Relationships between individual‐tree mortality and water‐balance variables indicate positive trends in water stress‐induced tree mortality across North America

Accounting for water stress‐induced tree mortality in forest productivity models remains a challenge due to uncertainty in stress tolerance of tree populations. In this study, logistic regression models were developed to assess species‐specific relationships between probability of mortality (P_m) and drought, drawing on 8.1 million observations of change in vital status (m) of individual trees across North America. Drought was defined by standardized (relative) values of soil water content (W_s,z) and reference evapotranspiration (ET_r,z) at each field plot. The models additionally tested for interactions between the water‐balance variables, aridity class of the site (AC), and estimated tree height (h). Considering drought improved model performance in 95 (80) per cent of the 64 tested species during calibration (cross‐validation). On average, sensitivity to relative drought increased with site AC (i.e. aridity). Interaction between water‐balance variables and estimated tree height indicated that drought sensitivity commonly decreased during early height development and increased during late height development, which may reflect expansion of the root system and decreasing whole‐plant, leaf‐specific hydraulic conductance, respectively. Across North America, predictions suggested that changes in the water balance caused mortality to increase from 1.1% yr^?1 in 1951 to 2.0% yr^?1 in 2014 (a net change of 0.9 ± 0.3% yr^?1). Interannual variation in mortality also increased, driven by increasingly severe droughts in 1988, 1998, 2006, 2007 and 2012. With strong confidence, this study indicates that water stress is a common cause of tree mortality. With weak‐to‐moderate confidence, this study strengthens previous claims attributing positive trends in mortality to increasing levels of water stress. This ‘learn‐as‐we‐go’ approach – defined by sampling rare drought events as they continue to intensify – will help to constrain the hydraulic limits of dominant tree species and the viability of boreal and temperate forest biomes under continued climate change.     

Reconciling estimates of the contemporary North American carbon balance among terrestrial biosphere models,atmospheric inversions,and a new approach for estimating net ecosystem exchange from inventory‐based data

We develop an approach for estimating net ecosystem exchange (NEE) using inventory‐based information over North America (NA) for a recent 7‐year period (ca. 2000–2006). The approach notably retains information on the spatial distribution of NEE, or the vertical exchange between land and atmosphere of all non‐fossil fuel sources and sinks of CO₂, while accounting for lateral transfers of forest and crop products as well as their eventual emissions. The total NEE estimate of a ?327 ± 252 TgC yr^?1 sink for NA was driven primarily by CO₂ uptake in the Forest Lands sector (?248 TgC yr^?1), largely in the Northwest and Southeast regions of the US, and in the Crop Lands sector (?297 TgC yr^?1), predominantly in the Midwest US states. These sinks are counteracted by the carbon source estimated for the Other Lands sector (+218 TgC yr^?1), where much of the forest and crop products are assumed to be returned to the atmosphere (through livestock and human consumption). The ecosystems of Mexico are estimated to be a small net source (+18 TgC yr^?1) due to land use change between 1993 and 2002. We compare these inventory‐based estimates with results from a suite of terrestrial biosphere and atmospheric inversion models, where the mean continental‐scale NEE estimate for each ensemble is ?511 TgC yr^?1 and ?931 TgC yr^?1, respectively. In the modeling approaches, all sectors, including Other Lands, were generally estimated to be a carbon sink, driven in part by assumed CO₂ fertilization and/or lack of consideration of carbon sources from disturbances and product emissions. Additional fluxes not measured by the inventories, although highly uncertain, could add an additional ?239 TgC yr^?1 to the inventory‐based NA sink estimate, thus suggesting some convergence with the modeling approaches.     

High retention of 15N‐labeled nitrogen deposition in a nitrogen saturated old‐growth tropical forest

The effects of increased reactive nitrogen (N) deposition in forests depend largely on its fate in the ecosystems. However, our knowledge on the fates of deposited N in tropical forest ecosystems and its retention mechanisms is limited. Here, we report the results from the first whole ecosystem ¹⁵N labeling experiment performed in a N‐rich old‐growth tropical forest in southern China. We added ¹⁵N tracer monthly as ¹⁵NH₄¹⁵NO₃ for 1 year to control plots and to N‐fertilized plots (N‐plots, receiving additions of 50 kg N ha^?1 yr^?1 for 10 years). Tracer recoveries in major ecosystem compartments were quantified 4 months after the last addition. Tracer recoveries in soil solution were monitored monthly to quantify leaching losses. Total tracer recovery in plant and soil (N retention) in the control plots was 72% and similar to those observed in temperate forests. The retention decreased to 52% in the N‐plots. Soil was the dominant sink, retaining 37% and 28% of the labeled N input in the control and N‐plots, respectively. Leaching below 20 cm was 50 kg N ha^?1 yr^?1 in the control plots and was close to the N input (51 kg N ha^?1 yr^?1), indicating N saturation of the top soil. Nitrogen addition increased N leaching to 73 kg N ha^?1 yr^?1. However, of these only 7 and 23 kg N ha^?1 yr^?1 in the control and N‐plots, respectively, originated from the labeled N input. Our findings indicate that deposited N, like in temperate forests, is largely incorporated into plant and soil pools in the short term, although the forest is N‐saturated, but high cycling rates may later release the N for leaching and/or gaseous loss. Thus, N cycling rates rather than short‐term N retention represent the main difference between temperate forests and the studied tropical forest.     

Tree growth and soil acidification in response to 30 years of experimental nitrogen loading on boreal forest

Relations among nitrogen load, soil acidification and forest growth have been evaluated based on short‐term (<15 years) experiments, or on surveys across gradients of N deposition that may also include variations in edaphic conditions and other pollutants, which confound the interpretation of effects of N per se. We report effects on trees and soils in a uniquely long‐term (30 years) experiment with annual N loading on an un‐polluted boreal forest. Ammonium nitrate was added to replicated (N=3) 0.09 ha plots at two doses, N1 and N2, 34 and 68 kg N ha^?1 yr^?1, respectively. A third treatment, N3, 108 kg N ha^?1 yr^?1, was terminated after 20 years, allowing assessment of recovery during 10 years. Tree growth initially responded positively to all N treatments, but the longer term response was highly rate dependent with no gain in N3, a gain of 50 m³ ha^?1 stemwood in N2 and a gain of 100 m³ ha^?1 stemwood in excess of the control (N0) in N1. High N treatments caused losses of up to 70% of exchangeable base cations (Ca²⁺, Mg²⁺, K⁺) in the mineral soil, along with decreases in pH and increases in exchangeable Al³⁺. In contrast, the organic mor‐layer (forest floor) in the N‐treated plots had similar amounts per hectare of exchangeable base cations as in the N0 treatment. Magnesium was even higher in the mor of N‐treated plots, providing evidence of up‐lift by the trees from the mineral soil. Tree growth did not correlate with the soil Ca/Al ratio (a suggested predictor of effects of soil acidity on tree growth). A boron deficiency occurred on N‐treated plots, but was corrected at an early stage. Extractable NH₄⁺ and NO₃^?were high in mor and mineral soils of on‐going N treatments, while NH₄+ was elevated in the mor only in N3 plots. Ten years after termination of N addition in the N3 treatment, the pH had increased significantly in the mineral soil; there were also tendencies of higher soil base status and concentrations of base cations in the foliage. Our data suggest the recovery of soil chemical properties, notably pH, may be quicker after removal of the N‐load than predicted. Our long‐term experiment demonstrated the fundamental importance of the rate of N application relative to the total amount of N applied, in particular with regard to tree growth and C sequestration. Hence, experiments adding high doses of N over short periods do not mimic the long‐term effects of N deposition at lower rates.     

Estimates of soil respiration and net primary production of three forests at different succession stages in South China

Soil respiration (heterotropic and autotropic respiration, R_g) and aboveground litter fall carbon were measured at three forests at different succession (early, middle and advanced) stages in Dinghushan Biosphere Reserve, Southern China. It was found that the soil respiration increases exponentially with soil temperature at 5 cm depth (T_s) according to the relation R_g=a exp(bT_s), and the more advanced forest community during succession has a higher value of a because of higher litter carbon input than the forests at early or middle succession stages. It was also found that the monthly soil respiration is linearly correlated with the aboveground litter carbon input of the previous month. Using measurements of aboveground litter and soil respiration, the net primary productions (NPPs) of three forests were estimated using nonlinear inversion. They are 475, 678 and 1148 g C m^?2 yr^?1 for the Masson pine forest (MPF), coniferous and broad‐leaf mixed forest (MF) and subtropical monsoon evergreen broad‐leaf forest (MEBF), respectively, in year 2003/2004, of which 54%, 37% and 62% are belowground NPP for those three respective forests if no change in live plant biomass is assumed. After taking account of the decrease in live plant biomass, we estimated the NPP of the subtropical MEBF is 970 g C m^?2 yr^?1 in year 2003/2004. Total amount of carbon allocated below ground for plant roots is 388 g C m^?2 yr^?1 for the MPF, 504 g C m^?2 yr^?1 for the coniferous and broad‐leaf MF and 1254 g C m^?2 yr^?1 for the subtropical MEBF in 2003/2004. Our results support the hypothesis that the amount of carbon allocation belowground increases during forest succession.     

The positive net radiative greenhouse gas forcing of increasing methane emissions from a thawing boreal forest‐wetland landscape

At the southern margin of permafrost in North America, climate change causes widespread permafrost thaw. In boreal lowlands, thawing forested permafrost peat plateaus (‘forest’) lead to expansion of permafrost‐free wetlands (‘wetland’). Expanding wetland area with saturated and warmer organic soils is expected to increase landscape methane (CH₄) emissions. Here, we quantify the thaw‐induced increase in CH₄ emissions for a boreal forest‐wetland landscape in the southern Taiga Plains, Canada, and evaluate its impact on net radiative forcing relative to potential long‐term net carbon dioxide (CO₂) exchange. Using nested wetland and landscape eddy covariance net CH₄ flux measurements in combination with flux footprint modeling, we find that landscape CH₄ emissions increase with increasing wetland‐to‐forest ratio. Landscape CH₄ emissions are most sensitive to this ratio during peak emission periods, when wetland soils are up to 10 °C warmer than forest soils. The cumulative growing season (May–October) wetland CH₄ emission of ~13 g CH₄ m^?2 is the dominating contribution to the landscape CH₄ emission of ~7 g CH₄ m^?2. In contrast, forest contributions to landscape CH₄ emissions appear to be negligible. The rapid wetland expansion of 0.26 ± 0.05% yr^?1 in this region causes an estimated growing season increase of 0.034 ± 0.007 g CH₄ m^?2 yr^?1 in landscape CH₄ emissions. A long‐term net CO₂ uptake of >200 g CO₂ m^?2 yr^?1 is required to offset the positive radiative forcing of increasing CH₄ emissions until the end of the 21st century as indicated by an atmospheric CH₄ and CO₂ concentration model. However, long‐term apparent carbon accumulation rates in similar boreal forest‐wetland landscapes and eddy covariance landscape net CO₂ flux measurements suggest a long‐term net CO₂ uptake between 49 and 157 g CO₂ m^?2 yr^?1. Thus, thaw‐induced CH₄ emission increases likely exert a positive net radiative greenhouse gas forcing through the 21st century.     

Environmental controls on net ecosystem-level carbon exchange and productivity in a Central American tropical wet forest

Difficulty in balancing the global carbon budget has lead to increased attention on tropical forests, which have been estimated to account for up to one third of global gross primary production. Whether tropical forests are sources, sinks, or neutral with respect to their carbon balance with the atmosphere remains unclear. To address this issue, estimates of net ecosystem exchange of carbon (NEE) were made for 3 years (1998–2000) using the eddy‐covariance technique in a tropical wet forest in Costa Rica. Measurements were made from a 42 m tower centred in an old‐growth forest. Under unstable conditions, the measurement height was at least twice the estimated zeroplane height from the ground. The canopy at the site is extremely rough; under unstable conditions the median aerodynamic roughness length ranged from 2.4 to 3.6 m. No relationship between NEE and friction velocity (u*) was found using all of the 30‐min averages. However, there was a linear relationship between the nighttime NEE and averaged u* (R² = 0.98). The diurnal pattern of flux was similar to that found in other tropical forests, with mean daytime NEE ca. ? 18 μ mol CO₂ m^?2 s^?1 and mean nighttime NEE 4.6 μ mol CO₂ m^?2 s^?1. However, because ～ 80% of the nighttime data in this forest were collected during low u* conditions ( < 0.2 m s^?1), nighttime NEE was likely underestimated. Using an alternative analysis, mean nighttime NEE increased to 7.05 μ mol CO₂ m^?2 s^?1. There were interannual differences in NEE, but seasonal differences were not apparent. Irradiance accounted for ～ 51% of the variation in the daytime fluxes, with temperature and vapour pressure deficit together accounting for another ～ 20%. Light compensation points ranged from 100 to 207 μ mol PPFD m^?2 s^?1. No was relationship was found between 30‐min nighttime NEE and tower‐top air temperature. A weak relationship was found between hourly nighttime NEE and canopy air temperature using data averaged hourly over the entire sampling period (Q₁₀ = 1.79, R² = 0.17). The contribution of below‐sensor storage was fairly constant from day to day. Our data indicate that this forest was a slight carbon source in 1998 (0.05 to ?1.33 t C ha^?1 yr^?1), a moderate sink in 1999 (?1.53 to ?3.14 t C ha^?1 yr^?1), and a strong sink in 2000 (?5.97 to ?7.92 t C ha^?1 yr^?1). This trend is interpreted as relating to the dissipation of warm‐phase El Niño effects over the course of this study.     

Declining trend of carbon in Finnish cropland soils in 1974–2009

Soil organic matter not only affects soil properties and productivity but also has an essential role in global carbon (C) cycle. We studied changes in the topsoil C content of Finnish croplands using a dataset produced in nationwide soil monitoring. The monitoring network consisting of fields on both mineral and organic soils was established in 1974 and resampled in 1987, 1998, and 2009. Over the monitoring period from 1974 to 2009, cultivated soils showed a continuous decline in C concentration (g kg^?1). In organic soils, C concentration decreased at a mean rate of 0.2–0.3% yr^?1 relative to the existing C concentration. In mineral soils, the relative decrease was 0.4% yr^?1 corresponding to a C stock (kg m^?2) loss of 220 kg ha^?1 yr^?1. The change in management practices in last decades toward increasing cultivation of annual crops has contributed to soil C losses noted in this study. The results, however, suggest that the C losses result partly from other processes affecting cultivated soils such as climatic change or the continuing long‐term effect of forest clearance. We estimated that Finnish cropland soils store 161 Tg carbon nationwide in the topmost 15 cm of which 117 Tg is in mineral soils. C losses from mineral soils can therefore total up to 0.5 Tg yearly.     

Tree seed rain and seed removal,but not the seed bank,impede forest recovery in bracken (Pteridium aquilinum (L.) Kuhn)‐dominated clearings in the African highlands

Considerable areas dominated by bracken Pteridium aquilinum (L.) Kuhn occur worldwide and are associated with arrested forest recovery. How forest recovery is impeded in these areas remains poorly understood, especially in the African highlands. The component processes that can lead to recruitment limitation—including low seed arrival, availability and persistence—are important determinants of plant communities and offer a potential explanation for bracken persistence. We investigated key processes that can contribute to recruitment limitation in bracken‐dominated clearings in the Bwindi Impenetrable National Park, Uganda. We examined if differences in seed rain (dispersal limitation), soil seed bank, or seed removal (seed viability and persistence) can, individually or in combination, explain the differences in tree regeneration found between bracken‐dominated areas and the neighboring forest. These processes were assessed along ten 50‐m transects crossing the forest–bracken boundary. When compared to the neighboring forest, bracken clearings had fewer seedlings (bracken 11,557 ± 5482 vs. forest 34,515 ± 6066 seedlings/ha), lower seed rain (949 ± 582 vs. 1605 ± 335 tree seeds m^?2 year^?1), comparable but sparse soil seed bank (304 ± 236 vs. 264 ± 99 viable tree seeds/m²), higher seed removal (70.1% ± 2.4% vs. 40.6% ± 2.4% over a 3‐day interval), and markedly higher rodent densities (25.7 ± 5.4 vs. 5.0 ± 1.6 rodents per 100 trapping sessions). Camera traps revealed that rodents were the dominant animals visiting the seeds in our seed removal study. Synthesis: Recruitment limitation contributes to both the slow recovery of forest in bracken‐dominated areas, and to the composition of the tree species that occur. Low seed arrival and low persistence of unburied seeds can both explain the reduced density of seedlings found in bracken versus neighboring forest. Seed removal, likely due to rodents, in particular appears sufficient to constrain forest recovery and impacts some species more severely than others.     

Effects of seasonality,transport pathway,and spatial structure on greenhouse gas fluxes in a restored wetland

Wetlands can influence global climate via greenhouse gas (GHG) exchange of carbon dioxide (CO₂), methane (CH₄), and nitrous oxide (N₂O). Few studies have quantified the full GHG budget of wetlands due to the high spatial and temporal variability of fluxes. We report annual open‐water diffusion and ebullition fluxes of CO₂, CH₄, and N₂O from a restored emergent marsh ecosystem. We combined these data with concurrent eddy‐covariance measurements of whole‐ecosystem CO₂ and CH₄ exchange to estimate GHG fluxes and associated radiative forcing effects for the whole wetland, and separately for open‐water and vegetated cover types. Annual open‐water CO₂, CH₄, and N₂O emissions were 915 ± 95 g C‐CO₂ m^?2 yr^?1, 2.9 ± 0.5 g C‐CH₄ m^?2 yr^?1, and 62 ± 17 mg N‐N₂O m^?2 yr^?1, respectively. Diffusion dominated open‐water GHG transport, accounting for >99% of CO₂ and N₂O emissions, and ~71% of CH₄ emissions. Seasonality was minor for CO₂ emissions, whereas CH₄ and N₂O fluxes displayed strong and asynchronous seasonal dynamics. Notably, the overall radiative forcing of open‐water fluxes (3.5 ± 0.3 kg CO₂‐eq m^?2 yr^?1) exceeded that of vegetated zones (1.4 ± 0.4 kg CO₂‐eq m^?2 yr^?1) due to high ecosystem respiration. After scaling results to the entire wetland using object‐based cover classification of remote sensing imagery, net uptake of CO₂ (?1.4 ± 0.6 kt CO₂‐eq yr^?1) did not offset CH₄ emission (3.7 ± 0.03 kt CO₂‐eq yr^?1), producing an overall positive radiative forcing effect of 2.4 ± 0.3 kt CO₂‐eq yr^?1. These results demonstrate clear effects of seasonality, spatial structure, and transport pathway on the magnitude and composition of wetland GHG emissions, and the efficacy of multiscale flux measurement to overcome challenges of wetland heterogeneity.     

Quantifying above‐ and belowground biomass carbon loss with forest conversion in tropical lowlands of Sumatra (Indonesia)

Natural forests in South‐East Asia have been extensively converted into other land‐use systems in the past decades and still show high deforestation rates. Historically, lowland forests have been converted into rubber forests, but more recently, the dominant conversion is into oil palm plantations. While it is expected that the large‐scale conversion has strong effects on the carbon cycle, detailed studies quantifying carbon pools and total net primary production (NPP_total) in above‐ and belowground tree biomass in land‐use systems replacing rainforest (incl. oil palm plantations) are rare so far. We measured above‐ and belowground carbon pools in tree biomass together with NPP_total in natural old‐growth forests, ‘jungle rubber’ agroforests under natural tree cover, and rubber and oil palm monocultures in Sumatra. In total, 32 stands (eight plot replicates per land‐use system) were studied in two different regions. Total tree biomass in the natural forest (mean: 384 Mg ha^?1) was more than two times higher than in jungle rubber stands (147 Mg ha^?1) and >four times higher than in monoculture rubber and oil palm plantations (78 and 50 Mg ha^?1). NPP_total was higher in the natural forest (24 Mg ha^?1 yr^?1) than in the rubber systems (20 and 15 Mg ha^?1 yr^?1), but was highest in the oil palm system (33 Mg ha^?1 yr^?1) due to very high fruit production (15–20 Mg ha^?1 yr^?1). NPP_total was dominated in all systems by aboveground production, but belowground productivity was significantly higher in the natural forest and jungle rubber than in plantations. We conclude that conversion of natural lowland forest into different agricultural systems leads to a strong reduction not only in the biomass carbon pool (up to 166 Mg C ha^?1) but also in carbon sequestration as carbon residence time (i.e. biomass‐C:NPP‐C) was 3–10 times higher in the natural forest than in rubber and oil palm plantations.     

Nitrogen deposition outweighs climatic variability in driving annual growth rate of canopy beech trees: Evidence from long‐term growth reconstruction across a geographic gradient

In this study, we investigated the role of climatic variability and atmospheric nitrogen deposition in driving long‐term tree growth in canopy beech trees along a geographic gradient in the montane belt of the Italian peninsula, from the Alps to the southern Apennines. We sampled dominant trees at different developmental stages (from young to mature tree cohorts, with tree ages spanning from 35 to 160 years) and used stem analysis to infer historic reconstruction of tree volume and dominant height. Annual growth volume (G_V) and height (G_H) variability were related to annual variability in model simulated atmospheric nitrogen deposition and site‐specific climatic variables, (i.e. mean annual temperature, total annual precipitation, mean growing period temperature, total growing period precipitation, and standard precipitation evapotranspiration index) and atmospheric CO₂ concentration, including tree cambial age among growth predictors. Generalized additive models (GAM), linear mixed‐effects models (LMM), and Bayesian regression models (BRM) were independently employed to assess explanatory variables. The main results from our study were as follows: (i) tree age was the main explanatory variable for long‐term growth variability; (ii) GAM, LMM, and BRM results consistently indicated climatic variables and CO₂ effects on G_V and G_H were weak, therefore evidence of recent climatic variability influence on beech annual growth rates was limited in the montane belt of the Italian peninsula; (iii) instead, significant positive nitrogen deposition (N_dep) effects were repeatedly observed in G_V and G_H; the positive effects of N_dep on canopy height growth rates, which tended to level off at N_dep values greater than approximately 1.0 g m^?2 y^?1, were interpreted as positive impacts on forest stand above‐ground net productivity at the selected study sites.     

Evaluation of terrestrial carbon cycle models for their response to climate variability and to CO2 trends

The purpose of this study was to evaluate 10 process‐based terrestrial biosphere models that were used for the IPCC fifth Assessment Report. The simulated gross primary productivity (GPP) is compared with flux‐tower‐based estimates by Jung et al. [Journal of Geophysical Research 116 (2011) G00J07] (JU11). The net primary productivity (NPP) apparent sensitivity to climate variability and atmospheric CO₂ trends is diagnosed from each model output, using statistical functions. The temperature sensitivity is compared against ecosystem field warming experiments results. The CO₂ sensitivity of NPP is compared to the results from four Free‐Air CO₂ Enrichment (FACE) experiments. The simulated global net biome productivity (NBP) is compared with the residual land sink (RLS) of the global carbon budget from Friedlingstein et al. [Nature Geoscience 3 (2010) 811] (FR10). We found that models produce a higher GPP (133 ± 15 Pg C yr^?1) than JU11 (118 ± 6 Pg C yr^?1). In response to rising atmospheric CO₂ concentration, modeled NPP increases on average by 16% (5–20%) per 100 ppm, a slightly larger apparent sensitivity of NPP to CO₂ than that measured at the FACE experiment locations (13% per 100 ppm). Global NBP differs markedly among individual models, although the mean value of 2.0 ± 0.8 Pg C yr^?1 is remarkably close to the mean value of RLS (2.1 ± 1.2 Pg C yr^?1). The interannual variability in modeled NBP is significantly correlated with that of RLS for the period 1980–2009. Both model‐to‐model and interannual variation in model GPP is larger than that in model NBP due to the strong coupling causing a positive correlation between ecosystem respiration and GPP in the model. The average linear regression slope of global NBP vs. temperature across the 10 models is ?3.0 ± 1.5 Pg C yr^?1 °C^?1, within the uncertainty of what derived from RLS (?3.9 ± 1.1 Pg C yr^?1 °C^?1). However, 9 of 10 models overestimate the regression slope of NBP vs. precipitation, compared with the slope of the observed RLS vs. precipitation. With most models lacking processes that control GPP and NBP in addition to CO₂ and climate, the agreement between modeled and observation‐based GPP and NBP can be fortuitous. Carbon–nitrogen interactions (only separable in one model) significantly influence the simulated response of carbon cycle to temperature and atmospheric CO₂ concentration, suggesting that nutrients limitations should be included in the next generation of terrestrial biosphere models.     

The role of harvest residue in rotation cycle carbon balance in loblolly pine plantations. Respiration partitioning approach

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 25 year 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 5 years of data from two adjacent loblolly pine plantations. We found that while the ecosystem is very productive (GEP up to 2900 g m^?2 yr^?1, NEE at maturity about 900 g C m^?2 yr^?1), the production of detritus does not offset the loss of soil C through heterotrophic respiration (R_H) 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 R_H. Contrary to expectations, the peak of R_H in relation to soil respiration (SR) did not occur immediately after the harvest disturbance, but in years 3 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 R_H from coarse woody debris lasted only 2 years. The postharvest increase in R_H 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 25–30% of the R_H over the rotation cycle.     

Reconstruction and attribution of the carbon sink of European forests between 1950 and 2000

European forests are an important carbon sink; however, the relative contributions to this sink of climate, atmospheric CO₂ concentration ([CO₂]), nitrogen deposition and forest management are under debate. We attributed the European carbon sink in forests using ORCHIDEE‐FM, a process‐based vegetation model that differs from earlier versions of ORCHIDEE by its explicit representation of stand growth and idealized forest management. The model was applied on a grid across Europe to simulate changes in the net ecosystem productivity (NEP) of forests with and without changes in climate, [CO₂] and age structure, the three drivers represented in ORCHIDEE‐FM. The model simulates carbon stocks and volume increment that are comparable – root mean square error of 2 m³ ha^?1 yr^?1 and 1.7 kg C m^?2 respectively – with inventory‐derived estimates at country level for 20 European countries. Our simulations estimate a mean European forest NEP of 175 ± 52 g C m^?2 yr^?1 in the 1990s. The model simulation that is most consistent with inventory records provides an upwards trend of forest NEP of 1 ± 0.5 g C m^?2 yr^?2 between 1950 and 2000 across the EU 25. Furthermore, the method used for reconstructing past age structure was found to dominate its contribution to temporal trends in NEP. The potentially large fertilizing effect of nitrogen deposition cannot be told apart, as the model does not explicitly simulate the nitrogen cycle. Among the three drivers that were considered in this study, the fertilizing effect of increasing [CO₂] explains about 61% of the simulated trend, against 26% to changes in climate and 13% only to changes in forest age structure. The major role of [CO₂] at the continental scale is due to its homogeneous impact on net primary productivity (NPP). At the local scale, however, changes in climate and forest age structure often dominate trends in NEP by affecting NPP and heterotrophic respiration.     

Methane emissions from rice paddies natural wetlands,lakes in China: synthesis new estimate

Sources of methane (CH₄) become highly variable for countries undergoing a heightened period of development due to both human activity and climate change. An urgent need therefore exists to budget key sources of CH₄, such as wetlands (rice paddies and natural wetlands) and lakes (including reservoirs and ponds), which are sensitive to these changes. For this study, references in relation to CH₄ emissions from rice paddies, natural wetlands, and lakes in China were first reviewed and then reestimated based on the review itself. Total emissions from the three CH₄ sources were 11.25 Tg CH₄ yr^?1 (ranging from 7.98 to 15.16 Tg CH₄ yr^?1). Among the emissions, 8.11 Tg CH₄ yr^?1 (ranging from 5.20 to 11.36 Tg CH₄ yr^?1) derived from rice paddies, 2.69 Tg CH₄ yr^?1 (ranging from 2.46 to 3.20 Tg CH₄ yr^?1) from natural wetlands, and 0.46 Tg CH₄ yr^?1 (ranging from 0.33 to 0.59 Tg CH₄ yr^?1) from lakes (including reservoirs and ponds). Plentiful water and warm conditions, as well as its large rice paddy area make rice paddies in southeastern China the greatest overall source of CH₄, accounting for approximately 55% of total paddy emissions. Natural wetland estimates were slightly higher than the other estimates owing to the higher CH₄ emissions recorded within Qinghai‐Tibetan Plateau peatlands. Total CH₄ emissions from lakes were estimated for the first time by this study, with three quarters from the littoral zone and one quarter from lake surfaces. Rice paddies, natural wetlands, and lakes are not constant sources of CH₄, but decreasing ones influenced by anthropogenic activity and climate change. A new progress‐based model used in conjunction with more observations through model‐data fusion approach could help obtain better estimates and insights with regard to CH₄ emissions deriving from wetlands and lakes in China.     

Detection and attribution of vegetation greening trend in China over the last 30 years

The reliable detection and attribution of changes in vegetation growth is a prerequisite for the development of strategies for the sustainable management of ecosystems. This is an extraordinary challenge. To our knowledge, this study is the first to comprehensively detect and attribute a greening trend in China over the last three decades. We use three different satellite‐derived Leaf Area Index (LAI) datasets for detection as well as five different process‐based ecosystem models for attribution. Rising atmospheric CO₂ concentration and nitrogen deposition are identified as the most likely causes of the greening trend in China, explaining 85% and 41% of the average growing‐season LAI trend (LAI_GS) estimated by satellite datasets (average trend of 0.0070 yr^?1, ranging from 0.0035 yr^?1 to 0.0127 yr^?1), respectively. The contribution of nitrogen deposition is more clearly seen in southern China than in the north of the country. Models disagree about the contribution of climate change alone to the trend in LAI_GS at the country scale (one model shows a significant increasing trend, whereas two others show significant decreasing trends). However, the models generally agree on the negative impacts of climate change in north China and Inner Mongolia and the positive impact in the Qinghai–Xizang plateau. Provincial forest area change tends to be significantly correlated with the trend of LAI_GS (P < 0.05), and marginally significantly (P = 0.07) correlated with the residual of LAI_GS trend, calculated as the trend observed by satellite minus that estimated by models through considering the effects of climate change, rising CO₂ concentration and nitrogen deposition, across different provinces. This result highlights the important role of China's afforestation program in explaining the spatial patterns of trend in vegetation growth.     

Limited effect of ozone reductions on the 20‐year photosynthesis trend at Harvard forest

Ozone (O₃) damage to leaves can reduce plant photosynthesis, which suggests that declines in ambient O₃ concentrations ([O₃]) in the United States may have helped increase gross primary production (GPP) in recent decades. Here, we assess the effect of long‐term changes in ambient [O₃] using 20 years of observations at Harvard forest. Using artificial neural networks, we found that the effect of the inclusion of [O₃] as a predictor was slight, and independent of O₃ concentrations, which suggests limited high‐frequency O₃ inhibition of GPP at this site. Simulations with a terrestrial biosphere model, however, suggest an average long‐term O₃ inhibition of 10.4% for 1992–2011. A decline of [O₃] over the measurement period resulted in moderate predicted GPP trends of 0.02–0.04 μmol C m^?2 s^?1 yr^?1, which is negligible relative to the total observed GPP trend of 0.41 μmol C m^?2 s^?1 yr^?1. A similar conclusion is achieved with the widely used AOT40 metric. Combined, our results suggest that ozone reductions at Harvard forest are unlikely to have had a large impact on the photosynthesis trend over the past 20 years. Such limited effects are mainly related to the slow responses of photosynthesis to changes in [O₃]. Furthermore, we estimate that 40% of photosynthesis happens in the shade, where stomatal conductance and thus [O₃] deposition is lower than for sunlit leaves. This portion of GPP remains unaffected by [O₃], thus helping to buffer the changes of total photosynthesis due to varied [O₃]. Our analyses suggest that current ozone reductions, although significant, cannot substantially alleviate the damages to forest ecosystems.     

Land use change emission scenarios: anticipating a forest transition process in the Brazilian Amazon

Following an intense occupation process that was initiated in the 1960s, deforestation rates in the Brazilian Amazon have decreased significantly since 2004, stabilizing around 6000 km² yr^?1 in the last 5 years. A convergence of conditions contributed to this, including the creation of protected areas, the use of effective monitoring systems, and credit restriction mechanisms. Nevertheless, other threats remain, including the rapidly expanding global markets for agricultural commodities, large‐scale transportation and energy infrastructure projects, and weak institutions. We propose three updated qualitative and quantitative land‐use scenarios for the Brazilian Amazon, including a normative ‘Sustainability’ scenario in which we envision major socio‐economic, institutional, and environmental achievements in the region. We developed an innovative spatially explicit modelling approach capable of representing alternative pathways of the clear‐cut deforestation, secondary vegetation dynamics, and the old‐growth forest degradation. We use the computational models to estimate net deforestation‐driven carbon emissions for the different scenarios. The region would become a sink of carbon after 2020 in a scenario of residual deforestation (~1000 km² yr^?1) and a change in the current dynamics of the secondary vegetation – in a forest transition scenario. However, our results also show that the continuation of the current situation of relatively low deforestation rates and short life cycle of the secondary vegetation would maintain the region as a source of CO₂ – even if a large portion of the deforested area is covered by secondary vegetation. In relation to the old‐growth forest degradation process, we estimated average gross emission corresponding to 47% of the clear‐cut deforestation from 2007 to 2013 (using the DEGRAD system data), although the aggregate effects of the postdisturbance regeneration can partially offset these emissions. Both processes (secondary vegetation and forest degradation) need to be better understood as they potentially will play a decisive role in the future regional carbon balance.     

Greenhouse gas emissions from dairy manure management: a review of field‐based studies

Livestock manure management accounts for almost 10% of greenhouse gas emissions from agriculture globally, and contributes an equal proportion to the US methane emission inventory. Current emissions inventories use emissions factors determined from small‐scale laboratory experiments that have not been compared to field‐scale measurements. We compiled published data on field‐scale measurements of greenhouse gas emissions from working and research dairies and compared these to rates predicted by the IPCC Tier 2 modeling approach. Anaerobic lagoons were the largest source of methane (368 ± 193 kg CH₄ hd^?1 yr^?1), more than three times that from enteric fermentation (~120 kg CH₄ hd^?1 yr^?1). Corrals and solid manure piles were large sources of nitrous oxide (1.5 ± 0.8 and 1.1 ± 0.7 kg N₂O hd^?1 yr^?1, respectively). Nitrous oxide emissions from anaerobic lagoons (0.9 ± 0.5 kg N₂O hd^?1 yr^?1) and barns (10 ± 6 kg N₂O hd^?1 yr^?1) were unexpectedly large. Modeled methane emissions underestimated field measurement means for most manure management practices. Modeled nitrous oxide emissions underestimated field measurement means for anaerobic lagoons and manure piles, but overestimated emissions from slurry storage. Revised emissions factors nearly doubled slurry CH₄ emissions for Europe and increased N₂O emissions from solid piles and lagoons in the United States by an order of magnitude. Our results suggest that current greenhouse gas emission factors generally underestimate emissions from dairy manure and highlight liquid manure systems as promising target areas for greenhouse gas mitigation.