Changes In Net Ecosystem Exchange Research Articles

Rapidly changing high-latitude seasonality: implications for the 21st century carbon cycle in Alaska

Seasonal variations in high-latitude terrestrial carbon (C) fluxes are predominantly driven by air temperature and radiation. At present, high-latitude net C uptake is largest during the summer. Recent observations and modeling studies have demonstrated that ongoing and projected climate change will increase plant productivity, microbial respiration, and growing season lengths at high-latitudes, but impacts on high-latitude C cycle seasonality (and potential feedbacks to the climate system) remain uncertain. Here we use ecosys, a well-tested and process-rich mechanistic ecosystem model that we evaluate further in this study, to explore how climate warming under an RCP8.5 scenario will shift C cycle seasonality in Alaska throughout the 21st century. The model successfully reproduced recently reported large high-latitude C losses during the fall and winter and yet still predicts a high-latitude C sink, pointing to a resolution of the current conflict between process-model and observation-based estimates of high-latitude C balance. We find that warming will result in surprisingly large changes in net ecosystem exchange (NEE; defined as negative for uptake) seasonality, with spring net C uptake overtaking summer net C uptake by year 2100. This shift is driven by a factor of 3 relaxation of spring temperature limitation to plant productivity that results in earlier C uptake and a corresponding increase in magnitude of spring NEE from −19 to −144 gC m−2 season−1 by the end of the century. Although a similar relaxation of temperature limitation will occur in the fall, radiation limitation during those months will limit increases in C fixation. Additionally, warmer soil temperatures and increased carbon inputs from plants lead to combined fall and winter C losses (163 gC m−2) that are larger than summer net uptake (123 gC m−2 season−1) by year 2100. However, this increase in microbial activity leads to more rapid N cycling and increased plant N uptake during the fall and winter months that supports large increases in spring NPP. Due to the large increases in spring net C uptake, the high-latitude atmospheric C sink is projected to sustain throughout this century. Our analysis disentangles the effects of key environmental drivers of high-latitude seasonal C balances as climate changes over the 21st century.

Warming and Increased Respiration Have Transformed an Alpine Steppe Ecosystem on the Tibetan Plateau From a Carbon Dioxide Sink Into a Source

AbstractCold region ecosystems store vast amounts of soil organic carbon (C), which upon warming and decomposition can affect the C balance and potentially change these ecosystems from C sinks to carbon dioxide (CO2) sources. We quantified the decadal year‐round CO2 flux from an alpine steppe‐ecosystem on the Tibetan Plateau using eddy covariance and automatic chamber approaches during a period of significant warming (0.13°C per 10 years; and 0.18°C in the non‐growing season alone: 1st October to next 30th April). The results showed that ongoing climate change, mainly warming within the topsoil layers, is the main reason for the site’s change from a sink for to a source of CO2 in the atmosphere. Non‐growing‐season ecosystem respiration accounted for 51% of the annual ecosystem respiration and has increased significantly. The growing seasons (1st May to 30th September) were consistent CO2 sink periods without significant changes over the study period. Observations revealed high‐emission events from the end of the non‐growing season to early in the growing season (1st March to fifteenth May), which significantly (p < 0.01) increased at a rate of 22.6 g C m−2 decade−1, ranging from 14.6 ± 10.7 g C m−2 yr−1 in 2012 to 35.3 ± 12.1 g C m−2 yr−1 in 2017. Structural equation modeling suggested that active layer warming was the key factor in explaining changes in ecosystem respiration, leading to significant changes in net ecosystem exchange over the period 2011–2020 and indicated that these changes have already transformed the ecosystem from a CO2 sink into a source. These results can be used to improve our understanding of the sensitivity of ecosystem respiration to increased warming during the non‐growing period.

Changes in net ecosystem exchange of CO2 in Arctic and their relationships with climate change during 2002–2017

Arctic warming leads to permafrost degradation, which can increase ecosystem respiration and release more greenhouse gas into the atmosphere. Meanwhile, climate warming also promotes the plant growth and increases carbon assimilation. Presently, it is largely unknown about the carbon budget and their responses to climate change in the Arctic regions. In this study, to investigate the seasonal and annual net ecosystem carbon exchange (NEE), we collected 71 observation stations for net ecosystem exchange (NEE) of CO2 in the high latitude permafrost regions during 2002–2017. The results showed that the annual NEE was −8.2 ± 4.1 g CO2 m−2 d−1 for forest, −3.3 ± 2.6 g CO2 m−2 d−1 for shrub, −4.8 ± 4.1 g CO2 m−2 d−1 for grassland, −3.6 ± 3.0 g CO2 m−2 d−1 for wetland and 0.02 ± 0.62 g CO2 m−2 d−1 for tundra, respectively. From 2002 to 2017, the CO2 emissions of grassland (carbon source) showed a decreasing trend, and the CO2 assimilation of shrub and forest (carbon sink) has been increased. The wetland and tundra are shifting from carbon sources to sinks. There were great variations in temperature sensitivities (Q10) of NEE in different seasons, with larger values in winter and lower values in summer. These findings indicate that the Arctic terrestrial ecosystem presently acts as a carbon sink, while there is a possibility that future warming, especially the warming in winter, may decrease the carbon sink of the Arctic terrestrial ecosystem.

Changes in net ecosystem exchange over Europe during the 2018 drought based on atmospheric observations.

The 2018 drought was one of the worst European droughts of the twenty-first century in terms of its severity, extent and duration. The effects of the drought could be seen in a reduction in harvest yields in parts of Europe, as well as an unprecedented browning of vegetation in summer. Here, we quantify the effect of the drought on net ecosystem exchange (NEE) using five independent regional atmospheric inversion frameworks. Using a network of atmospheric CO2 mole fraction observations, we estimate NEE with at least monthly and 0.5° × 0.5° resolution for 2009–2018. We find that the annual NEE in 2018 was likely more positive (less CO2 uptake) in the temperate region of Europe by 0.09 ± 0.06 Pg C yr−1 (mean ± s.d.) compared to the mean of the last 10 years of −0.08 ± 0.17 Pg C yr−1, making the region close to carbon neutral in 2018. Similarly, we find a positive annual NEE anomaly for the northern region of Europe of 0.02 ± 0.02 Pg C yr−1 compared the 10-year mean of −0.04 ± 0.05 Pg C yr−1. In both regions, this was largely owing to a reduction in the summer CO2 uptake. The positive NEE anomalies coincided spatially and temporally with negative anomalies in soil water. These anomalies were exceptional for the 10-year period of our study.This article is part of the theme issue ‘Impacts of the 2018 severe drought and heatwave in Europe: from site to continental scale’.

How Simulations of the Land Carbon Sink Are Biased by Ignoring Fluvial Carbon Transfers: A Case Study for the Amazon Basin

Summary Land-surface models are important tools for simulation of the past, present, and future capacity of terrestrial ecosystems to absorb anthropogenic CO2 emissions. However, fluvial carbon (C) transfers are presently neglected in these models. Using the Amazon basin as a case study, we show that this negligence leads to significant underestimation of the net uptake of atmospheric C while terrestrial C storage changes are overestimated. These biases arise from the fact that C—in reality, leached from soils and exported through the river network—is instead represented as partly being respired and partly being stored in soils. Moreover, these biases scale mainly to the fluvial C export to the coast, despite aquatic CO2 emission to the atmosphere being the major pathway of riverine C exports. We further show that fluvial C transfers may change significantly in response to changes in either hydrology or in atmospheric C uptake by vegetation.

Changes in net ecosystem exchange of CO2, latent and sensible heat fluxes in a recently clear-cut spruce forest in western Russia: results from an experimental and modeling analysis

Ecosystem carbon dioxide, energy, and water fluxes were measured using eddy covariance in a fresh clear-cut surrounded by a mixed spruce-birch-aspen forest in the boreal zone of European Russia. Measurements were initiated in spring 2016 following timber harvest and continued for five months. The influence of surrounding forest on air flow and turbulent fluxes within the clear-cut were examined using a process-based two-dimensional (2D) hydrodynamic turbulent exchange model. The clear-cut was a source of CO2 to the atmosphere prior to onset of vegetation growth during early spring. During this period the mean daily latent (LE) and sensible (H) heat fluxes were very similar and the Bowen ratio (β = H/LE) averaged about 1.0. Daily net ecosystem exchange of CO2 (NEE) was around 0 gC m−2 d−1 following onset of vegetation growth from mid-spring through summer, while β declined to 0.6–0.7. There was strong diurnal variability in NEE, LE and H over the measurement period that was governed by solar radiation and temperature as well as the leaf area index (LAI) of regrown vegetation. Modeled vertical CO2 and H2O fluxes along a transect that crossed the clear-cut and coincided with the dominate wind direction showed that the clear-cut strongly influenced turbulent fluxes within the atmospheric surface layer. Furthermore, modeled atmospheric dynamics suggested that the clear-cut had a large influence on turbulent fluxes in the downwind forest, but little impact on the upwind side. An aggregated approach including field measurements and process-based models can be a useful approach to estimate energy, water and carbon dioxide fluxes in non-uniform forest landscapes.

Carbon exchange between the atmosphere and subtropical forested cypress and pine wetlands

Abstract. Carbon dioxide exchange between the atmosphere and forested subtropical wetlands is largely unknown. Here we report a first step in characterizing this atmospheric–ecosystem carbon (C) exchange, for cypress strands and pine forests in the Greater Everglades of Florida as measured with eddy covariance methods at three locations (Cypress Swamp, Dwarf Cypress and Pine Upland) for 2 years. Links between water and C cycles are also examined at these three sites, as are methane emission measured only at the Dwarf Cypress site. Each forested wetland showed net C uptake from the atmosphere both monthly and annually, as indicated by the net ecosystem exchange (NEE) of carbon dioxide (CO2). For this study, NEE is the difference between photosynthesis and respiration, with negative values representing uptake from the atmosphere that is retained in the ecosystem or transported laterally via overland flow (unmeasured for this study). Atmospheric C uptake (NEE) was greatest at the Cypress Swampp (−900 to −1000 g C m2 yr−1), moderate at the Pine Upland (−650 to −700 g C m2 yr−1) and least at the Dwarf Cypress (−400 to −450 g C m2 yr−1). Changes in NEE were clearly a function of seasonality in solar insolation, air temperature and flooding, which suppressed heterotrophic soil respiration. We also note that changes in the satellite-derived enhanced vegetation index (EVI) served as a useful surrogate for changes in NEE at these forested wetland sites.

The effect of atmospheric aerosol particles and clouds on net ecosystem exchange in the Amazon

Abstract. Carbon cycling in the Amazon is closely linked to atmospheric processes and climate in the region as a consequence of the strong coupling between the atmosphere and biosphere. This work examines the effects of changes in net radiation due to atmospheric aerosol particles and clouds on the net ecosystem exchange (NEE) of CO2 in the Amazon region. Some of the major environmental factors affecting the photosynthetic activity of plants, such as air temperature and relative humidity, were also examined. An algorithm for clear-sky irradiance was developed and used to determine the relative irradiance, f, which quantifies the percentage of solar radiation absorbed and scattered due to atmospheric aerosol particles and clouds. Aerosol optical depth (AOD) was calculated from irradiances measured with the MODIS (Moderate Resolution Imaging Spectroradiometer) sensor, onboard the Terra and Aqua satellites, and was validated with ground-based AOD measurements from AERONET (Aerosol Robotic Network) sun photometers. Carbon fluxes were measured using eddy covariance technique at the Large-Scale Biosphere-Atmosphere Experiment in Amazonia (LBA) flux towers. Two sites were studied: the Jaru Biological Reserve (RBJ), located in Rondonia, and the Cuieiras Biological Reserve at the K34 LBA tower (located in a preserved region in the central Amazon). Analysis was performed continuously from 1999 to 2009 at K34 and from 1999 to 2002 at RBJ, and includes wet, dry and transition seasons. In the Jaru Biological Reserve, a 29% increase in carbon uptake (NEE) was observed when the AOD ranged from 0.10 to 1.5 at 550 nm. In the Cuieiras Biological Reserve, the aerosol effect on NEE was smaller, accounting for an approximate 20% increase in NEE. High aerosol loading (AOD above 3 at 550 nm) or high cloud cover leads to reductions in solar flux and strong decreases in photosynthesis up to the point where NEE approaches zero. The observed increase in NEE is attributed to an enhancement (~50%) in the diffuse fraction of photosynthetic active radiation (PAR). The enhancement in diffuse PAR can be done through increases in aerosols and/or clouds. In the present study, it was not possible to separate these two components. Significant changes in air temperature and relative humidity resulting from changes in solar radiation fluxes under high aerosol loading were also observed at both sites. Considering the long-range transport of aerosols in the Amazon, the observed changes in NEE for these two sites may occur over large areas in the Amazon, significantly altering the carbon balance in the largest rainforest in the world.

SIMULATION AND ANALYSIS ON FUTURE CARBON BALANCE OF THREE DECIDUOUS FORESTS IN BEIJING MOUNTAIN AREA, WARM TEMPERATE ZONE OF CHINA

Aims Climate change is expected to cause changes of carbon cycling in forest ecosystems, and prediction research suggests there is dramatic spatial heterogeneity and uncertainty in responses of carbon balance of forest ecosystems to climate change. The goals of our research were to predict and analyze impacts of climate change on the carbon cycling of warm temperate forests in Beijing mountain area in the next 100 years and to understand the heterogeneity and uncertainty on the ecosystem scale with LPJ-GUESS model. Methods The ecosystem model was used to learn how forest ecosystem productivity and carbon bal-ance change in a long-term time scale and to learn about differences of carbon balance among various ecosystems by comparing ecosystem components of carbon balance. The dynamic vegetation model (LPJ-GUESS), used for the first time in China and driven by A2 and B2 greenhouse gas emission sce-narios of the Special Report on Emissions Scenarios (SRES) of IPCC, projected climate change impacts on carbon balance for three typical warm temperate forest ecosystems (oak, birch and Chinese pine forests) in the Dongling Mountain area of Beijing, China. Important findings Net ecosystem primary productivity (NPP) and heterotrophic respiration (Rh) will increase in the three forests, and the A2 scenario was associated with larger changes in NPP and Rh than the B2 scenario. Because of differences in species composition among the forests, increases in NPP and Rh were different and changes in net ecosystem exchange (NEE) were different among the forests: oakforest switched from a sink to a small source of carbon, birch forest remained as a smaller sink of carbon, and Chinese pine forest became a larger carbon sink over the next 100 years. Also, carbon biomass will generally increase in the forests by 2100. Comparing SRES A2 with B2, there was larger carbon storage in the relative lower emission scenario (B2). Projected differences in carbon balance among these forests in the same area were more dependent on species composition than climate factors (A2 and B2 scenarios) under future climate change.

Closing the carbon budget of estuarine wetlands with tower‐based measurements and MODIS time series

Since the publication of Yan et al. (2008) earlier this month, Dr. Asko Noormets of North Carolina State University has communicated with us about potential errors involved in this publication. Consequently, we found that the units used for several variables were wrong. This corrigendum provides the following corrections: The label of the horizontal axis in Figure 2 (Page 1693) should read PAR (μmol m−2 s−1). GPP, NEE and PAR in 3-5 as well as Table 3 were incorrectly presented because of unit conversion. For GPP and NEE, the original data needs to be timed by (1800(s) × 12(g mol−1)/1 00 0000) and for PAR, it needs to be timed by (1800(s)/10 00 000). Here, we provide the correct versions of these figures and the table. Seasonal changes in net ecosystem exchange (NEE) of CO2 and gross primary production (GPP) (a–c); photosynthetically active radiation (PAR) and mean air temperature (T) (d–f); vegetation indices (NDVI, normalized difference vegetation index; EVI, enhanced vegetation index and LSWI, land surface water index) (g–i) in the 2005 growing season at Dongtan wetlands, Shanghai, China. Comparisons between vegetation indices (EVI, NDVI) and gross primary production (GPP) among three sites at Dongtan, China (a, Site D; b, Site M; c, Site S). Simple linear regression models between GPP and vegetation (NDVI, EVI) are shown with data combined from all three sites (d). The solid line represents the relationship between GPP and EVI (GPP=19.31 × EVI+1.25, R2=0.520) and the dashed line between GPP and NDVI (GPP=11.90 × NDVI+0.84, R2=0.460). Comparisons of seasonal changes in gross primary production from eddy covariance towers (GPPEC), the MODIS-based model (GPPMODIS) and the hybrid model (GPP′) Eqn (10) for the 2005 growing season at three sites (a, Site D; b, Site M; c, Site S). Simple linear relationships between GPPEC, GPPMODIS and GPP′ are presented with data combined from all three sites (GPPEC=3.29 × GPPMODIS+1.47, R2=0.550; GPPEC=1.03 × GPP′−0.10, R2=0.876) (d). Because of the above errors, some results in the paper need to be changed accordingly. These include: Abstract (Page 1690): … The average GPP calculated from eddy covariance between March and November was 5.65 g C m−2 day−1, whereas that from the LUE model was 1.27 g C m−2 day−1: … The average GPP from the modified model increased to 5.69 g C m−2 day−1, … Results (Page 1696): … The Spartina-dominated wetland near the dike (Site D) had the highest peak NEE (−12.16 g C m−2 day−1) and GPP (14.46 g C m−2 day−1) and exhibited the most obvious seasonal pattern. Average values of NEE (GPP) were −4.36 (6.38), −3.97 (5.98) and −3.68 (4.61) g C m−2 day−1 for sites D, M and S, respectively, … … The standard error of the estimate was 1.27 when all three variables (TH, LSWI and ET) were considered … The citation of Eqn (9) in the title of Table 2 should read Eqn (6). We incorrectly referred Davidson et al. (2006) (Page 1695), as the authors did not publish the corresponding equation. Therefore, this citation should be removed. The latent heat of vaporization (Page 1694) should read 44 kJ mol−1. These corrections, however, do not affect the overall conclusions of the paper. The authors apologize for any confusion caused.

The fate of assimilated carbon during drought: impacts on respiration in Amazon rainforests.

Interannual variations in CO2 exchange across Amazonia, as deduced from atmospheric inversions, correlate with El Niño occurrence. They are thought to result from changes in net ecosystem exchange and fire incidence that are both related to drought intensity. Alterations to net ecosystem production (NEP) are caused by changes in gross primary production (GPP) and ecosystem respiration (Reco). Here, we analyse observations of the components of Reco (leaves, live and dead woody tissue, and soil) to provide first estimates of changes in Reco during short-term (seasonal to interannual) moisture limitation. Although photosynthesis declines if moisture availability is limiting, leaf dark respiration is generally maintained, potentially acclimating upwards in the longer term. If leaf area is lost, then short-term canopy-scale respiratory effluxes from wood and leaves are likely to decline. Using a moderate short-term drying scenario where soil moisture limitation leads to a loss of 0.5 m2 m−2 yr−1 in leaf area index, we estimate a reduction in respiratory CO2 efflux from leaves and live woody tissue of 1.0 (±0.4) t C ha−1 yr−1. Necromass decomposition declines during drought, but mortality increases; the median mortality increase following a strong El Niño is 1.1% (n=46 tropical rainforest plots) and yields an estimated net short-term increase in necromass CO2 efflux of 0.13–0.18 t C ha−1 yr−1. Soil respiration is strongly sensitive to moisture limitation over the short term, but not to associated temperature increases. This effect is underestimated in many models but can lead to estimated reductions in CO2 efflux of 2.0 (±0.5) t C ha−1 yr−1. Thus, the majority of short-term respiratory responses to drought point to a decline in Reco, an outcome that contradicts recent regional-scale modelling of NEP. NEP varies with both GPP and Reco but robust moisture response functions are clearly needed to improve quantification of the role of Reco in influencing regional-scale CO2 emissions from Amazonia.

Influence of vegetation and seasonal forcing on carbon dioxide fluxes across the Upper Midwest, USA: Implications for regional scaling

Carbon dioxide fluxes were examined over the growing seasons of 2002 and 2003 from 14 different sites in Upper Midwest (USA) to assess spatial variability of ecosystem–atmosphere CO 2 exchange. These sites were exposed to similar temperature/precipitation regimes and spanned a range of vegetation types typical of the region (northern hardwood, mixed forest, red pine, jack pine, pine barrens and shrub wetland). The hardwood and red pine sites also spanned a range of stand ages (young, intermediate, mature). While seasonal changes in net ecosystem exchange (NEE) and photosynthetic parameters were coherent across the 2 years at most sites, changes in ecosystem respiration (ER) and gross ecosystem production (GEP) were not. Canopy height and vegetation type were important variables for explaining spatial variability of CO 2 fluxes across the region. Light-use efficiency (LUE) was not as strongly correlated to GEP as maximum assimilation capacity (Amax). A bottom-up multi-tower land cover aggregated scaling of CO 2 flux to a 2000 km 2 regional flux estimate found June to August 2003 NEE, ER and GEP to be −290 ± 89, 408 ± 48, and 698 ± 73 gC m −2, respectively. Aggregated NEE, ER and GEP were 280% larger, 32% smaller and 3% larger, respectively, than that observed from a regionally integrating 447 m tall flux tower. However, when the tall tower fluxes were decomposed using a footprint-weighted influence function and then re-aggregated to a regional estimate, the resulting NEE, ER and GEP were within 11% of the multi-tower aggregation. Excluding wetland and young stand age sites from the aggregation worsened the comparison to observed fluxes. These results provide insight on the range of spatial sampling, replication, measurement error and land cover accuracy needed for multi-tiered bottom-up scaling of CO 2 fluxes in heterogeneous regions such as the Upper Midwest, USA.

Factors controlling CO2 exchange on timescales from hourly to decadal at Harvard Forest

We analyzed 13 years (1992−2004) of CO2 flux data, biometry, and meteorology from a mixed deciduous forest in central Massachusetts. Annual net uptake of CO2 ranged from 1.0 to 4.7 Mg‐C ha−1yr−1, with an average of 2.5 Mg‐C ha−1yr−1. Uptake rates increased systematically, nearly doubling over the period despite forest age of 75–110 years; there were parallel increases in midsummer photosynthetic capacity at high light level (21.5−31.5 μmole m−2s−1), woody biomass (101−115 Mg‐C ha−1 from 1993−2005, mostly due to growth of one species, red oak), and peak leaf area index (4.5−5.5 from 1998–2005). The long‐term trends were interrupted in 1998 by sharp declines in photosynthetic capacity, net ecosystem exchange (NEE) of CO2, and other parameters, with recovery over the next 3 years. The observations were compared to empirical functions giving the mean responses to temperature and light, and to a terrestrial ecosystem model (IBIS2). Variations in gross ecosystem exchange of CO2 (GEE) and NEE on hourly to monthly timescales were represented well as prompt responses to the environment, but interannual variations and long‐term trends were not. IBIS2 simulated mean annual NEE, but greatly overpredicted the amplitude of the seasonal cycle and did not predict the decadal trend. The drivers of interannual and decadal changes in NEE are long‐term increases in tree biomass, successional change in forest composition, and disturbance events, processes not well represented in current models.

Climatic versus biotic constraints on carbon and water fluxes in seasonally drought‐affected ponderosa pine ecosystems

We investigated the relative importance of climatic versus biotic controls on gross primary production (GPP) and water vapor fluxes in seasonally drought‐affected ponderosa pine forests. The study was conducted in young (YS), mature (MS), and old stands (OS) over 4 years at the AmeriFlux Metolius sites. Model simulations showed that interannual variation of GPP did not follow the same trends as precipitation, and effects of climatic variation were smallest at the OS (<10%), largest at the MS (>50%), and intermediate at the YS (<20%). In the young, developing stand, interannual variation in leaf area has larger effects on fluxes than climate, although leaf area is a function of climate in that climate can interact with age‐related shifts in carbon allocation and affect whole‐tree hydraulic conductance. Older forests, with well‐established root systems, appear to be better buffered from effects of seasonal drought and interannual climatic variation. Interannual variation of net ecosystem exchange (NEE) was also lowest at the OS, where NEE is controlled more by interannual variation of ecosystem respiration, 70% of which is from soil, than by the variation of GPP, whereas variation in GPP is the primary reason for interannual changes in NEE at the YS and MS. Across spatially heterogeneous landscapes with high frequency of younger stands resulting from natural and anthropogenic disturbances, interannual climatic variation and change in leaf area are likely to result in large interannual variation in GPP and NEE.

Seasonal changes in photosynthesis, respiration and NEE of a mixed temperate forest

We analysed 6 years (1997–2002) of continuous eddy covariance CO 2 fluxes over a mixed temperate forest in the Belgian Campine region and present the long-term mean seasonal changes in net ecosystem exchange (NEE), gross primary production (GPP) and total ecosystem respiration (RE). Seasonal changes in NEE, GPP and RE were tightly correlated, with extremes in July and December/January. Nonetheless, maximum rates of RE slightly lagged behind the maximum rates of GPP. We observed a substantial increase in NEE at optimum light conditions (reduced uptake) above 24 °C. Because this increase is unlikely to be related only to the increase in RE, we hypothesize that it also indicates a significant decrease in GPP under dry and warm meteorological conditions. Because this increase in NEE was better correlated with atmospheric vapour pressure deficit than with temperature, we further hypothesize that the observed change in NEE was predominantly related to the effect of high atmospheric evaporative demand on GPP via stomatal closure than to a negative-temperature response. Because dry conditions were rare in the maritime climate of this study site, its impact on the annual GPP or even on NEE was small (<1% and 10%, respectively).

Response of the seasonal carbon cycle in high latitudes to climate anomalies

Many observational studies have demonstrated that the amplitude of the seasonal carbon cycle in high latitudes is increasing and advancing in phase. Several hypotheses related to natural and anthropogenic landscape disturbance and climate change have been proposed to explain these observations. This study employs a regional atmosphere‐biosphere model (the Arctic Regional Climate System Model (ARCSyM)) to investigate the impacts of the climatic effects of elevated carbon dioxide on terrestrial carbon exchange over Alaska. A perturbation experiment, driven by a “transient” global climate scenario at the point at which atmospheric carbon dioxide has doubled, is compared to a validated present‐day simulation. The focus of this paper is on the role of physical climate in seasonal carbon exchange between the atmosphere and the terrestrial biosphere at regional scales. In the present‐day simulation the ARCSyM reproduces most observed features of climate and the surface energy budget and simulates a clearly defined growing season in the seasonal cycle of carbon dioxide exchange. In the perturbation experiment, the model atmosphere shows a generally warmer and wetter climate, with increased permafrost thaw. Two dominant biomes, tundra and boreal forest, are analyzed in detail for their responses to the perturbed climate. The amplitudes of the seasonal carbon dioxide cycle for Alaskan boreal forest and tundra increase 12 and 8%, respectively, with phase advances of ∼7 and 2 days, respectively. The forcing of photosynthetic productivity by soil moisture changes is a dominant contributor to the simulated changes in net ecosystem exchange. Boreal forest ecosystem is more sensitive to moisture anomalies due to a drier soil, whereas in the tundra regions, which are often close to saturation, changes are forced primarily by temperature variations. Thus in the event of a northward migration of tree line, sensitivity of regional carbon exchange to climate perturbations could increase.