Higher productivities for loblolly pine ( Pinus taeda L.) growing at exotic locations (e.g., Hawaii, Brazil) indicate that the full growth potential of this southern pine species has yet to be reached in plantations across the southeastern United States (US). The higher productivity of Hawaii-grown loblolly pine has been attributed to more favorable climate conditions. To date, physical, anatomical, and chemical property data for loblolly pine wood from exotic locations are scarce. Mid-infrared spectroscopy coupled with multivariate analysis was used to screen for chemical differences between wood samples from Hawaii (HI), North Carolina (NC), and Mississippi (MS). The principal component analysis scores plot showed that the MS samples formed a distinct cluster apart from overlapping clusters for the other two sites. Using standard wet chemistry procedures, we found that the mean lignin content for the MS site (30.1%) was significantly lower than that determined for the NC (34.5%) and HI (33.9%) sites; said lower lignin value was offset by significantly higher glucose and mannose contents. Results on the US mainland were seemingly consistent with the greater formation of latewood at sites with longer growing seasons, and the lower lignin content of latewood relative to earlywood shown for pines. Given the low percent latewood for the HI site (37.2%) relative to both mainland sites (MS, 54.8%; NC, 53.1%), yet similar lignin contents between the NC and HI sites, it appears that the relative lignin value for these exotically-grown loblolly pine trees cannot be solely attributed to the proportion of latewood.

Longleaf pine demonstrated general resistance to reduced soil moisture and increased VPD, but results highlight the soil and atmospheric conditions that could trigger declines in longleaf pine function and productivity. Low soil moisture and high atmospheric vapor pressure deficit (VPD) independently limit tree function and forest productivity. However, questions remain about how large, established trees respond to dry soil and high VPD over longer time periods. We carried out a 3-year throughfall reduction experiment in a young (12–14-year-old) longleaf pine plantation in west Georgia (USA). We hypothesized that throughfall reduction would reduce soil moisture, leaf-scale stomatal conductance (gs), and net photosynthesis (Pnet), but increase intrinsic water-use efficiency (iWUE). We also hypothesized that throughfall reduction would reduce canopy conductance (Gs) at a reference VPD of 1 kPa and Gs sensitivity to VPD. In addition, we used Gs data collected across both treatments to identify breakpoints in the relative control of soil moisture and VPD on Gs. Throughfall reduction decreased soil moisture and caused small reductions in gs ( – 21

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.

Lignin contents of mature wood growth rings formed under soil moisture extremes were determined to investigate the possibility of drought-caused reductions in lignin deposition for the southern pines. A well-defined set of slash (Pinus elliottii Engelm.), longleaf (Pinus palustris Mill.) and loblolly pine (Pinus taeda L.) increment cores were processed to excise growth rings formed during multiyear periods of above- (wet) or below-normal (drought) soil moisture. The average acid-insoluble (Klason) lignin content of the resultant drought-formed slash pine wood samples was significantly lower (31.4% vs. 34.1%; P = 0.0005) than that for the corresponding wood samples from growth rings formed under conditions of ample soil moisture; respectively higher glucan (40.6% vs. 36.9%; P = 0.0010) and mannan (11.7 vs. 10.4%; P = 0.0010) values were also observed. Small, but significant differences for other hemicellulose derived sugars (xylan, arabinan) were determined for the longleaf and loblolly pine wood samples. Altogether, these data represent the first wet chemical results showing differences in lignin and polysaccharide sugar contents for mature wood formed in pine trees during annual droughts. Among the southern pines evaluated here, slash pine is the least drought tolerant; the lower lignin content observed for the slash pine drought-formed wood is particularly intriguing since it occurred with the species most susceptible to water stress.

Loblolly pine ( Pinus taeda L.) productivity over the past century has increased significantly from genetic improvements and more intensive management practices. The current study concludes a series of assessments of loblolly pine growth/physiological responses to continuous resource management treatments of weed control (W), weed control plus irrigation (WI), and weed control plus irrigation and fertigation (WIF). Increment cores were analyzed by X-ray densitometry to assess treatment impacts on wood properties. Plotting the wood property data against assigned years allowed results to be compared with available weather data. Mean values for all wood property determinations were similar between the W and WI treatments. Increased ring width for the WIF treatment was consistent with other studies demonstrating substantial increases in loblolly pine productivity by fertilization. Since decreases in ring specific gravity (SG) from fertilization can be offset by increases in ring SG from irrigation, gains in productivity were achieved without reducing wood quality.

Abstract Studies linking wood properties of the southern pines to climate parameters and/or irrigation treatments have generally used seedlings or mid-rotation age trees, the latter comprised primarily of juvenile wood. To investigate possible drought-induced effects on mature wood physical properties, densitometry data from 50-year-old slash (Pinus elliottii Engelm.), longleaf (Pinus palustris Mill.), and loblolly (taeda L.) pine trees were matched with annual soil moisture values. Each of two growth ring groupings per increment core had a two-year period of ample moisture followed by a two-year period of drought; these were centered at ages of 20 and 38 years. For slash pine, the latewood width was 30% lower (p = 0.011) for the drought period at age 20. Seemingly similar results were obtained for longleaf pine, but the probability (p = 0.051) just exceeded the threshold for significance (α = 0.05). No differences were observed for either earlywood or total ring widths. Ring specific gravity (SG) values that were 11% lower for slash pine and 7% lower for longleaf pine can be attributed to drought-related reductions in latewood formation. Unlike other studies with younger trees, both percent latewood and ring SG values for mature loblolly pine were unaffected by drought.

Considering the temporal responses of carbon isotope discrimination (Δ13C) to local water availability in the spatial analysis of Δ13C is essential for evaluating the contribution of environmental and genetic facets of plant Δ13C. Using tree-ring Δ13C from years with contrasting water availability at 76 locations across the natural range of loblolly pine, we decomposed site-level Δ13C signals to maximum Δ13C in well-watered conditions (Δ13Cmax) and isotopic drought sensitivity (m) as a change in Δ13C per unit change of Palmer's Drought Severity Index (PDSI). Site water status, especially the tree lifetime average PDSI, was the primary factor affecting Δ13Cmax. The strong spatial correlation exhibited by m was related to both genetic and environmental factors. The long-term average water availability during the period relevant to trees as indicated by lifetime average PDSI correlated with Δ13Cmax, suggesting acclimation in tree gas-exchange traits, independent of incident water availability. The positive correlation between lifetime average PDSI and m indicated that loblolly pines were more sensitive to drought at mesic than xeric sites. The m was found to relate to a plant's stomatal control and may be employed as a genetic indicator of efficient water use strategies. Partitioning Δ13C to Δ13Cmax and m provided a new angle for understanding sources of variation in plant Δ13C, with several fundamental and applied implications.

Short-rotation woody crops have maintained global prominence as biomass feedstocks for bioenergy, in part due to their fast growth and coppicing ability. However, the water usage efficiency of some woody biomass crops suggests potential adverse hydrological impacts. Monitoring tree water use in large-scale plantations would be very time-consuming and cost-prohibitive because it would typically require the installation and maintenance of sap flux sensors and dataloggers or other instruments. We developed a model to estimate the sap flux of eastern cottonwood (Populus deltoides. Bartr. ex Marsh.)) grown in bioenergy plantations. This model is based on adjusted vapor pressure deficit (VPD) using Structural Thinking and Experiential Learning Laboratory with Animation (STELLA) software (Architect Version 1.8.2), and is validated using the sap flux data collected from a 4-year-old eastern cottonwood biomass production plantation. With R-2 values greater than 0.79 and Nash Sutcliffe coefficients greater than 0.69 and p values < 0.001, a strong agreement was obtained between measured and predicted diurnal sap flux patterns and annual sap flux cycles. We further validated the model using eastern cottonwood sap flux data from Aiken, South Carolina, USA with a good agreement between method predictions and field measurements. The model was able to predict a typical diurnal pattern, with sap flux density increasing during the day and decreasing at night for a 5-year-old cottonwood plantation. We found that a 10% increase in VPD due to climate change increased the sap flux of eastern cottonwood by about 5%. Our model also forecasted annual sap flux characteristics of measured cycles that increased in the spring, reached a maximum in the summer, and decreased in the fall. The model developed here can be adapted to estimate sap flux of other trees species in a time- and cost-effective manner.

Net primary productivity (NPP) and net ecosystem production (NEP) are often used interchangeably, as their difference, heterotrophic respiration (soil heterotrophic CO 2 efflux, R SH = NPP−NEP), is assumed a near‐fixed fraction of NPP. Here, we show, using a range‐wide replicated experimental study in loblolly pine ( Pinus taeda ) plantations that R SH responds differently than NPP to fertilization and drought treatments, leading to the divergent responses of NPP and NEP. Across the natural range of the species, the moderate responses of NPP (+11%) and R SH (−7%) to fertilization combined such that NEP increased nearly threefold in ambient control and 43% under drought treatment. A 13% decline in R SH under drought led to a 26% increase in NEP while NPP was unaltered. Such drought benefit for carbon sequestration was nearly twofold in control, but disappeared under fertilization. Carbon sequestration efficiency, NEP:NPP, varied twofold among sites, and increased up to threefold under both drought and fertilization.

Abstract In recent decades, conservation objectives have driven changes to the management of some pine forests in the southeastern United States. Forest thinning and frequent burning of old‐field and plantation pine forests have resulted in an open loblolly–shortleaf pine forest community which resembles the original longleaf pine forest. It is, however, unclear how the structure, composition, and function of the loblolly–shortleaf forest compare to natural longleaf pine forest, and whether it represents an alternative stable state, or simply a transitional state. Understanding the stability of open loblolly–shortleaf pine forest is critical, particularly because several threatened and endangered species are now reliant on it for habitat. The structure and composition of loblolly–shortleaf forest and natural longleaf pine forest were compared using data from permanent forest plots at Fort Benning, Georgia, USA. To assess the stability of the loblolly–shortleaf pine forest and determine whether it is an alternative stable or transitional state, the LANDIS‐II forest landscape simulation model was used to simulate changes in forest type cover under no disturbance, and a frequent‐fire regime at Fort Benning. Under both management scenarios, nearly all loblolly–shortleaf pine forest converted to mixed hardwood forest over the course of the simulation, with most conversion occurring within 60 yr. In contrast, longleaf pine forest cover increased under frequent fire. Several important structural and compositional differences may have contributed to the instability of loblolly–shortleaf pine forest compared to longleaf pine forest. These include, among other factors, higher densities of resprouting hardwood trees and shrubs in loblolly–shortleaf pine forest, including sweetgum, a resilient broadleaf species capable of transforming ecosystem structure. These results highlight the instability of the open loblolly–shortleaf pine forest community and confirm that is a transitional state, destined for mixed hardwood forest in the coming decades under either no disturbance or frequent fire alone. Future forest planning should consider an active transition from the loblolly–shortleaf pine forest in the coming decades if open pine forest is to be conserved for wildlife and conservation objectives.

Forests in the Southeast USA are predicted to experience a moderate decrease in precipitation inputs over this century that may result in soil water deficiency during the growing season. The potential impact of a drier climate on the productivity of managed loblolly pine (Pinus taeda L.) plantations in the Southeast USA is uncertain. Access to water reserves in deep soil during drought periods may help buffer these forests from the effects of water deficits. To better understand the potential impact of drought on deep soil water, we studied the combined effects of throughfall reduction and fertilization on soil water usage in a clay rich Piedmont Ultisol to a depth of 3 m. In a 6-year-old loblolly pine plantation, we applied a throughfall reduction treatment (ambient vs. similar to 30% throughfall reduction) and a fertilization treatment (no fertilization vs. fertilization). Over 28 months, throughfall reduction lowered soil moisture for all depths and differences were significant in the surface soils (0-0.3 m) (1.2-3.6%) and deep soils (below 2 m) (2.6-3.6%). Fertilization also lowered soil moisture for all depths and differences were significant at 0.3-0.6 m (2.9%) and 1.94-3.06 m (4.5%). Fertilization when combined with the throughfall reduction treatment significantly decreased soil water at 0.1-0.9 m depth. Soils of all depths were rarely depleted of plant available water with the exception of 0-0.1 m, mainly during the growing season. Under throughfall reduction treatment, soil below 0.9 m consistently accounted for more than half of the change in plant available water during months when transpiration exceeded precipitation. When considering the whole soil profile in this clay rich Ultisol, soil water storage buffered transpirational demand in the face of decreasing throughfall input.

Longleaf pine (Pins palustris Mill.) forests are thought to be drought tolerant and if so, planting longleaf pine presents a forest management strategy for climate change adaptation in the southeastern United States (U.S.). To better understand how longleaf pine copes with drought, leaf water relations, sap flow, canopy stomatal conductance (Gs), and growth were studied over three growing seasons in response to ambient throughfall (ambient treatment) versus an approximate 40% reduction in throughfall (drought treatment) in a 13-year-old plantation. An exceptional drought occurred the first year of the study and decreased mean predawn (Psi(PD)) and midday (Psi(MD)) leaf water potential to -2.9 MPa and -3.6 MPa, respectively, and decreased average monthly midday Gs to near zero for at least one month in both treatments. Stomatal closure occurred at a Psi(MD) of -3.0 MPa in both treatments. Leaf water potentials and transpiration recovered quickly following significant rain events that terminated the drought and mortality was similar among years and treatments (2.8%). Longleaf pine responded to drought treatment with greater stomatal control of plant water loss rather than adjustments in leaf area, the sapwood to leaf area ratio, or leaf water potential at the turgor loss point (Psi(TLP)). Annual transpiration per unit leaf area was reduced 16% by drought treatment, but greater stomatal control of water loss in response to drought treatment was associated with decreases in growth efficiency and volume, and no improvement in water use efficiency.

There is an increasing interest in estimating biomass for longleaf pine (Pinus palustris Mill.), an important tree species in the southeastern U.S. Most of the individual-tree allometric models available for the species are local, relying on stem diameter outside bark at breast height (DBH) and total tree height (HT), but seldom include stand-level variables such as stand age, basal area or stand density. Using the biomass dataset of 296 longleaf pine trees sampled in the southeastern U.S. by different forestry research institutions, we developed a set of local and general systems of tree biomass equations to predict total tree total above-stump biomass, bole biomass outside bark, live branch biomass and live foliage biomass. The local systems were based on DBH or DBH and HT, and the general systems included in addition to DBH and HT, stand-level variables such as age, basal area and stand density. This paper reports the first set of general allometric equations reported for longleaf pine trees. These systems of biomass equations provide tools to support managers in making management decisions for the species in a variety of ecological, silvicultural and economics applications. The systems can be applied to trees growing over a large geographical area and having a wide range of ages and stand characteristics.

Storage of belowground carbon (C) is an important component of total forest C. However, belowground C changes temporally due to forest growth and tree mortality (natural and via harvesting) and these fluctuations are critical for modeling C in forests under varying management regimes. To date, little progress has been made in quantifying the rate of decay of southern pines in general, and specifically in longleaf pine (Pinus palustris Mill.) coarse root systems. Decomposition rates of lateral roots and tap roots of longleaf pine were quantified in situ under field conditions across the species' range to create a model for necromass loss. The roots of 37 longleaf pine stumps were excavated from Florida, Georgia, Louisiana, and North Carolina. The age of the trees when cut ranged from 14 to 260 years, and the time since cut ranged from 5 to 70 years. Remaining lateral roots to a 1 m depth plus the entire tap root were removed, dried, weighed and analyzed for C and nitrogen (N) content. Total dry necromass of harvested roots ranged from 8 to 195 kg tree(-1). Soil C and N content at 15 cm depth were significantly higher near the stump compared to half-way between and adjacent to the nearest living longleaf pine. A regression model was developed to predict necromass loss. The final model included years since cut, stump diameter, and average minimum monthly air temperature as predictors (R-2 = 0.83). For example, a 100 year-old tree would have a predicted root decomposition rate (k) of -0.120 for lateral roots and -0.038 per year for tap roots. Results suggest that longleaf pine coarse roots persist in the environment longer than the tap roots of loblolly pine.

Silvicultural practices, particularly fertilization, may counteract or accentuate the effects of climate change on carbon cycling in planted pine ecosystems, but few studies have empirically assessed the potential effects. In the southeastern United States, we established a factorial throughfall reduction (D) x fertilization (F) experiment in 2012 in four loblolly pine (Paws taeda L.) plantations encompassing the climatic range of the species in Florida (FL), Georgia (GA), Oklahoma (OK), and Virginia (VA). Net primary productivity (NPP) was estimated from tree inventories for four consecutive years, and net ecosystem productivity (NEP) as NPP minus heterotrophic respiration (R-H). Soil respiration (R-S) was measured biweekly-monthly for at least one year at each site and simultaneous measurements of R-S & R-H were taken five to eight times through the year for at least one year during the experiment. Reducing throughfall by 30% decreased available soil water at the surface and for the 0-90 cm soil profile. Fertilization increased NPP at all sites and D decreased NPP (to a lesser extent) at the GA and OK sites. The F + D treatment did not affect NPP. Mean annual NPP under F ranged from 10.01 +/- 0.21 MgC center dot ha(-1).yr(-1) at VA (mean +/- SE) to 17.20 +/- 0.50 MgC center dot ha(-1).yr(-1) at FL, while the lowest levels were under the D treatment, ranging from 8.63 +/- 0.21 MgC center dot ha(-1).yr(-1) at VA to 14.97 +/- 0.50 MgC center dot ha(-1).yr(-1) at FL. R-S and R-H were, in general, decreased by F and D with differential responses among sites, leading to NEP increases under F. Throughfall reduction increased NEP at FL and VA due to a negative effect on R-H and no effect on NPP. Mean annual NEP ranged from 1.63 +/- 0.59 MgC center dot ha(-1).yr(-1) in the control at OK to 8.18 +/- 0.82 MgC center dot ha(-1).yr(-1) under F + D at GA. These results suggest that fertilization will increase NEP under a wide range of climatic conditions including reduced precipitation, but either NPP or R-H could be the primary driver because F can increase stand growth, as well as suppress R-S and R-H. Moreover, D and F never significantly interacted for an annual C flux, potentially simplifying estimates of how fertilization and drought will affect C cycling in these ecosystems.

Soil carbon dynamics remains a key uncertainty in terrestrial carbon cycle models. While our understanding of the dynamism of soil carbon transformations has evolved significantly in the past two decades, it is not reflected in the mechanistic models that describe the interactions between soil, vegetation and the atmosphere. Processes that control the input, stabilization and loss of carbon in soil remain poorly defined and need to be refined in order to better constrain projections of the magnitude of carbon sequestration in terrestrial ecosystems under changing environmental conditions.

Longleaf pine forests are currently being restored in the southern U.S. To aid in the deployment of longleaf pine under current and future climate conditions, we tested the hypothesis that genetic variability in foliar carbon isotope composition (δ13C) exists in this species. Foliar δ13C, height and diameter were measured at ages of 5 and 6 years, and needle length, specific leaf weight (SLW) and foliar N concentration were measured at an age of 6 years in 16 longleaf pine families representing a large portion of the species’ range. Families were grown in common garden tests in North Carolina and Mississippi and grouped for analysis into six provenances based on climate, soils, and discontinuities in the species’ range. No genetic by environment interactions were observed. Greater foliar δ13C was observed in trees from the provenance consisting of the Piedmont and Montane Uplands than from the provenances representing the western and eastern Gulf Coastal Plains. Foliar δ13C was not significantly correlated to height at age 6, suggesting that it may be possible to select for improved foliar δ13C without sacrificing growth. These results represent a first step in identifying potential genetic variation in leaf water use efficiency and drought tolerance of longleaf pine.

High productivity of fertilized loblolly pine (Pinus taeda L.) plantations in the southern United States is related to increased leaf area index (LAI), but higher evaporative leaf surface area may increase drought vulnerability. To determine if the benefits of fertilization are affected by water availability or the effects of drought are exacerbated by fertilization, the interactive effects of throughfall treatment (ambient throughfall versus throughfall reduction) and fertilization treatment (no fertilization versus one-time fertilization) on a loblolly pine plantation were examined over five growing seasons. Enhancement of LAI and growth from fertilization was unaffected by throughfall treatment, and reductions in LAI, tree height, and stand volume increment in response to throughfall reduction were unaffected by fertilization treatment. Leaf-level stomatal conductance (gS) was decreased and water use efficiency was increased by fertilization and by throughfall reduction. Lower gS was associated with decreased leaf predawn water potential in response to throughfall reduction. In contrast, lower gs in response to fertilization was associated with a reduction in the hydraulic allometry index, a measure of the ability of sapwood to supply water to leaves. These results suggest that fertilization may enhance LAI and growth even under mild or moderate drought.

Longleaf pine (Pinus palustris Miller) forests in the southern United States are being restored and actively managed for a variety of goals including: forest products, biodiversity, C sequestration and forest resilience in the face of repeated disturbances from hurricanes and climate change. Managed southern pine forests can be sinks for atmospheric CO2 in forest biomass; however, the persistence of biomass in the environment or in forest products is limited, thus making soil C the primary long-term pool. Little is known about the size of extant soil C pools, residence time of soil C or the role that frequent burning plays in C stabilization in longleaf pine ecosystems. We sampled soil from a chronosequence of longleaf pine stands ranging in age from 5 to 87 years to quantify the vertical distribution of soil organic carbon (SOC) stocks; both oxidizable (SOCox) and oxidation resistant (SOCR) fractions, pyrogenic carbon (PyC) and the mean residence time (MRT) of SOC and its associated fractions. SOC stocks (0-1 m) ranged from 44.1 to 98.1 ((x) over bar = 77.0) Mg C ha(-1), and no effect of stand age or biomass accumulation on SOC stocks was detected. Soil C accumulation was associated with elevated clay and extractable Fe contents. While SOC concentration declined with soil depth, the proportion of SOCR in SOC increased with depth. PyC was a minor component of soil C, representing 5-7% of SOC and the proportion was not depth dependent. The MRT of SOC was hundreds of years near the surface and many thousands of years at depth. Though SOCR was less abundant than SOCox, SOCR MRT was an order of magnitude greater than SOCox MRT and had a strong influence on bulk SOC MRT. The majority of the PyC was in the less persistent SOCox and not associated with long-term C storage in soil. Despite the flow of C from biomass in the form of decay products, litter fall, root turnover and pulses of PyC, these soils preserve little of recent inputs, which may be rapidly oxidized, lost to the atmosphere from periodic fires or, in the case of PyC, may be transported out of the system via erosion. Our results indicate that these soils were not strong sinks for atmospheric CO2, especially when compared to C accumulation in biomass. Published by Elsevier B.V.

Forests can partially offset greenhouse gas emissions and contribute to climate change mitigation, mainly through increases in live biomass. We quantified carbon (C) density in 20 managed longleaf pine (Pinus palustris Mill.) forests ranging in age from 5 to 118 years located across the southeastern United States and estimated above- and belowground C trajectories. Ecosystem C stock (all pools including soil C) and aboveground live tree C increased nonlinearly with stand age and the modeled asymptotic maxima were 168 Mg C/ha and 80 Mg C/ha, respectively. Accumulation of ecosystem C with stand age was driven mainly by increases in aboveground live tree C, which ranged from <1 Mg C/ha to 74 Mg C/ha and comprised <1% to 39% of ecosystem C. Live root C (sum of below-stump C, ground penetrating radar measurement of lateral root C, and live fine root C) increased with stand age and represented 4-22% of ecosystem C. Soil C was related to site index, but not to stand age, and made up 39-92% of ecosystem C. Live understory C, forest floor C, downed dead wood C, and standing dead wood C were small fractions of ecosystem C in these frequently burned stands. Stand age and site index accounted for 76% of the variation in ecosystem C among stands. The mean root-to-shoot ratio calculated as the average across all stands (excluding the grass-stage stand) was 0.54 (standard deviation of 0.19) and higher than reports for other conifers. Long-term accumulation of live tree C, combined with the larger role of belowground accumulation of lateral root C than in other forest types, indicates a role of longleaf pine forests in providing disturbance-resistant C storage that can balance the more rapid C accumulation and C removal associated with more intensively managed forests. Although other managed southern pine systems sequester more C over the short-term, we suggest that longleaf pine forests can play a meaningful role in regional forest C management.