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Carbon (C) allocation and nonstructural carbon (NSC) dynamics play essential roles in plant growth and survival under stress and disturbance. However, quantitative understanding of these processes remains limited. Here we propose a framework where we connect commonly measured carbon cycle components (eddy covariance fluxes of canopy CO

Hurricanes are one of the most significant threats to coastal plain forest ecosystems and urban communities of the southeastern U.S., but their implications for watershed hydrology are unclear. Hurricanes have the potential to alter water balances, causing extensive flooding, biogeochemical cycle disruption, and water quality degradation, saltwater intrusion, and increased nutrient sedimentation export in coastal watersheds. This case study focused on Hurricane Michael, a recent catastrophic event that impacted the Gulf coast, the Florida panhandle, southwestern Georgia, and southeastern Alabama. Through empirical (Double Mass Curve) and process-based ecohydrological modeling (WaSSI model) on long-term streamflow data, we explored whether vegetation damage caused by this hurricane resulted in an increase in streamflow two years after the extreme event. We found that monthly streamflow from the Chipola River watershed with an area of 2023 km2 did not change (

Background and aims Fine roots can be functionally classified into an absorptive fine root pool (AFR) and a transport fine root pool (TFR) and their production, mortality and decomposition play a critical role in forest soil carbon (C) cycling. Different methods give significantly different estimates. However, how methodological difference affects AFT and TFR production, mortality, and decomposition estimates remains unclear, impeding us to accurately construct fine root C budgets. Methods We used dynamic-flow model, a model based on measurements of litterbags and soil cores, and balanced-hybrid model, a model based on measurements of minirhizotrons and soil cores, to quantify AFT and TFR production, mortality, and decomposition in a planted loblolly pine forest. Results Temporal changes in AFR and TFR production, mortality and decomposition estimates were generally the same between the two models. Annual production, mortality, and decomposition were comparable between AFRs and TFRs when measured using the dynamic-flow model but significantly higher for AFRs than for TFRs when measured using the balanced-hybrid model. Annual production, mortality and decomposition estimates using the balanced-hybrid model were 75%, 71% and 69% higher than those using the dynamic-flow model, respectively, for AFRs, but 12%, 6% and 5% higher than those using the dynamic-flow model, respectively, for TFRs. Model test showed that the balanced-hybrid model had greater accuracy than the dynamic-flow model in estimating AFR and TFR production, mortality and decomposition. Lower AFR estimates using the dynamic-flow model than using the balanced-hybrid model appeared to result from the underestimated AFR mass loss rate induced by the litterbag method. Conclusions The balanced-hybrid model is more reliable than the dynamics-flow model. Methodological difference has greater effects on AFR estimates than TFR estimates. Effects of methodological difference must be taken into consideration when quantifying fine root production, mortality and decomposition by functional groups.

Key Messages: * Soil salinization in the coastal Southeastern United States is becoming more prevalent as storm surges increase in frequency,and sea levels rise. * Salinization reduces the productivity of working lands and can prevent crops from growing. * Resources are lacking for landowners to understand coastal salinization and how to manage for resilience. * Action must be taken if the land is to remain profitable as conditions change. This manual describes the impacts and includes adaptation measures that can be taken to maintain productivity in working lands.

Globally, soils store two to three times as much carbon as currently resides in the atmosphere, and it is critical to understand how soil greenhouse gas (GHG) emissions and uptake will respond to ongoing climate change. In particular, the soil‐to‐atmosphere CO2 flux, commonly though imprecisely termed soil respiration (R S), is one of the largest carbon fluxes in the Earth system. An increasing number of high‐frequency R S measurements (typically, from an automated system with hourly sampling) have been made over the last two decades; an increasing number of methane measurements are being made with such systems as well. Such high frequency data are an invaluable resource for understanding GHG fluxes, but lack a central database or repository. Here we describe the lightweight, open‐source COSORE (COntinuous SOil REspiration) database and software, that focuses on automated, continuous and long‐term GHG flux datasets, and is intended to serve as a community resource for earth sciences, climate change syntheses and model evaluation. Contributed datasets are mapped to a single, consistent standard, with metadata on contributors, geographic location, measurement conditions and ancillary data. The design emphasizes the importance of reproducibility, scientific transparency and open access to data. While being oriented towards continuously measured R S, the database design accommodates other soil‐atmosphere measurements (e.g. ecosystem respiration, chamber‐measured net ecosystem exchange, methane fluxes) as well as experimental treatments (heterotrophic only, etc.). We give brief examples of the types of analyses possible using this new community resource and describe its accompanying R software package., Here we describe the lightweight, open source COSORE (COntinuous SOil REspiration) database and software. COSORE focuses on automated, continuous and long‐term greenhouse gas flux datasets, and is intended to serve as a community resource for earth sciences, climate change syntheses and model evaluation.

Wetlands store large carbon (C) stocks and play important roles in biogeochemical C cycling. However, the effects of environmental and anthropogenic pressures on C dynamics in lower coastal plain forested wetlands in the southern U.S. are not well understood. We established four eddy flux stations in two post-harvest and newly-planted loblolly pine plantations (YP2–6, 2–6 yrs old; YP2–8, 2–8 yrs old), a rotation-aged loblolly pine plantations (MP, 15–27 yrs old), and a mixed bottomland hardwood forest (BHF, >100 yrs old) in the lower coastal plain of North Carolina, USA. We analyzed the gross primary productivity (GPP), ecosystem respiration (RE) and net ecosystem exchange (NEE) for age-related trends, interannual variability in response to climate forcing, and management-related disturbances from 2005 – 2017. For the first few years after being harvested, pine plantations were net C sources (NEE = 1133 and 897 g C m–2 yr–1 in YP2–6 and YP2–8, respectively). The MP was a strong C sink (–369 to –1131 g C m–2 yr–1) over the entire study period. In contrast, BHF was a C source (NEE = 87 g C m–2 yr–1 to 759 g C m–2 yr–1) in most years, although in the first year it did show a net C uptake (NEE = –368 g C m–2 yr–1). The source activity of BHF may have been related to increasing overstory tree mortality and diameter growth suppression. Decreases in relative extractable water in pine plantations enhanced GPP and RE. Pine plantations regained status as C sinks 5–8 years after harvest and recovered C equivalent to post-harvest losses at 8–14 years. Thus, coastal pine plantations have a net C uptake for only about half the 25-year rotation period, suggesting that they have decreased climate mitigation potential in comparison to protecting primary forests. However, primary forests in this area may be vulnerable to ecosystem transition, and subsequent C loss, due to the changing environmental conditions at the land-ocean interface.

Forest water use efficiency (WUE), the ratio of gross primary productivity (GPP) to evapotranspiration (ET), is an important variable to understand the coupling between water and carbon cycles, and to assess resource use, ecosystem resilience, and commodity production. Here, we determined WUE for managed loblolly pine plantations over the course of a rotation on the coastal plain of North Carolina in the eastern U.S. We found that the forest annual GPP, ET, and WUE increased until age ten, which stabilized thereafter. WUE varied annually (2–44%), being higher at young plantation (YP, 3.12 ± 1.20 g C kg−1 H2O d−1) compared to a mature plantation (MP, 2.92 ± 0.45 g C kg−1 H2O d−1), with no distinct seasonal patterns. Stand age was strongly correlated with ET (R2 = 0.71) and GPP (R2 = 0.64). ET and GPP were tightly coupled (R2 = 0.86). Radiation and air temperature significantly affected GPP and ET (R2 = 0.71 − R2 = 0.82) at a monthly scale, but not WUE. Drought affected WUE (R2 = 0.35) more than ET (R2 = 0.25) or GPP (R2 = 0.07). A drought enhanced GPP in MP (19%) and YP (11%), but reduced ET 7% and 19% in MP and YP, respectively, resulting in a higher WUE (27–32%). Minor seasonal and interannual variation in forest WUE of MP (age &gt, 10) suggested that forest functioning became stable as stands matured. We conclude that carbon and water cycles in loblolly pine plantations are tightly coupled, with different characteristics in different ages and hydrologic regimes. A stable WUE suggests that the pine ecosystem productivity can be readily predicted from ET and vice versa. The tradeoffs between water and carbon cycling should be recognized in forest management to achieve multiple ecosystem services (i.e., water supply and carbon sequestration).

Accurate measurement of total fine root decomposition (the amount of dead fine roots decomposed per unit soil volume) is essential for constructing a soil carbon budget. However, the ingrowth/soil core-based models are dependent on the assumptions that fine roots in litterbags/intact cores have the same relative decomposition rate as those in intact soils and that fine root growth and death rates remain constant over time, while minirhizotrons cannot quantify the total fine root decomposition. To improve the accuracy of estimates for total fine root decomposition, we propose a new method (balanced hybrid) with two models that integrate measurements of soil coring and minirhizotrons into a mass balance model. Model input parameters were fine root biomass, necromass and turnover rate for Model 1, and fine root biomass, necromass and death rate for Model 2. We tested the balanced hybrid method in a loblolly pine plantation forest in coastal North Carolina, USA. The total decomposition rate of absorptive fine roots (ARs) (a combination of first- and second-order fine roots) using Models 1 and 2 was 107 ± 13 g m−2 year−1 and 129 ± 12 g m−2 year−1, respectively. Monthly total AR decomposition was highest from August to November, which corresponded with the highest monthly total ARs mortality. The ARs imaged by minirhizotrons well represent those growing in intact soils, evident by a significant and positive relationship between the standing biomass and the standing length. The total decomposition estimate in both models was sensitive to changes in fine root biomass, turnover rate and death rate but not to change in necromass. Compared with Model 2, Model 1 can avoid the technical difficulty of deciding dead time of individual fine roots but requires greater time and effort to accurately measure fine root biomass dynamics. The balanced hybrid method is an improved technique for measuring total fine root decomposition in plantation forests in which the estimates are based on empirical data from soil coring and minirhizotrons, moving beyond assumptions of traditional approaches.

International audience; Forested wetlands are important in regulating regional hydrology and climate. However, long-term studies on the hydrologic impacts of converting natural forested wetlands to pine plantations are rare for the southern US. From 2005-2018, we quantified water cycling in two post-harvest and newly-planted loblolly pine (Pinus taeda) plantations (YP2-7, 2-7 yrs old; YP2-8, 2-8 yrs old), a rotation-age loblolly pine plantation (MP, 15-28 yrs old), and a natural bottomland hardwood forest (BHF, > 100 yrs old) along the lower coastal plain of North Carolina. We quantified the differences in inter-annual and seasonal water balance and trends of evapotranspiration (ET) using eddy covariance over 37 site-years and assessed key climatic and biological drivers of ET. We found that the rotation-age plantation (MP) had higher annual ET (933 +/- 63 mm) than the younger plantations (776 +/- 74 mm for YP2-7 and 638 +/- 190 mm for YP2-8), and the BHF (743 +/- 172 mm), owing to differences in stand age, canopy cover, and micrometeorology. Chronosequence analysis of the pine sites showed that ET increased with stand age up to 10 years, then gradually stabilized for the remainder of the rotation of 28 - 30 years. YP2-8 was sensitive to water availability, decreasing ET by 30 - 43 % during the extreme 2007 - 2008 drought, but reductions in ET at MP were only 8 - 11 %. Comparing to BHF, ditching with management enhanced drainage at YP2-7 and YP2-8, while drainage was lower at the mature pine site. This study provides insight into land use-hydrology-climate interactions that have important implications for forested wetland management in a time of rapidly changing environmental conditions of the LCP of the southern US.

The area of planted pine in the southern U.S. is predicted to increase by over 70% by 2060, potentially altering the natural hydrologic cycle and water balance at multiple scales. To better account for potential shifts in water yield, land managers and resource planners must accurately quantify water budgets from the stand to the regional scale. The amount of precipitation as rainfall intercepted by forest canopies is an important component of evapotranspiration in forested ecosystems, yet there is little information about intra- and inter-annual canopy interception variability in southern pine plantations. To address this knowledge gap, canopy rainfall interception was measured between 2005 and 2014 in a North Carolina coastal plain loblolly pine ( L.) plantation to quantify the range of annual and seasonal variability in interception rates (IRs) as influenced by stand thinning and natural variation in rainfall rates and intensities. Over the study period, biweekly measured canopy IRs averaged 19% across all years, with a range of 14% to 23%. However, at the annual scale, IRs averaged 12% and ranged from 2% to 17%. Thinning resulted in a 5% decrease in rainfall interception, but IRs quickly returned to pre-thin levels. Across years, the amount of annual rainfall intercepted by the canopy averaged 15% of total evapotranspiration, with a range of 2% to 24%. The decade-long data indicate that inter-annual variability of canopy interception is higher than reported in short-term studies. Local and regional hydrological models must describe the variability of canopy interception to accurately predict the hydrologic impacts of forest management and climate change.

The USDA Forest Service hosted a two-day drought adaptation workshop in Atlanta, Georgia in January 2017 to share state-of-science information on drought and climate effects in the region and to develop management response strategies. The workshop was attended by regional experts from the Forest Service Southern Region, Southern Research Station, and Office of Sustainability and Climate; the USDA Southeast Regional Climate Hub; and state and federal climate offices. They met to address challenges, cultivate opportunities, and develop and expand the collective understanding of the most effective management strategies to adapt to and mitigate the effects of drought in the region. The workshop focused on the effects of, and management responses to drought in forest, riparian, and aquatic ecosystems. This fact sheet is a synopsis of the workshop.

International audience; Throughout the southern US, past forest management practices have replaced large areas of native forests with loblolly pine plantations and have resulted in changes in forest response to extreme weather conditions. However, uncertainty remains about the response of planted versus natural species to drought across the geographical range of these forests. Taking advantage of a cluster of unmanaged stands (85-130 year-old hardwoods) and managed plantations (17-20 year-old loblolly pine) in coastal and Piedmont areas of North Carolina, tree water use, cavitation resistance, whole-tree hydraulic (K-tree) and stomatal (G(s)) conductances were measured in four sites covering representative forests growing in the region. We also used a hydraulic model to predict the resilience of those sites to extreme soil drying. Our objectives were to determine: (1) if K-tree and stomatal regulation in response to atmospheric and soil droughts differ between species and sites; (2) how ecosystem type, through tree water use, resistance to cavitation and rooting profiles, affects the water uptake limit that can be reached under drought; and (3) the influence of stand species composition on critical transpiration that sets a functional water uptake limit under drought conditions. The results show that across sites, water stress affected the coordination between K-tree and G(s). As soil water content dropped below 20% relative extractable water, K-tree declined faster and thus explained the decrease in G(s) and in its sensitivity to vapor pressure deficit. Compared to branches, the capability of roots to resist high xylem tension has a great impact on tree-level water use and ultimately had important implications for pine plantations resistance to future summer droughts. Model simulations revealed that the decline in K-tree due to xylem cavitation aggravated the effects of soil drying on tree transpiration. The critical transpiration rate (E-crit), which corresponds to the maximum rate at which transpiration begins to level off to prevent irreversible hydraulic failure, was higher in managed forest plantations than in their unmanaged counterparts. However, even with this higher E-crit, the pine plantations operated very close to their critical leaf water potentials (i.e. to their permissible water potentials without total hydraulic failure), suggesting that intensively managed plantations are more drought-sensitive and can withstand less severe drought than natural forests.

International audience; Managed and natural coastal plain forests in the humid southeastern United States exchange large amounts of water and energy with the atmosphere through the evapotranspiration (ET) process. ET plays an important role in controlling regional hydrology, climate, and ecosystem productivity. However, long-term studies on the impacts of forest management and climatic variability on forest ET are rare, and our understanding of both external and internal drivers on seasonal and interannual ET variability is incomplete. Using techniques centered on an eddy covariance method, the present study measured year-round ET flux and associated hydrometeorological variables in a drained loblolly pine (Pinus taeda L.) plantation on the lower coastal plain of North Carolina, U.S. We found that annual ET was relatively stable (1076 ± 104 mm) in comparison to precipitation (P) (1168 ± 216 mm) during the 10-year study period when the site experienced extreme climate (2007–2008) and forest thinning (2009). At the seasonal time scale, mean ET/P varied between 0.41 and 1.51, with a mean value of 1.12 ± 0.23 and 0.72 ± 0.16 for the growing and dormant seasons, respectively. The extreme drought during 2007–2008 (mean annual P, 854 mm) only resulted in a slight decrease (∼8%) in annual ET owing to the shallow groundwater common to the study area. Although changes in leaf area index and canopy structure were large after the stand was 50% thinned in the fall of 2009, mean annual ET was similar and averaged 1055 mm and 1104 mm before (2005, 2006 and 2009) and after (2010–2015) thinning, respectively. Data suggested that annual ET recovered within two years of the thinning as a result of rapid canopy closure and growth of understory. Further analysis indicated that available energy was the key driver of ET: approximately 69% and 61% of the monthly variations in ET were explained by net radiation during the dormant and growing seasons, respectively. Overall, we concluded that drought and forest thinning had limited impacts on seasonal and annual ET in this energy limited forest ecosystem with shallow groundwater. The results from this study help to better understand regional ecohydrological processes and projecting potential effects of forest management and extreme climate on water and carbon cycles.

International audience; Deep root water uptake and hydraulic redistribution (HR) have been shown to play a major role in forest ecosystems during drought, but little is known about the impact of climate change, fertilization and soil characteristics on HR and its consequences on water and carbon fluxes. Using data from three mid-rotation loblolly pine plantations, and simulations with the process-based model MuSICA, this study indicated that HR can mitigate the effects of soil drying and had important implications for carbon uptake potential and net ecosystem exchange (NEE), especially when N fertilization is considered. At the coastal site (C), characterized by deep organic soil, HR increased dry season tree transpiration (T) by up to 40%, and such an increase affected NEE through major changes in gross primary productivity (GPP). Deep-rooted trees did not necessarily translate into a large volume of HR unless soil texture allowed large water potential gradients to occur, as was the case at the sandy site (S). At the Piedmont site (P) characterized by a shallow clay-loam soil, HR was low but not negligible, representing up to 10% of T. In the absence of HR, it was predicted that at the C, S and P sites, annual GPP would have been diminished by 19, 7 and 9%, respectively. Under future climate conditions HR was predicted to be reduced by up to 25% at the C site, reducing the resilience of trees to precipitation deficits. The effect of HR on T and GPP was predicted to diminish under future conditions by 12 and 6% at the C and P sites, respectively. Under future conditions, T was predicted to stay the same at the P site, but to be marginally reduced at the C site and slightly increased at the S site. Future conditions and N fertilization would decrease T by 25% at the C site, by 15% at the P site and by 8% at the S site. At the C and S sites, GPP was estimated to increase by 18% and by > 70% under future conditions, respectively, with little effect of N fertilization. At the P site, future conditions would stimulate GPP by only 12%, but future conditions plus N fertilization would increase GPP by 24%. As a consequence, in all sites, water use efficiency was predicted to improve dramatically with future conditions. Modeling the effect of reduced annual precipitation indicated that limited water availability would decrease all carbon fluxes, including NEE and respiration. Our simulations highlight the interactive effects of nutrients and elevated CO2, and showed that the effect of N fertilization would be greater under future climate conditions.

During 2005–2007, we used the eddy covariance and associated hydrometric methods to construct energy and water budgets along a chronosequence of loblolly pine ( Pinus taeda ) plantations that included a mid-rotation stand (LP) (i.e., 13–15 years old) and a recently established stand on a clearcut site (CC) (i.e., 4–6 years old) in Eastern North Carolina. Our central objective was to quantify the differences in both energy and water balances between the two contrasting stands and understand the underlining mechanisms of environmental controls. We found that the LP site received about 20% more net radiation ( R n ) due to its lower averaged albedo ( α ) of 0.25, compared with that at the CC ( α = 0.34). The mean monthly averaged Bowen ratios ( β ) at the LP site were 0.89 ± 0.7, significantly ( p = 0.02) lower than at the CC site (1.45 ± 1.2). Higher net radiation resulted in a 28% higher ( p = 0.02) latent heat flux (LE) for ecosystem evapotranspiration at the LP site, but there was no difference in sensible heat flux ( H ) between the two contrasting sites. The annual total evapotranspiration (ET) at the LP site and CC site was estimated as 1011–1226 and 755–855 mm year −1 , respectively. The differences in ET rates between the two contrasting sites occurred mostly during the non-growing seasons and/or dry periods, and they were small during peak growing seasons or wet periods. Higher net radiation and biomass in LP were believed to be responsible to the higher ET. The monthly ET/Grass Reference ET ratios differed significantly across site and season. The annual ET/ P ratio for the LP and CC were estimated as 0.70–1.13 and 0.60–0.88, respectively, indicating higher runoff production from the CC site than the LP site. This study implied that reforestation practices reduced surface albedos and thus increased available energy, but they did not necessarily increase energy for warming the atmosphere in the coastal plain region where soil water was generally not limited. This study showed the highly variable response of energy and water balances to forest management due to climatic variability.

Full accounting of ecosystem carbon (C) pools and fluxes in coastal plain ecosystems remains less studied compared with upland systems, even though the C stocks in these systems may be up to an order of magnitude higher, making them a potentially important component in regional C cycle. Here, we report C pools and CO2 exchange rates during three hydrologically contrasting years (i.e. 2005‐2007) in a coastal plain loblolly pine plantation in North Carolina, USA. The daily temperatures were similar among the study years and to the long-term (1971‐2000) average, whereas the amount and timing of precipitation differed significantly. Precipitation was the largest in 2005 (147mm above normal), intermediate in 2006 (48mm below) and lowest in 2007 (486mm below normal). The forest was a strong C sink during all years, sequestering 361 � 67 (2005), 835 � 55 (2006) and 724 � 55 (2007)gCm � 2 yr � 1 according to eddy covariance measurements of net ecosystem CO2 exchange (NEE). The interannual differences in NEE were traced to drought-induced declines in canopy and whole tree hydraulic conductances, which declined with growing precipitation deficit and decreasing soil volumetric water content (VWC). In contrast, the interannual differences were small in gross ecosystem productivity (GEP) and ecosystem respiration (ER), both seemingly insensitive to drought. However, the drought sensitivity of GEP was masked by higher leaf area index and higher photosynthetically active radiation during the dry year. Normalizing GEP by these factors enhanced interannual differences, but there were no signs of suppressed GEP at low VWC during any given year. Although ER was very consistent across the 3 years, and not suppressed by low VWC, the total respiratory cost as a fraction of net primary production increased with annual precipitation and the contribution of heterotrophic respiration (Rh) was significantly higher during the wettest year, exceeding new litter inputs by 58%. Although the difference was smaller during the other 2 years (Rh:litterfall ratio was 1.05 in 2006 and 1.10 in 2007), the soils lost about 109gCm � 2 yr � 1

The study examined the relationships between whole tree hydraulic conductance (K(tree)) and the conductance in roots (K(root)) and leaves (K(leaf)) in loblolly pine trees. In addition, the role of seasonal variations in K(root) and K(leaf) in mediating stomatal control of transpiration and its response to vapour pressure deficit (D) as soil-dried was studied. Compared to trunk and branches, roots and leaves had the highest loss of conductivity and contributed to more than 75% of the total tree hydraulic resistance. Drought altered the partitioning of the resistance between roots and leaves. As soil moisture dropped below 50%, relative extractable water (REW), K(root) declined faster than K(leaf). Although K(tree) depended on soil moisture, its dynamics was tempered by the elongation of current-year needles that significantly increased K(leaf) when REW was below 50%. After accounting for the effect of D on g(s), the seasonal decline in K(tree) caused a 35% decrease in g(s) and in its sensitivity to D, responses that were mainly driven by K(leaf) under high REW and by K(root) under low REW. We conclude that not only water stress but also leaf phenology affects the coordination between K(tree) and g(s) and the acclimation of trees to changing environmental conditions.

Mass and carbon load estimates, such as those from forest soil organic matter (duff and litter), inform forestry decisions. The US Forest Inventory and Analysis (FIA) Program systematically collects data nationwide: a down woody material protocol specifies discrete duff and litter depth measurements, and a soils protocol specifies mass and carbon of duff and litter combined. Sampling duff and litter separately via the soils protocol would increase accuracy of subsequent bulk density calculations and mass and carbon estimates that use them. At 57 locations in North Carolina, Virginia, and West Virginia, we measured depth, mass, and carbon of duff and litter separately. Duff depth divided by total depth varied from 20% to 56%, duff was 1–4 times denser than litter, and the calculated median carbon-to-mass ratio for hardwood duff (0.37) was less than that for litter (0.45). Using FIA depth measurements, we calculated mass from (1) our mean density values, (2) a mass versus depth regression model we developed, and (3) published density values. Model mass calculations were lower than those using our mean densities, possibly because the latter ignore density differences with layer thickness. Our model could provide valuable mass and carbon estimates if fully developed with future FIA data (duff and litter separated).

The declining health of high-elevation red spruce (Picea rubens Sarg.) and Fraser fir (Abies fraseri (Pursh) Poir.) in the southern Appalachian region has long been linked to nitrogen (N) deposition. Recently, N deposition has also been proposed as a source of negative health impacts in lower elevation deciduous forests. In 1998 we established 46 plots on six sites in North Carolina and Virginia dominated by American beech (Fagus grandifolia Ehrh.), sugar maple (Acer saccharum Marsh.), and yellow birch (Betula alleghaniensis Britt). We evaluated several response variables across an N deposition gradient, including annual basal area growth; foliage percent N, Al, P, K, Mg, and Ca; and forest floor percent N, Mg, and C, pH, and potential net nitrification and N mineralization rates. We found a significant linear relationship between N deposition and basal area growth in sugar maple, but not in American beech or yellow birch. In addition, we found a significant relationship between N deposition and foliar chemistry (foliar %N in all species, foliar Mg/N and %K in sugar maple, and %P in sugar maple and yellow birch). Foliar %N of the three studied species was high relative to values reported in other studies in the United States and Canada. Several forest floor response variables (%N, C/N, pH, Mg/N, and potential net nitrification and N mineralization rates and nitrification/mineralization fractions) were also correlated with N deposition. The correlations between the above response variables and N deposition are consistent with the influence of chronic N deposition on forested ecosystems measured in other regions and suggest that chronic N deposition may be influencing forest structure and chemistry within the southern region.

Evapotranspiration (ET) is a key component of the hydrologic cycle in terrestrial ecosystems and accurate description of ET processes is essential for developing reliable ecohydrological models. This study investigated the accuracy of ET prediction by the DRAINMOD-FOREST after its calibration/validation for predicting commonly measured hydrological variables. The model was tested by conducting an eight year simulation of drainage and shallow groundwater dynamics in a managed mid-rotation loblolly pine (Pinus taeda L.) plantation located in the coastal plain of North Carolina, USA. Modeled daily ET rates were compared to those measured in the field using the eddy covariance technique. In addition, the wavelet transform and coherence analysis were used to compare ET predictions and measurements on the time-frequency domain. Results showed that DRAINMOD-FOREST accurately predicted annual and monthly ET after a successful calibration and validation using measured drainage rates and water table depth. The model under predicted ET on an annual basis by 2%, while the Nash-Sutcliffe coefficient of model predictions on a monthly basis was 0.78. Results from wavelet transform and coherence analysis demonstrated that the model reasonably captured the high power spectra of ET at an annual scale with significantly high model-data coherency. These results suggested that the calibrated DRAINMOD-FOREST collectively captured key factors and mechanisms controlling ET dynamics in the drained pine plantation. However, the global power spectrum revealed that the model over predicted the power spectrum of ET at an annual scale, suggesting the model may have under predicted canopy conductance during non- growing seasons. In addition, this study also suggested that DRAINMOD-FOREST did not properly capture the seasonal dynamics of ET under extreme drought conditions with deeper water table depths. These results suggested further refinement to parameters, particularly vegetation related, and structures of DRAINMOD-FOREST to achieve better agreement between ET predictions and measurements in the time-frequency domain. Published by Elsevier B.V.

Dead fuel loads were measured on six distinct forest management compartments in North Carolina’s Uwharrie national forest, Croatan national forest and the Alligator River National Wildlife Refuge. Average 1-, 10-, 100- and 1000-hour fuels loads were analyzed within and between each of the three research areas and compared to National Fire Danger Rating System fuel model estimates of dead fuel load. Mean dead fuel load measurements were significantly different within and between most research areas and differences tended to increase with fuel class size. While there was good agreement within and between research areas for woody fuels, the addition of litter and duff generally resulted in larger variability and significantly different dead fuel load measurements. NFDRS fuel load estimates compared well with some classes of measured fuel load, but no one model provided estimates comparable with measured fuel load across all fuel size classes within a site. The models tended to estimate 1- and 10-hour fuels well, but generally underestimated 100- and 1000-hour fuels. Large differences between 100- and 1000-hour fuels were mostly the result of high duff and litter measurements, especially on the sites with deep peat soils. This important component of forest fuel loads may not be well represented in the current NFDRS. As forests become more fragmented and managed for different resource objectives, finer scale fuel load estimates may be necessary to accurately assess fire danger and minimize the loss of life and property.

This report describes the database used to compile, store, and manage intensive ground-based biometric data collected at research sites in Colorado, Minnesota, New Hampshire, New Jersey, North Carolina, and Wyoming, supporting research activities of the U.S. North American Carbon Program (NACP). This report also provides details of each site, the sampling design and collection standards for biometric measurements, the database design, data summary examples, and the uses of intensive ground-based biometric data. Additional information on location descriptions, data, databases, and documentation may be accessed at http://www.nrs.fs.fed.us/data/lcms.

International audience; Timber harvests remove a significant portion of ecosystem carbon. While some of the wood products moved off-site may last past the harvest cycle of the particular forest crop, the effect of the episodic disturbances on long-term on-site carbon sequestration is unclear. The current study presents a 25year carbon budget estimate for a typical commercial loblolly pine plantation in North Carolina, USA, spanning the entire rotation cycle. We use a chronosequence approach, based on 5years of data from two adjacent loblolly pine plantations. We found that while the ecosystem is very productive (GEP up to 2900gm-2yr-1, NEE at maturity about 900g Cm-2yr-1), the production of detritus does not offset the loss of soil C through heterotrophic respiration (RH) on an annual basis. The input of dead roots at harvest may offset the losses, but there remain significant uncertainties about both the size and decomposition dynamics of this pool. The pulse of detritus produced at harvest resulted in a more than 60% increase in RH. Contrary to expectations, the peak of RH in relation to soil respiration (SR) did not occur immediately after the harvest disturbance, but in years3 and 4, suggesting that a pool of roots may have remained alive for the first few years. On the other hand, the pulse of aboveground RH from coarse woody debris lasted only 2years. The postharvest increase in RH was offset by a decrease in autotrophic respiration such that the total ecosystem respiration changed little. The observed flux rates show that even though the soil C pool may not necessarily decrease in the long-term, old soil C is definitely an active component in the site C cycle, contributing about 2530% of the RH over the rotation cycle.

International audience; Increasing variability of rainfall patterns requires detailed understanding of the pathways of water loss from ecosystems to optimize carbon uptake and management choices. In the current study we characterized the usability of three alternative methods of different rigor for quantifying stand-level evapotranspiration (ET), partitioned ET into tree transpiration (T), understory transpiration, interception, and soil evaporation (ES) and determined their sensitivity to drought, and evaluated the reliability of soil moisture measurements by taking into account deep soil moisture dynamic. The analyses were conducted in an early- and in a mid-rotation stand of loblolly pine, the predominant species of southern US forest plantations. The three alternative methods for estimating ET were the eddy covariance measurements of water vapor fluxes (ETEC), the water table fluctuation (ETWT), and the soil moisture fluctuation (ETSM). On annual and monthly scales, the three methods agreed to within 10-20%, whereas on a daily scale, the values of ETSM and ETEC differed by up to 50% and ETSM and ETWT differed by up to 100%. The differences between the methods were attributed to root water extraction below measurement depth and to the sampling at different spatial scales. Regardless of the method used, ET at the early-rotation site was 15-30% lower than that at the mid-rotation site. The dry years did not affect ET at the mid-rotation site but reduced significantly ET at the early-rotation site. Soil moisture trends revealed the importance of measuring water content at several depths throughout the rooting zone because less than 20% of the water is stored in the top 30 cm of soil. Annually, ES represented approximately 9 and 14% of ETEC at the mid-rotation site and the early-rotation site, respectively. At the mid-rotation site, T accounted for approximately 70% of ETEC. Canopy interception was estimated to be 5-10% of annual precipitation and 6-13% of total ETEC. At the early-rotation site, T accounted for only 35% of ETEC. At this site, transpiration from subdominant trees and shrubs represented 40-45% of ETEC, indicating that understory was a significant part of the water budget. We concluded that the eddy covariance method is best for estimating ET at the fine temporal scale (i.e., daily), but other soil moisture and water table-based methods were equally reliable and cost-effective for quantifying seasonal ET dynamics.

*Hydraulic redistribution (HR) of water via roots from moist to drier portions of the soil occurs in many ecosystems, potentially influencing both water use and carbon assimilation. *By measuring soil water content, sap flow and eddy covariance, we investigated the temporal variability of HR in a loblolly pine (Pinus taeda) plantation during months of normal and below-normal precipitation, and examined its effects on tree transpiration, ecosystem water use and carbon exchange. *The occurrence of HR was explained by courses of reverse flow through roots. As the drought progressed, HR maintained soil moisture above 0.15 cm(3) cm(-3) and increased transpiration by 30-50%. HR accounted for 15-25% of measured total site water depletion seasonally, peaking at 1.05 mm d(-1). The understory species depended on water redistributed by the deep-rooted overstory pine trees for their early summer water supply. Modeling carbon flux showed that in the absence of HR, gross ecosystem productivity and net ecosystem exchange could be reduced by 750 and 400 g C m(-2) yr(-1), respectively. *Hydraulic redistribution mitigated the effects of soil drying on understory and stand evapotranspiration and had important implications for net primary productivity by maintaining this whole ecosystem as a carbon sink.

Soils store about 1.5x10^16 g of carbon (C), about as much as terrestrial vegetation and atmosphere combined 1. The complex interplay between factors that regulate C release from soil through respiration is not completely understood, but could potentially exert strong influence on global radiation balance and climate change2. Respiration exerts strong effect on the spatial heterogeneity of terrestrial C cycle 3 and its temporal variation remains more poorly understood compared to gross ecosystem production (GEP) 4. This variation can analytically be attributed to changes in environmental factors, but forecasting individual deviations remains a challenge 4. Here we propose that deviations of the typical covariance pattern of primary environmental drivers (temperature, T, and moisture, presented in this study as volumetric water content, VWC) may affect the deviations of respiratory C loss. Typically, T and VWC are inversely related, with warm periods being generally drier and vice versa, and therefore the stimulating effect of one factor is counterbalanced by unfavorable levels of the other 5. However, should the driving variables be positively related, respiratory carbon release can increase significantly (Supplementary Fig. 1a). This hypothesis is supported by two consecutive years of ecosystem-level and soil carbon exchange data that differed in rain fall periodicity and T-VWC-covariance. With changing climate patterns, including the intensity and frequency of rainfall events, there is the possibility that the covariance patterns of T and VWC may change, and more frequent periods of positive T-VWC covariance may lead to greater loss of soil carbon, and contribute to greater radiative forcing on Earth’s energy budget.

Soils store about 1.5x10^16 g of carbon (C), about as much as terrestrial vegetation and atmosphere combined 1. The complex interplay between factors that regulate C release from soil through respiration is not completely understood, but could potentially exert strong influence on global radiation balance and climate change2. Respiration exerts strong effect on the spatial heterogeneity of terrestrial C cycle 3 and its temporal variation remains more poorly understood compared to gross ecosystem production (GEP) 4. This variation can analytically be attributed to changes in environmental factors, but forecasting individual deviations remains a challenge 4. Here we propose that deviations of the typical covariance pattern of primary environmental drivers (temperature, T, and moisture, presented in this study as volumetric water content, VWC) may affect the deviations of respiratory C loss. Typically, T and VWC are inversely related, with warm periods being generally drier and vice versa, and therefore the stimulating effect of one factor is counterbalanced by unfavorable levels of the other 5. However, should the driving variables be positively related, respiratory carbon release can increase significantly (Supplementary Fig. 1a). This hypothesis is supported by two consecutive years of ecosystem-level and soil carbon exchange data that differed in rain fall periodicity and T-VWC-covariance. With changing climate patterns, including the intensity and frequency of rainfall events, there is the possibility that the covariance patterns of T and VWC may change, and more frequent periods of positive T-VWC covariance may lead to greater loss of soil carbon, and contribute to greater radiative forcing on Earth’s energy budget.

In Sun et al. (2010) we reported the albedos (A) for two loblolly pine plantations in the range of 0.22?0.38 (Table 5 in Sun et al., 2010).