Net Carbon Source Research Articles

Forests out of balance

Carbon Cycle![Figure][1] Twelve years of data show that tropical forests are a net carbon source. PHOTO: MVALIGURSKY/ISTOCKPHOTO Are tropical forests a net source or net sink of atmospheric carbon dioxide? As fundamental a question as that is, there still is no agreement about the answer,

Tropical forests are a net carbon source based on aboveground measurements of gain and loss.

The carbon balance of tropical ecosystems remains uncertain, with top-down atmospheric studies suggesting an overall sink and bottom-up ecological approaches indicating a modest net source. Here we use 12 years (2003 to 2014) of MODIS pantropical satellite data to quantify net annual changes in the aboveground carbon density of tropical woody live vegetation, providing direct, measurement-based evidence that the world's tropical forests are a net carbon source of 425.2 ± 92.0 teragrams of carbon per year (Tg C year-1). This net release of carbon consists of losses of 861.7 ± 80.2 Tg C year-1 and gains of 436.5 ± 31.0 Tg C year-1 Gains result from forest growth; losses result from deforestation and from reductions in carbon density within standing forests (degradation or disturbance), with the latter accounting for 68.9% of overall losses.

Improving predictions of tropical forest response to climate change through integration of field studies and ecosystem modeling.

Tropical forests play a critical role in carbon and water cycles at a global scale. Rapid climate change is anticipated in tropical regions over the coming decades and, under a warmer and drier climate, tropical forests are likely to be net sources of carbon rather than sinks. However, our understanding of tropical forest response and feedback to climate change is very limited. Efforts to model climate change impacts on carbon fluxes in tropical forests have not reached a consensus. Here, we use the Ecosystem Demography model (ED2) to predict carbon fluxes of a Puerto Rican tropical forest under realistic climate change scenarios. We parameterized ED2 with species-specific tree physiological data using the Predictive Ecosystem Analyzer workflow and projected the fate of this ecosystem under five future climate scenarios. The model successfully captured interannual variability in the dynamics of this tropical forest. Model predictions closely followed observed values across a wide range of metrics including aboveground biomass, tree diameter growth, tree size class distributions, and leaf area index. Under a future warming and drying climate scenario, the model predicted reductions in carbon storage and tree growth, together with large shifts in forest community composition and structure. Such rapid changes in climate led the forest to transition from a sink to a source of carbon. Growth respiration and root allocation parameters were responsible for the highest fraction of predictive uncertainty in modeled biomass, highlighting the need to target these processes in future data collection. Our study is the first effort to rely on Bayesian model calibration and synthesis to elucidate the key physiological parameters that drive uncertainty in tropical forests responses to climatic change. We propose a new path forward for model-data synthesis that can substantially reduce uncertainty in our ability to model tropical forest responses to future climate.

Evaluation of the DNDC model for simulating soil temperature, moisture and respiration from monoculture and rotational corn, soybean and winter wheat in Canada

Process-based modelling studies can help to interpret experimental results and gain more insights into CO2 fluxes in agro-ecosystems. The objective of this study was to evaluate the ability of the DeNitrification-DeComposition (DNDC) model to predict field-measured soil temperature, moisture, and respiration (daily and seasonal) in monoculture versus rotational cropping systems. Field measurements (2003–2005) from monoculture cropping (corn, Zea mays L.; soybean, Glycine max L.; and winter wheat, Triticum aestivum L.), as well as from 2-yr (corn-soybean) and 3-yr (corn-soybean-winter wheat) crop rotations in southwestern Ontario, Canada were used for model calibration and evaluation. Model simulated soil (0–10cm) temperature (nRMSE=8–9%, d=0.97 and EF=0.87–0.90) and moisture (nRMSE=15–22%, d≥0.7 and EF≥0) agreed well with the growing season field measurements. Predicted daily soil CO2 fluxes were accurate for corn (under both monoculture and rotation) and winter wheat (under monoculture) (nRMSE=56–70%, d>0.7, slope=0.60–0.88, R2=0.36–0.48). The overestimation in daily soil CO2 fluxes for winter wheat phase under the 3-yr rotation could be addressed by decreasing the root: shoot ratio. The simulated temporal offset in the soil CO2 fluxes for soybean could be improved by adjusting crop parameters (thermal degree days for crop maturity and root: shoot ratio). The simulated cumulative seasonal soil CO2 fluxes were not statistically different from the measured CO2 fluxes for corn, soybean and winter wheat as indicated by the paired-t test (p=0.17–1.00). The impacts of monoculture and rotation cropping on seasonal root autotrophic respiration (RA), soil heterotrophic respiration (RH) and RH/RS ratios for corn, soybean and winter wheat were attributed to the different biomass production and residue C input management systems. The RH contributed more to total soil respiration for corn (during the growing season) under monoculture (RH/RS=42%) than when it was grown in rotation (26–29%), while ratios of RH/RS were similar among monocultural and rotational soybean (41–49%) and winter wheat (31–40%). The current litter C level and consequently annual RH were driven by the litter C input carried over from previous crop. When carbon in the harvested grain was taken into account in the carbon budget, corn would act as a strong net carbon sink (3year average of −975, −2675 and −3151kgCha−1yr−1 under monoculture, 2-yr and 3-yr rotations, respectively), while soybean and winter wheat would act as weak net carbon source under both monoculture and rotation. The DNDC model was found to be able to simulate soil temperature, moisture, and soil respiration for corn and winter wheat under both monoculture and rotation in humid southwestern Ontario, Canada but crop parameter adjustments are required to simulate soybean.

River sequesters atmospheric carbon and limits the CO2 degassing in karst area, southwest China

CO2 fluxes across water-air interfaces of river systems play important roles in regulating the regional and global carbon cycle. However, great uncertainty remains as to the contribution of these inland water bodies to the global carbon budget. Part of the uncertainty stems from limited understanding of the CO2 fluxes at diurnal and seasonal frequencies caused by aquatic metabolism. Here, we measured surface water characteristics (temperature, pH, and DO, DIC, Ca2+ concentrations) and CO2 fluxes across the air-water interface at two transects of Guijiang River, southwest China to assess the seasonal and diurnal dynamics of fluvial carbon cycling and its potential role in regional and global carbon budgets. The two transects had differing bedrock; DM transect is underlain by carbonate and detrital rock and PY is underlain by pure carbonate. Our results show that the river water both degasses CO2 to and absorbs CO2 from the atmosphere in both summer and winter, but the degassing and absorption varied between the two transects. Further, CO2 fluxes evolve through diurnal cycles. At DM, the river evaded CO2 from early morning through noon and absorbed CO2 from afternoon through early morning. At PY in summer, the CO2 evasion decreased during the daytime and increased at night while in winter at night, CO2 uptake increased in the morning and decreased in the afternoon but remained relatively stable at night. Although the river is a net source of carbon to the atmosphere (~15mMm−2day−1), the evasion rate is the smallest of all reported world's inland water bodies reflecting sequestration of atmospheric carbon through the carbonate dissolution and high primary productivity. These results emphasize the need of seasonal and diurnal monitoring of CO2 fluxes across water-air interface, particularly in highly productive rivers, to reduce uncertainty in current estimates of global riverine CO2 emission.

Carbon and Water Budgets in Multiple Wheat-Based Cropping Systems in the Inland Pacific Northwest US: Comparison of CropSyst Simulations with Eddy Covariance Measurements

Accurate carbon and water flux simulations for croplands are greatly dependent on high quality representation of management practices and meteorological conditions, which are key drivers of the surface-atmosphere exchange processes. Fourteen site-years of carbon and water fluxes were simulated using the CropSyst model over four agricultural sites in the inland Pacific Northwest US from October 1, 2011 to September 30, 2015. Model performance for field-scale net ecosystem exchange of CO2 (NEE) and evapotranspiration (ET) was evaluated by comparing simulations with long-term eddy covariance measurements. The model captured the temporal variations of NEE and ET reasonably well with an overall r of 0.78 and 0.80, and a low RMSE of 1.82 g C m-2 d-1 and 0.84 mm d-1 for NEE and ET, respectively. The model slightly underestimated NEE and ET by 0.51 g C m-2 d-1 and 0.09 mm d-1, respectively. ET simulations showed better agreement with eddy covariance measurements than NEE. The model performed much better for the sites with detailed initial conditions (e.g. SOC content) and management practice information (e.g. tillage type). The CropSyst results showed that the winter wheat fields could be annual net carbon sinks or close to neutral with the net ecosystem carbon balance (NECB) ranging from 92 to -17 g C m-2, while the spring crop fields were net carbon sources or neutral with an annual NECB of -327 to -3 g C m-2. Simulations for the paired tillage sites showed that the no-till site resulted in lower CO2 emissions for the crop rotations of winter wheat-spring garbanzo, but had higher carbon loss into the atmosphere for spring canola compared to the conventional tillage site. Water budgets did not differ significantly between the two tillage systems. Winter wheat in the high-rainfall area had higher crop yields and water use efficiency but emitted larger amounts of CO2 into the atmosphere than in the low-rainfall area. Based on model evaluations in this study, CropSyst appears promising as a tool to simulate field-scale carbon and water budgets and assess the effects of different management practices and local meteorological conditions for the wheat-based cropping systems in this region.

Net CO2 emissions from a primary boreo-nemoral forest over a 10 year period

Primary forests play an important role in the global carbon balance. With little to no human intervention, primary forests are shaped and characterised by disturbances such as weather extremes, fire, insect and pathogen attacks. Such disturbances have a direct impact on the volume of coarse woody debris (CWD) which contributes to the total ecosystem respiration (Re). There are currently few studies that present continuous long term measurements of the carbon balance of northern primary forests. We used the eddy covariance method to measure continuous carbon dioxide (CO2) fluxes from a Swedish primary boreo-nemoral forest over a ten year period. By mapping the measured CO2 fluxes to the forest ecosystem we could indicate that small areas that had some form of disturbance and areas with significant levels of CWD within the eddy covariance footprint contributed to the total Re resulting in the forest being a net carbon source. A weighing algorithm was used to account for directional ecosystem heterogeneity and to estimate a representative carbon balance for the ecosystem. The forest ecosystem was a continuous source of CO2 to the atmosphere, losing around 25Mg per hectare of CO2 over a ten year period.

Carbon dioxide emissions from an oligotrophic temperate lake: An eddy covariance approach

In recent years, several studies have suggested that most lakes are net carbon sources to the atmosphere rather than carbon sinks. Quantifying and understanding the environmental drivers of carbon dioxide (CO2) flux from lakes is important in order to have a better understanding of the current and future greenhouse-gas budget of aquatic systems and the global ecosystem as a whole. In this study, we present observations of CO2 fluxes in an oligotrophic lake in Northern Michigan during two full growing seasons. We used the eddy covariance technique to measure continuous fluxes of CO2 and calculate the advective fluxes between the lake and the surrounding forest. We found that, at our measurement location far from shore, to the effects of horizontal advection were significantly lower than EC-observed vertical turbulent fluxes and contributed minimally to estimate of the seasonal totals. We found that during the summers the lake was an overall net carbon source, though at rates at much lower magnitude than nearby terrestrial ecosystems. Using a hierarchical modelling approach, we determined that net carbon flux from the lake is primarily correlated with wind speed, indicating the key role of mixing in the upper water layer. Variables indicative of microbial activity and lake gas storage were more highly correlated with the positive fraction of carbon flux than with carbon uptake.

The impact of cultivation on CO2 and CH4 fluxes over organic soils in Sweden

Cultivated organic soils are large contributors to global greenhouse gas emissions. We measured carbon dioxide (CO2) and methane (CH4) fluxes from two organic soil plots in central Sweden between 2012 and 2016 using the eddy covariance technique. One site was cultivated whilst the other was set aside with no cultivation occurring. The aim was to establish the impact that cultivation had on the carbon balance. Over the five year period the set aside field acted as a net carbon source whilst the cultivated site was seen to be a small carbon sink. The set aside was further seen to act as a small methane sink over a 36 month period, which reduced the CO2 source by ca. 10% in terms of CO2 equivalents. Considering exported biomass, both sites were carbon sources in terms of Net Biome Production (NBP).

Experimental warming reveals positive feedbacks to climate change in the Eurasian Steppe.

Identifying soil microbial feedbacks to increasing temperatures and moisture alterations is critical for predicting how terrestrial ecosystems will respond to climate change. We performed a 5-year field experiment manipulating warming, watering and their combination in a semiarid temperate steppe in northern China. Warming stimulated the abundance of genes responsible for degrading recalcitrant soil organic matter (SOM) and reduced SOM content by 13%. Watering, and warming plus watering also increased the abundance of recalcitrant SOM catabolism pathways, but concurrently promoted plant growth and increased labile SOM content, which somewhat offset SOM loss. The treatments also increased microbial biomass, community complexity and metabolic potential for nitrogen and sulfur assimilation. Both microbial and plant community composition shifted with the treatment conditions, and the sample-to-sample compositional variations of the two communities (pairwise β-diversity distances) were significantly correlated. In particular, microbial community composition was substantially correlated with the dominant plant species (~0.54 Spearman correlation coefficient), much more than with measured soil indices, affirming a tight coupling between both biological communities. Collectively, our study revealed the direction and underlying mechanisms of microbial feedbacks to warming and suggested that semiarid regions of northern steppes could act as a net carbon source under increased temperatures, unless precipitation increases concurrently.

Effects of irrigation and addition of nitrogen fertiliser on net ecosystem carbon balance for a grassland.

The ability to quantify the impacts of changing management practices on the components of net ecosystem carbon balance (NB) is required to forecast future changes in soil carbon stocks and potential feedbacks on atmospheric CO2 concentrations. In this study we investigated seasonal changes on the components of net ecosystem carbon balance resulting from the application of irrigation and nitrogen fertiliser to a temperate grassland in New Zealand where we simulated grazing events. We made seasonal measurements of the components of NB using chamber measurements in field plots with and without irrigation and addition of nitrogen fertiliser. We developed models to determine the physiological responses of gross canopy photosynthesis (A), leaf respiration (RL) and soil respiration (RS) to soil and air temperature, soil water content and irradiance and we estimated annual NB for the first year after treatments were applied. Overall, irrigation and nitrogen addition had a synergistic effect to increase annual estimates of above-ground components of carbon balance (A, RL and carbon exported through simulated grazing, Fexport), but there was no effect from adding nitrogen alone. Annual RS remained unchanged between treatments. The treatments resulted in increases in above-ground biomass production, but, with the high intensity of simulated grazing, these were not sufficient to offset ecosystem carbon losses, so all treatments remained a net source of carbon. There were no significant differences between treatments and annual NB ranged from -540gCm-2y-1 for the treatment with no irrigation and no nitrogen addition and -284gCm-2y-1 for the treatment with irrigation and nitrogen addition. Our findings from the first year of the treatments quantify the net benefits of addition of irrigation and nitrogen on increasing above-ground production for animal feed but show that this did not lead to a net increase carbon input to the soil.

Air–Water CO2 Fluxes and Net Ecosystem Production Changes in a Baja California Coastal Lagoon During the Anomalous North Pacific Warm Condition

The present study examines the temporal variability of air–water CO2 fluxes (FCO2) and seawater carbonate chemistry in a Baja California coastal lagoon during an exceptionally warm anomaly that was developed in Northeast Pacific coasts during 2014. This oceanographic condition led to a summer-like season (weak upwelling condition) during the study period, which reached a maximum surface temperature anomaly of 2 °C in September 2014. San Quintin Bay acts as a source of CO2 to the atmosphere in 2014 (3.3 ± 4.8 mmol C m−2 day−1) with the higher positive fluxes mainly observed in summer months (9.0 ± 5.3 mmol C m−2 day−1). Net ecosystem production (NEP) switched seasonally between net heterotrophy and net autotrophy during the study period, with an annual average of 2.2 ± 7.1 mmol C m−2 day−1, which indicates that San Quintin Bay was a net autotrophic system during the atypical warm oceanographic condition in 2014. This pattern of seasonal variations in the carbon balance at San Quintin Bay appears to be linked to the life cycle of benthic communities, which play an important role in the whole-ecosystem metabolism. Under the limited input from external sources coupled with an increase in seawater temperatures, the recycled benthic carbon and nutrient fluxes play a major role to sustain water-column processes within the bay. Since the upwelling condition may influence the magnitude of the air–water CO2 fluxes, our results clearly indicated that San Quintin Bay is a net source of carbon to the atmosphere regardless of the adjacent oceanic conditions. Our study sheds light on the carbon dynamics and its metabolic implications in a shallow coastal ecosystem under a regional warm anomaly and contributes potentially relevant information in view of the likely future scenario of global climate change.

The terrestrial carbon budget of South and Southeast Asia

Accomplishing the objective of the current climate policies will require establishing carbon budget and flux estimates in each region and county of the globe by comparing and reconciling multiple estimates including the observations and the results of top-down atmospheric carbon dioxide (CO2) inversions and bottom-up dynamic global vegetation models. With this in view, this study synthesizes the carbon source/sink due to net ecosystem productivity (NEP), land cover land use change (ELUC), fires and fossil burning (EFIRE) for the South Asia (SA), Southeast Asia (SEA) and South and Southeast Asia (SSEA = SA + SEA) and each country in these regions using the multiple top-down and bottom-up modeling results. The terrestrial net biome productivity (NBP = NEP – ELUC – EFIRE) calculated based on bottom-up models in combination with EFIRE based on GFED4s data show net carbon sinks of 217 ± 147, 10 ± 55, and 227 ± 279 TgC yr−1 for SA, SEA, and SSEA. The top-down models estimated NBP net carbon sinks were 20 ± 170, 4 ± 90 and 24 ± 180 TgC yr−1. In comparison, regional emissions from the combustion of fossil fuels were 495, 275, and 770 TgC yr−1, which are many times higher than the NBP sink estimates, suggesting that the contribution of the fossil fuel emissions to the carbon budget of SSEA results in a significant net carbon source during the 2000s. When considering both NBP and fossil fuel emissions for the individual countries within the regions, Bhutan and Laos were net carbon sinks and rest of the countries were net carbon source during the 2000s. The relative contributions of each of the fluxes (NBP, NEP, ELUC, and EFIRE, fossil fuel emissions) to a nation’s net carbon flux varied greatly from country to country, suggesting a heterogeneous dominant carbon fluxes on the country-level throughout SSEA.

Measuring and modeling the impact of a severe drought on terrestrial ecosystem CO2 and water fluxes in a subtropical forest

AbstractA severe drought occurred in central and southern China in the summer of 2013. The precipitation dropped to less than 25% of the long‐term average, and temperatures were abnormally high for more than 2 months with return periods of 125 years and 301 years, respectively, which induced significant changes in the terrestrial ecohydrological cycle. In this study, the impact of the severe drought on a subtropical forest ecosystem was investigated using measurements from a newly established flux tower and simulations performed by the Community Land Model version 4.5 (CLM4.5). Based on in situ observations, we found that both gross primary production (GPP) and evapotranspiration experienced strong reductions of 76% and 40%, respectively, during the prolonged dry and hot spell. There was an exponential relationship between ecosystem respiration (Reco) and temperatures when soil moisture was not too dry, but Reco decreased along with GPP when the temperature exceeded 32°C and soil moisture was below 0.14 m3 m−3. The ecosystem even switched to a net source of carbon in late August. The model captured the variations in water vapor fluxes well, with correlation coefficients r > 0.88, but overestimated net ecosystem CO2 exchange because it did not adequately represent carbon fluxes responses to water stress and failed to capture the nonlinear relationship between GPP and Reco during the drought period. The long‐term simulation suggested that water availability severely limited carbon sequestration, and both the underlying water use efficiency and inherent water use efficiency reached their maximum values.

Effect of tillage practices on net carbon flux and economic parameters from farmland on the Loess Plateau in China

It is important to evaluate the effect of tillage practices on carbon flow and economic parameters in order to better understand soil carbon sequestration and mitigation of carbon release into the atmosphere, and to increase income for farmers. A 2-year field experiment was conducted in a winter wheat-summer corn rotation system on dryland farmland on the Loess Plateau in Northwest China with four treatments; zero tillage with straw mulching (zero tillage), rotary tillage with straw incorporation (rotary tillage), chisel plow tillage with straw incorporation (chisel plow), and conventional mouldboard plow tillage without crop straw (conventional tillage). Results showed that zero tillage reduced carbon emissions, mainly through reduction of emissions from tillage practices, and served as a net carbon sink, as did rotary tillage and chisel plow, while conventional tillage served as a net carbon source. Specifically, zero tillage reduced carbon emissions from farm inputs by 168.8 and 75.0 kg C ha−1 yr−1 compared with conventional tillage and rotary tillage (or chisel plow), respectively. The difference in annual rate of carbon sequestration between 2013 and 2015 was 1.21-fold higher with zero tillage than with conventional tillage, suggesting zero tillage released less CO2 and was therefore better able to mitigate against global warming. Chisel plow significantly (P < 0.05) improved net return values by 10.7%–63.2% in the 2013-14 cropping year and by 12.8%–93.7% in the 2014–15 cropping year, respectively when compared to other tillage treatments. Zero tillage was the most effective method for reducing carbon inputs and increasing carbon sequestration, and chisel plow was the best option for provision of economic parameters on dryland farmland in this region. Therefore, the application of conservation (zero, rotary and chisel plow) tillage could be a C-smart and effective economic agricultural practice for winter wheat-summer corn ecosystems under rain fed condition.

Regional carbon fluxes from land use and land cover change in Asia, 1980–2009

We present a synthesis of the land-atmosphere carbon flux from land use and land cover change (LULCC) in Asia using multiple data sources and paying particular attention to deforestation and forest regrowth fluxes. The data sources are quasi-independent and include the U.N. Food and Agriculture Organization-Forest Resource Assessment (FAO-FRA 2015; country-level inventory estimates), the Emission Database for Global Atmospheric Research (EDGARv4.3), the ‘Houghton’ bookkeeping model that incorporates FAO-FRA data, an ensemble of 8 state-of-the-art Dynamic Global Vegetation Models (DGVM), and 2 recently published independent studies using primarily remote sensing techniques. The estimates are aggregated spatially to Southeast, East, and South Asia and temporally for three decades, 1980–1989, 1990–1999 and 2000–2009. Since 1980, net carbon emissions from LULCC in Asia were responsible for 20%–40% of global LULCC emissions, with emissions from Southeast Asia alone accounting for 15%–25% of global LULCC emissions during the same period. In the 2000s and for all Asia, three estimates (FAO-FRA, DGVM, Houghton) were in agreement of a net source of carbon to the atmosphere, with mean estimates ranging between 0.24 to 0.41 Pg C yr−1, whereas EDGARv4.3 suggested a net carbon sink of −0.17 Pg C yr−1. Three of 4 estimates suggest that LULCC carbon emissions declined by at least 34% in the preceding decade (1990–2000). Spread in the estimates is due to the inclusion of different flux components and their treatments, showing the importance to include emissions from carbon rich peatlands and land management, such as shifting cultivation and wood harvesting, which appear to be consistently underreported.

The influence of drought and heat stress on long-term carbon fluxes of bioenergy crops grown in the Midwestern USA.

Perennial grasses are promising feedstocks for bioenergy production in the Midwestern USA. Few experiments have addressed how drought influences their carbon fluxes and storage. This study provides a direct comparison of ecosystem-scale measurements of carbon fluxes associated with miscanthus (Miscanthus × giganteus), switchgrass (Panicum virgatum), restored native prairie and maize (Zea mays)/soybean (Glycine max) ecosystems. The main objective of this study was to assess the influence of a naturally occurring drought during 2012 on key components of the carbon cycle and plant development relative to non-extreme years. The perennials reached full maturity 3-5 years after establishment. Miscanthus had the highest gross primary production (GPP) and lowest net ecosystem exchange (NEE) in 2012 followed by similar values for switchgrass and prairie, and the row crops had the lowest GPP and highest NEE. A post-drought effect was observed for miscanthus. Over the duration of the experiment, perennial ecosystems were carbon sinks, as indicated by negative net ecosystem carbon balance (NECB), while maize/soybean was a net carbon source. Our observations suggest that perennial ecosystems, and in particular miscanthus, can provide a high yield and a large potential for CO2 fixation even during drought, although drought may negatively influence carbon uptake in the following year, questioning the long-term consequence of its maintained productivity.

Land‐use change to bioenergy: grassland to short rotation coppice willow has an improved carbon balance

AbstractThe effect of a transition from grassland to second‐generation (2G) bioenergy on soil carbon and greenhouse gas (GHG) balance is uncertain, with limited empirical data on which to validate landscape‐scale models, sustainability criteria and energy policies. Here, we quantified soil carbon, soilGHGemissions and whole ecosystem carbon balance for short rotation coppice (SRC) bioenergy willow and a paired grassland site, both planted at commercial scale. We quantified the carbon balance for a 2‐year period and captured the effects of a commercial harvest in theSRCwillow at the end of the first cycle. Soil fluxes of nitrous oxide (N2O) and methane (CH4) did not contribute significantly to theGHGbalance of these land uses. Soil respiration was lower inSRCwillow (912 ± 42 g C m−2 yr−1) than in grassland (1522 ± 39 g C m−2 yr−1). Net ecosystem exchange (NEE) reflected this with the grassland a net source of carbon with meanNEEof 119 ± 10 g C m−2 yr−1andSRCwillow a net sink, −620 ± 18 g C m−2 yr−1. When carbon removed from the ecosystem in harvested products was considered (Net Biome Productivity),SRCwillow remained a net sink (221 ± 66 g C m−2 yr−1). Despite theSRCwillow site being a net sink for carbon, soil carbon stocks (0–30 cm) were higher under the grassland. There was a largerNEEand increase in ecosystem respiration in theSRCwillow after harvest; however, the site still remained a carbon sink. Our results indicate that once established, significant carbon savings are likely inSRCwillow compared with the minimally managed grassland at this site. Although these observed impacts may be site and management dependent, they provide evidence that land‐use transition to 2G bioenergy has potential to provide a significant improvement on the ecosystem service of climate regulation relative to grassland systems.

Changing temperature response of respiration turns boreal forest from carbon sink into carbon source

Seventeen years (1997–2013) of carbon dioxide (CO2) fluxes were measured in a boreal forest stand in northern Sweden using the eddy covariance technique. During the measurement period the forest turned from a net carbon sink into a net carbon source. The net ecosystem exchange (NEE) was separated using values from periods of darkness into the gross components of total ecosystem respiration (TER) and gross primary productivity (GPP), which was calculated as GPP=−NEE+TER. From the gross components we could determine that an increase in TER during the autumn (September to end of November) and spring (March to end of May) periods resulted in the forest becoming a net source of CO2. We observed no increase in the GPP from the eddy covariance measurements. This was further supported by measurements of tree growth rings. The increased TER was attributed to a change in the forest’s temperature response at lower temperatures (−5 to 10°C) rather than to a temperature increase. This study shows that changes in ecosystem functioning can have a larger impact on the carbon balance than climate warming per se.

Climate indices strongly influence old-growth forest carbon exchange

We present a decade and a half (1998–2013) of carbon dioxide fluxes from an old-growth stand in the American Pacific Northwest to identify ecosystem-level responses to Pacific teleconnection patterns, including the El Niño/Southern Oscillation (ENSO). This study provides the longest, continuous record of old-growth eddy flux data to date from one of the longest running Fluxnet stations in the world. From 1998 to 2013, average annual net ecosystem exchange (FNEE) at Wind River AmeriFlux was −32 ± 84 g C m−2 yr−1 indicating that the late seral forest is on average a small net sink of atmospheric carbon. However, interannual variability is high (>300 g C m−2 yr−1) and shows that the stand switches from net carbon sink to source in response to climate drivers associated with ENSO. The old-growth forest is a much stronger sink during La Niña years (mean FNEE = −90 g C m−2 yr−1) than during El Niño when the stand turns carbon neutral or into a small net carbon source (mean FNEE = +17 g C m−2 yr−1). Forest inventory data dating back to the 1930s show a similar correlation with the lower frequency Pacific North American (PNA) and Pacific Decadal Oscillation (PDO) whereby higher aboveground net primary productivity (FANPP) is associated with cool phases of both the PNA and PDO. These measurements add evidence that carbon exchange in old-growth stands may be more sensitive to climate variability across shorter time scales than once thought.