine (Pinus elliottii Engelm.), both species reached a similar average C stock at age 75years, but when averaged across the whole rotation, slash pine sequestered more C. Nevertheless, for medium quality sites, C sequestration was similar between thinned 75-year rotation longleaf pine and unthinned 25-year rotation slash pine. This longleaf pine plantation C sequestration model, based on empirical and biological relationships, provides an important new tool for developing testable research hypotheses, estimating C stocks for regional assessments or C credit verification, and for guiding future longleaf pine management.

Restoring Longleaf Pine: Effects of Seasonal Prescribed Fire and Overstory Density on Vegetation Structure of a Young Longleaf Pine Plantation

Young longleaf pine (Pinus palustris Mill.) plantations provide an important starting point for restoration of longleaf pine ecosystems in the southeastern United States, with management goals often emphasizing restoration of vegetation composition and structure, as well as reestablishment of important ecological processes such as fire. We evaluated the influence of seasonal prescribed fire and overstory density on vegetation structure across overstory, midstory, and understory vegetation strata of a 23-year-old longleaf pine plantation in west-central Georgia. After three prescribed fire cycles through 8 years, we found significantly fewer broad-leaved woody stems in the midstory on both winter- and summer-burned plots compared to unburned plots, as well as fourfold greater herbaceous vegetation cover on burned plots compared to unburned plots. While we predicted that summer burning may be more important in controlling woody plants on low-density overstory plots compared to high-density plots, we found no meaningful interactions between fire seasonality and overstory density. Reductions in litter cover via prescribed fire appeared to be an important mechanism regulating herbaceous vegetation, suggesting that restoration should emphasize management of the forest floor to reduce litter cover and enhance opportunities for herbaceous plant establishment.

Impact of cogongrass management strategies on generalist predators in cogongrass-infested longleaf pine plantations.

Cogongrass (Imperata cylindrica Beav.) is an aggressive, invasive weed with a global distribution. In North America, it threatens the integrity of southeastern pine agroecosystems, including longleaf pine (Pinus palustris Mill.). While studies have examined the impacts of cogongrass and various vegetation management strategies on longleaf pine understory plant communities, little is known about how they impact associated insect communities. To understand the effect of cogongrass management strategies on arthropod natural enemies and bark beetles, a split-plot design was used to test fire (whole-plot) and four subplot treatments (control, herbicide, seeding and herbicide plus seeding). Arthropods were sampled using pitfall traps and sweep samples. After 2 years of sampling, total natural enemies were not significantly affected by subplot treatment but were affected by burn treatment. Upon subdividing natural enemies into groups, only spiders were significantly affected by subplot treatment, but predatory beetles and ants were significantly affected by burn treatment. The abundance of root-feeding bark beetles (Hylastes spp.) was not significant by subplot or whole-plot treatments. Multiple applications of herbicide remain the most effective way to manage cogongrass in longleaf pine. In this study, we found limited evidence that cogongrass management with herbicides would negatively impact arthropod natural enemies associated with longleaf pine or locally increase root-feeding bark beetles.

The Role of Habitat in the Persistence of Fire Ant Populations

The association of the exotic fire ant, Solenopsis invicta with man-modified habitats has been amply demonstrated, but the fate of such populations if ecological succession proceeds has rarely been investigated. Resurvey of a fire ant population in a longleaf pine plantation after 25 years showed that the recovery of the site from habitat disturbance was associated with a large fire ant population decline. Most of the persisting colonies were associated with the disturbance caused by vehicle tracks. In a second study, mature monogyne fire ant colonies that had been planted in experimental plots in native groundcover of the north Florida longleaf pine forest had mostly vanished six years later. These observations and experiments show that S. invicta colonies rarely persist in the native habitat of these pine forests, probably because they are not replaced when they die. A single site harbored a modest population of polygyne fire ants whose persistence was probably facilitated by reproduction through colony fission.

Modeling Survival, Yield, Volume Partitioning and Their Response to Thinning for Longleaf Pine Plantations

Longleaf pine (Pinus palustris Mill.) is an important tree species of the southeast U.S. Currently there is no comprehensive stand-level growth and yield model for the species. The model system described here estimates site index (SI) if dominant height (Hdom) and stand age are known (inversely, the model can project Hdom at any given age if SI is known). The survival (N) equation was dependent on stand age and Hdom, predicting greater mortality on stands with larger Hdom. The function that predicts stand basal area (BA) for unthinned stands was dependent on N and Hdom. For thinned stands BA was predicted with a competition index that was dependent on stand age. The function that best predicted stand stem volume (outside or inside bark) was dependent on BA and Hdom. All functions performed well for a wide range of stand ages and productivity, with coefficients of determination ranging between 0.946 (BA) and 0.998 (N). We also developed equations to estimate merchantable volume yield consisting of different combinations of threshold diameter at breast height and top diameter for longleaf pine stands. The equations presented in this study performed similarly or slightly better than other reported models to estimate future N, Hdom and BA. The system presented here provides important new tools for supporting future longleaf pine management and research.

Long‐term patterns of fruit production in five forest types of the South Carolina upper coastal plain

AbstractFleshy fruit is a key food resource for many vertebrates and may be particularly important energy source to birds during fall migration and winter. Hence, land managers should know how fruit availability varies among forest types, seasons, and years. We quantified fleshy fruit abundance monthly for 9 years (1995–2003) in 56 0.1‐ha plots in 5 forest types of South Carolina's upper Coastal Plain, USA. Forest types were mature upland hardwood and bottomland hardwood forest, mature closed‐canopy loblolly (Pinus taeda) and longleaf pine (P. palustris) plantation, and recent clearcut regeneration harvests planted with longleaf pine seedlings. Mean annual number of fruits and dry fruit pulp mass were highest in regeneration harvests (264,592 ± 37,444 fruits; 12,009 ± 2,392 g/ha), upland hardwoods (60,769 ± 7,667 fruits; 5,079 ± 529 g/ha), and bottomland hardwoods (65,614 ± 8,351 fruits; 4,621 ± 677 g/ha), and lowest in longleaf pine (44,104 ± 8,301 fruits; 4,102 ± 877 g/ha) and loblolly (39,532 ± 5,034 fruits; 3,261 ± 492 g/ha) plantations. Fruit production was initially high in regeneration harvests and declined with stand development and canopy closure (1995–2003). Fruit availability was highest June–September and lowest in April. More species of fruit‐producing plants occurred in upland hardwoods, bottomland hardwoods, and regeneration harvests than in loblolly and longleaf pine plantations. Several species produced fruit only in 1 or 2 forest types. In sum, fruit availability varied temporally and spatially because of differences in species composition among forest types and age classes, patchy distributions of fruiting plants both within and among forest types, fruiting phenology, high inter‐annual variation in fruit crop size by some dominant fruit‐producing species, and the dynamic process of disturbance‐adapted species colonization and decline, or recovery in recently harvested stands. Land managers could enhance fruit availability for wildlife by creating and maintaining diverse forest types and age classes. © 2012 The Wildlife Society.

Overstory and understory relationships in longleaf pine plantations 14 years after thinning and woody control

To develop silvicultural strategies for restoring longleaf pine ( Pinus palustris Mill.) savannas, mortality and growth of overstory pines and midstory hardwoods and abundance and species richness of herbs were studied for 14 years after pine thinning and nonpine woody control. Pine cover in thinned stands was about half of that in nonthinned stands through year 5, but it lagged by only 8% and 3% in years 9 and 14, respectively, because of vigorous crown responses. Despite a cumulative mortality of 64% of hardwood stems from prescribed fires in years 0, 4, and 9, hardwood basal area in thinned stands (2.1 m2·ha–1) was three times that in nonthinned stands (0.7 m2·ha–1) in year 14. Thinning was associated with 13%–22% more cover and six to eight more species of herbs in years 3–8 but only 6% more cover and two more species in year 14 because of accelerated growth of pine cover and hardwood basal area. However, similar increases in cover and richness of herb species in the woody control treatment were retained through year 14 because it had sustained reductions in hardwood and shrub abundance. Silvicultural strategies that substantially delay encroachment by pines, hardwoods, and shrubs will be those most effective at retaining herb species in longleaf pine savannas, including planting pines at wide spacing, periodic thinning and woody control, and frequent burning.

Partitioning of applied 15N fertilizer in a longleaf pine and native woody ornamental intercropping system

The cultivation of ornamentals to produce woody floral products—the fresh or dried stems that are used for decorative purposes—may be an attractive option for southeastern landowners looking to generate income from small landholdings. Since many shrubs native to the understory of the longleaf pine (Pinus palustris Mill.) ecosystem have market potential, one possibility is the intercropping of select species in the between-row spacing of young longleaf pine plantations. The objective of this study was to evaluate how interspecific competition affects the fate of 15N fertilizer when American beautyberry (Callicarpa americana L.), wax myrtle (Morella cerifera (L.) Small) and inkberry (Ilex glabra (L.) A.Gray) are intercropped with longleaf pine. Nitrogen derived from fertilizer (NDF), utilization of fertilizer N (UFN) and recovery of fertilizer N (RFNsoil) were compared between agroforestry and monoculture (treeless) treatments to assess the effects of competition. Results varied by species, with NDF being higher for C. americana foliage and lower for all M. cerifera tissues in the agroforestry treatment. No effect was observed for I. glabra. UFN was lower for all species in the agroforestry treatment. RFNsoil was higher in the agroforestry treatment for I. glabra, but no treatment effects were observed for C. americana or M. cerifera. Overall, while it is clear that interspecific competition was present in the agroforestry treatment, the inefficiency of fertilizer use suggests that nitrogen was not the most limiting resource. Management interventions, particularly those that address competition for water, will likely be critical to the success of this system.

A Height–Diameter Curve for Longleaf Pine Plantations in the Gulf Coastal Plain

Abstract Tree height is a critical component of a complete growth-and-yield model because it is one of the primary components used in volume calculation. To develop an equation to predict total height from dbh for longleaf pine (Pinus palustris Mill.) plantations in the West Gulf region, many different sigmoidal curve forms, weighting functions, and ways of expressing height and diameter were explored. Most of the functional forms tried produced very similar results, but ultimately the form developed by Levakovic was chosen as best. Another useful result was that scaling diameters by the quadratic mean diameter on a plot and height by the average height of dominant and codominant trees in the target stand resulted in dramatically better fits than using these variables in their raw forms.