Annual Carbon Uptake Research Articles

The effect of static pressure-wind covariance on vertical carbon dioxide exchange at a windy subalpine forest site

Accounting for turbulent temperature and water vapor fluctuations on the vertical flux of CO2 measured by an open-path infrared gas analyzer are commonly known as the Webb–Pearman–Leuning (WPL) corrections. Static pressure fluctuations also affect air sample density, but the magnitude and effect of these changes on CO2 flux is not usually considered. In our study, the turbulent static pressure p and the vertical wind component w covariance w′p′¯ from just-above a Rocky Mountain subalpine forest are examined. The magnitude of w′p′¯ was highly correlated to the mean horizontal wind speed U with similar characteristics during the daytime and nighttime. The pressure term was calculated from w′p′¯ and compared to other terms in the equation for the vertical net ecosystem exchange of CO2 (NEE). We found the following: (1) for U≲6 m s−1, the pressure term was small, (2) as U increased beyond 6 m s−1, the pressure term became more and more important reaching a magnitude of ≈ 2 μmol m−2 s−1 at U of 12 m s−1 (for leaf area index LAI ≈ 2.6 m2 m−2), (3) for a more dense forest (LAI ≈ 4.8 m2 m−2), the magnitude of the pressure term at U of 12 m s−1 was ≈ 1 μmol m−2 s−1, or about half the open forest value, and (4) based on 14 years of measurements, the interannual mean and standard deviation of the yearly cumulative pressure and NEE terms were −32.8± 24.5 g C m−2 year−1 and −147.4± 231.7 g C m−2 year−1, respectively. Therefore, on average, including the pressure term in the NEE calculation reduces the annual carbon uptake by the GLEES forest from 147.4 g C m−2 year−1 to 114.6 g C m−2 year−1. This implies that carbon uptake by forests in windy locations that ignore the pressure term can be overestimated by as much as 20%. However, the year-to-year variability of both NEE and the pressure term was large due to changes in the forest structure and LAI from beetle attack and tree die-off. Finally, we present results from sensor-manipulation experiments examining the effect of tilting the quad-disk pressure ports on the turbulent static pressure.

Annual aboveground carbon uptake enhancements from assisted gene flow in boreal black spruce forests are not long-lasting

Assisted gene flow between populations has been proposed as an adaptive forest management strategy that could contribute to the sequestration of carbon. Here we provide an assessment of the mitigation potential of assisted gene flow in 46 populations of the widespread boreal conifer Picea mariana, grown in two 42-year-old common garden experiments and established in contrasting Canadian boreal regions. We use a dendroecological approach taking into account phylogeographic structure to retrospectively analyse population phenotypic variability in annual aboveground net primary productivity (NPP). We compare population NPP phenotypes to detect signals of adaptive variation and/or the presence of phenotypic clines across tree lifespans, and assess genotype‐by‐environment interactions by evaluating climate and NPP relationships. Our results show a positive effect of assisted gene flow for a period of approximately 15 years following planting, after which there was little to no effect. Although not long lasting, well-informed assisted gene flow could accelerate the transition from carbon source to carbon sink after disturbance.

Spatial variations in terrestrial net ecosystem productivity and its local indicators

Abstract. Multiple lines of evidence have demonstrated the persistence of global land carbon (C) sink during the past several decades. However, both annual net ecosystem productivity (NEP) and its inter-annual variation (IAVNEP) keep varying over space. Thus, identifying local indicators for the spatially varying NEP and IAVNEP is critical for locating the major and sustainable C sinks on land. Here, based on daily NEP observations from FLUXNET sites and large-scale estimates from an atmospheric-inversion product, we found a robust logarithmic correlation between annual NEP and seasonal carbon uptake–release ratio (i.e. U ∕ R). The cross-site variation in mean annual NEP could be logarithmically indicated by U ∕ R, while the spatial distribution of IAVNEP was associated with the slope (i.e. β) of the logarithmic correlation between annual NEP and U ∕ R. Among biomes, for example, forests and croplands had the largest U ∕ R ratio (1.06 ± 0.83) and β (473 ± 112 g C m−2 yr−1), indicating the highest NEP and IAVNEP in forests and croplands, respectively. We further showed that these two simple indicators could directly infer the spatial variations in NEP and IAVNEP in global gridded NEP products. Overall, this study provides two simple local indicators for the intricate spatial variations in the strength and stability of land C sinks. These indicators could be helpful for locating the persistent terrestrial C sinks and provide valuable constraints for improving the simulation of land–atmospheric C exchanges.

The effect of tree mortality on CO2 fluxes in an old-growth spruce forest

Long-term (2009–2019) field studies of the CO2 fluxes over the Valday upland in north-western European Russia were performed in an old-growth spruce-dominated forest subject to the combined effects of climate change, bark beetle attacks and windfall events. The annual carbon uptake within the study area decreased from − 300 g C m−2 yr−1 in 2010–2011 to − 95 g C m−2 yr−1 in 2018 as a result of progressive tree mortality. However, mortality did not cause a significant reduction in specific ecosystem respiration. The respiration of the damaged forest was formed by efflux from soil (64.8%), living vegetation (15.5%), hotspots under dry standing trees (12.1%) and decomposition of woody debris (7.6%). High correlations between net ecosystem exchange and dry standing spruce stocks and average soil CO2 efflux were found. The decrease in the carbon sink was followed by a decline in evapotranspiration from 0.0142 ± 0.0003 g H2O m−2 s−1 for May–October 2010 to 0.0116 ± 0.0002 g H2O m−2 s−1 in 2018. We assumed that the decrease in carbon uptake was due to both the reduction in primary tree production and the decrease in the area of the unaffected stands. Our estimates show that an increase in tree mortality up to 27% of a stand area could turn an old-growth spruce forest into a net source of CO2. This should be taken into account when considering human-induced and climate-related effects on boreal forests.

Seasonal variations in photosynthetic functions of the urban landscape tree species Gingko biloba: photoperiod is a key trait

Seasonal variations in the leaf photosynthetic traits of an urban tree, Ginkgo biloba, were almost synchronized with the photoperiod. Non-stomatal limitations were a cue for photosynthesis in Ginkgo biloba. Photosynthetic functions, which are key traits in determining the carbon uptake of urban trees, exhibit significant seasonal variations in temperate zones. It is essential to clarify the seasonal dynamics of photosynthesis to evaluate the CO2 uptake in urban areas. We investigated seasonal variations in the photosynthetic traits of Ginkgo biloba, which is a major urban deciduous tall tree often planted in Japan. Seasonal variations in the leaf photosynthetic traits, including the maximum photosynthesis rate, maximum carboxylation rate, and mesophyll and stomatal conductance, were well fitted to quadratic models, in which they peaked around the summer solstice and then declined with time. Seasonal variations in the environmental variables, such as photoperiod, temperature, and solar radiation, were compared to those of the leaf photosynthetic traits, in which the photoperiod explained well variations in the leaf photosynthetic traits. Seasonal variations in photosynthesis were largely governed by non-stomatal limitations, i.e., mesophyll and biochemical limitations. The high synchrony of the photoperiod and photosynthetic traits during leaf maturation may cause an enhancement in the daily carbon uptake of G. biloba leaves around the summer solstice, which has the longest photoperiod, and thus, will lead to an increase in the annual carbon uptake.

Modeled Impacts of Sea Ice Exchange Processes on Arctic Ocean Carbon Uptake and Acidification (1980–2015)

AbstractA regional Arctic ice ocean model incorporating biogeochemical processes occurring inside the sea ice and water column is used to assess changes to the Arctic Ocean's carbon system, including oceanic carbon uptake and ocean acidification, over the recent period of Arctic sea ice decline (1980–2015). Two novel modifications are the following: (1) incorporation of carbon uptake by sea ice algae and (2) modification of the sea ice carbon pump to allow for vertical transport by brine plumes with high concentrations of dissolved inorganic carbon (DIC) and total alkalinity (TA) to the bottom of the mixed layer. The simulated carbon uptake north of 66.5°N increases by 20%, from 110 to 135 Tg C yr−1 from 1980–2015, and the mean pan‐Arctic sea surface pH decreases from 8.1 to 8.0. There is substantial regional and seasonal variability, highlighting potential problems with interpolating sparse measurements. Two sensitivity studies assess the effects of modifications to the near‐surface carbonate system. First, excluding the sea ice carbon pump results in a marked decrease in seasonal variability of pan‐Arctic‐mean sea surface DIC (∼25% less than the standard run) and TA (∼10% less), suggesting that neglecting the ice carbon pump results in models overestimating the saturation state in summer. Second, neglecting the sea ice algae results in an underestimation of the Arctic Ocean's annual carbon uptake (∼3% per year), indicating that the accumulating year‐on‐year underestimation of annual carbon uptake, as a result of neglecting sea ice algae, would lead to increasing error over multidecadal runs of polar ocean models.

Hot moments in ecosystem fluxes: High GPP anomalies exert outsized influence on the carbon cycle and are differentially driven by moisture availability across biomes

The ‘hot spot-hot moment’ concept is a long-standing and popular framework often invoked to explain spatially or temporally variable rates of biogeochemical cycling. However, this concept has been rarely extended to ecosystem fluxes such as gross primary productivity (GPP), in part due to the lack of a quantitative definition of hot moments that can be applied to large flux datasets. Here, we develop a general statistical framework for quantifying hot moments in GPP and identify their spatial patterns and climatic drivers. Using 308 site-years of eddy covariance data from the FLUXNET2015 dataset spanning 32 U.S. sites, we found hot moments in GPP to comprise a disproportionate percentage of annual carbon (C) uptake relative to the frequency of their occurrence. For example, at five sites over 12% of annual C uptake occurred during the ∼2% most extreme half-hourly or hourly observations of GPP. Hot moments were most quantitatively important for the C cycle in short-stature, arid ecosystem such as grasslands, woody savannas, and open shrublands, where these positive anomalies in GPP were caused by increases in moisture availability. In contrast, hot moments were less important for annual C uptake in more mesic ecosystems, where their occurrence was largely determined by high temperature and light availability. Our results point to a need to consider how short-term spikes in environmental conditions exert an outsized influence on annual GPP, and how future shifts in these conditions could impact the terrestrial C cycle.

Can more carbon be captured by grasslands? A case study of Inner Mongolia, China

Grasslands cover a large part of the Earth's surface and play an important role in the global carbon cycle. Previous studies have indicated that nearly half of the grassland vegetation cover has experienced degradation on a global scale; if this degradation is reversed, grasslands can act as potential carbon sinks. However, the question of how much more carbon (carbon gap) could be sequestrated by grassland vegetation by regulating human activities remains unanswered. Here, we present an innovative approach to assess the achievable carbon gap through focal analysis of long-term Moderate Resolution Imaging Spectroradiometer (MODIS) Net Primary Production (NPP) dataset or observed NPP (ONPP). In focal analysis, region segmentation was done to produce spatially homogeneous patches of the same types of soil, topography, and vegetation, referred to as S-T-V units, to minimize the variation in environmental conditions and their impacts on the NPP. Then, the ONPP within each S-T-V unit was rectified by offsetting the variations in potential NPP determined by the climate-oriented Miami NPP model. Hence, spatial variations in the climate-rectified ONPP (ONPPCR) in an S-T-V unit were solely determined by different human activities across locations. In a case study of the Inner Mongolia grassland of China, three focal statistics, namely mean (Mean), 95% percentile threshold (95%PCT), and maximum (Max) within each S-T-V unit were computed for ONPPCR for each year from 2000 to 2014 to assess the annual carbon uptake that was achievable by updating grassland management practices. The carbon gaps were assessed to be 11.8, 58.9, and 74.6 gC/m2 per year based on Mean, 95%PCT, and Max, respectively, compared to 65.0 gC/m2 per year based on the traditional pixel-based approach. We conclude that the carbon gap patterns identified from focal analysis are practically achievable and are more valuable in formulating policy-related decisions for grassland management. Implementing sustainable management practices that are currently being practiced at locations with high ONPPCR in neighboring degraded areas is expected to increase the carbon sequestration by grassland vegetation by one-third.

Winter CO2 Efflux From Sagebrush Shrublands Distributed Across the Rain‐to‐Snow Transition Zone

AbstractThe projected shifts in winter weather and snowpack conditions are expected to impact carbon storage in western U.S. rangelands. Sagebrush shrublands comprise much of the western United States, yet contribution of winter CO2 efflux to the overall carbon budget of these ecosystems remains uncertain. We explored factors controlling winter CO2 efflux measured using eddy covariance at five sagebrush‐dominated sites along an elevation/climate transect extending from 1,425 to 2,111 m. Results showed that winter CO2 efflux was modest but had important impacts on annual carbon budgets, and its impact increased in high‐elevation, snow‐dominated ecosystems compared to low, rain‐dominated ones. Observed cumulative winter CO2 efflux accounted for 8–30% of annual gross ecosystem production (GEP) and roughly approximated annual net carbon uptake. Omission of winter periods would have increased net uptake by 1.5 to 2.2 times. Within‐site variability in observed 30‐min winter CO2 efflux was related to soil temperature and moisture. Between‐site variability was attributed to available carbon stocks, including soil organic carbon and the previous year's GEP. At low elevations, lack of snow cover to insulate soil from freezing, coupled with lower carbon stocks, limited CO2 efflux. Conversely, large carbon stocks and deep snowpack that prevented soil freezing at high elevation led to increased CO2 efflux. These results show how climate and biota exert strong controls on winter ecosystem respiration and extend our understanding of how state factors influence winter CO2 efflux. Collectively, our findings suggest that an upward climatic shift in the rain‐to‐snow transition elevation may alter the carbon budget of sagebrush shrublands.

Cushion bogs are stronger carbon dioxide net sinks than moss-dominated bogs as revealed by eddy covariance measurements on Tierra del Fuego, Argentina

Abstract. The near-pristine bog ecosystems of Tierra del Fuego in southernmost Patagonia have so far not been studied in terms of their current carbon dioxide (CO2) sink strength. CO2 flux data from Southern Hemisphere peatlands are scarce in general. In this study, we present CO2 net ecosystem exchange (NEE) fluxes from two Fuegian bog ecosystems with contrasting vegetation communities. One site is located in a glaciogenic valley and developed as a peat moss-dominated raised bog, and the other site is a vascular plant-dominated cushion bog located at the coast of the Beagle Channel. We measured NEE fluxes with two identical eddy covariance (EC) setups at both sites for more than 2 years. With the EC method, we were able to observe NEE fluxes on an ecosystem level and at high temporal resolution. Using a mechanistic modeling approach, we estimated daily NEE models to gap fill and partition the half-hourly net CO2 fluxes into components related to photosynthetic uptake (gross primary production, GPP) and to total ecosystem respiration (TER). We found a larger relative variability of annual NEE sums between both years at the moss-dominated site. A warm and dry first year led to comparably high TER sums. Photosynthesis was also promoted by warmer conditions but less strongly than TER with respect to absolute and relative GPP changes. The annual NEE carbon (C) uptake was more than 3 times smaller in the warm year. Close to the sea at the cushion bog site, the mean temperature difference between both observed years was less pronounced, and TER stayed on similar levels. A higher amount of available radiation in the second observed year led to an increase in GPP (5 %) and NEE (35 %) C uptake. The average annual NEE-C uptake of the cushion bog (-122±76 gm-2a-1, n=2) was more than 4 times larger than the average uptake of the moss-dominated bog (-27±28 gm-2a-1, n=2).

Effects of even-aged and uneven-aged management on carbon dynamics and timber yield in boreal Norway spruce stands: a forest ecosystem model approach

Abstract We used a gap-type forest ecosystem model to study how even- and uneven-aged management affected the carbon dynamics and timber production in boreal Norway spruce stands. In business-as-usual management, the intensity of thinnings (from below) and single-tree selective cuttings followed those recommended for even-aged (BT) and uneven-aged management (BSC) in practical forestry in Finland. Moreover, higher or lower basal area thresholds, and shorter or longer production cycles, were used in simulations. We found that, the mean annual carbon uptake, volume growth, and carbon stock in trees and harvested timber, were nearly the same under even-aged (BT) and uneven-aged (BSC) management, when assuming full seed crop in latter one. However, the carbon stock in the soil and ecosystem and the mean annual net ecosystem exchange were slightly smaller under BT. The carbon retention time was longer under BSC. The net present value (NPV with interest rate of 3 per cent) of timber production was clearly lower under BT, when the calculation was initiated at planting on clear-cutting area, in opposite to when initiating calculation a few years before the second thinning. Higher basal area thresholds and longer production cycles increased carbon stocks, carbon retention and timber yield, regardless of management system. On the other hand, the results of uneven-aged management (BSC) were very sensitive to the success of natural regeneration and ingrowth of seedlings, as a reduction of the seed crop by 25–75 per cent from the full seed crop decreases the volume growth by 44–74 per cent and timber yield up to 46 per cent.

남부지방 조경수종의 탄소저감과 계량모델†: - 동백나무, 배롱나무 및 가시나무를 대상으로 -

This study quantified, through a direct harvesting method, storage and annual uptake of carbon from open-grown trees for three landscape tree species frequently planted in the southern region of Korea, and developed quantitative models to easily estimate the carbon reduction by tree growth for each species. The tree species for the study included <italic>Camellia japonica, Lagerstroemia indica</italic>, and <italic>Quercus myrsinaefolia</italic>, for which no information on carbon storage and uptake was available. Ten tree individuals for each species (a total of 30 individuals) were sampled considering various stem diameter sizes at given intervals. The study measured biomass for each part of the sample trees to quantify the total carbon storage per tree. Annual carbon uptake per tree was computed by analyzing the radial growth rates of the stem samples at breast height or ground level. Quantitative models were developed using stem diameter as an independent variable to easily calculate storage and annual uptake of carbon per tree for study species. All the quantitative models showed high fitness with <italic>r</italic>² values of 0.94-0.98. The storage and annual uptake of carbon from a <italic>Q. myrsinaefolia</italic> tree with dbh of 10 cm were 24.0 kg and 4.5 kg/yr, respectively. A <italic>C. japonica</italic> tree and <italic>L. indica</italic> tree with dg of 10 cm stored 11.2 kg and 8.1 kg of carbon and annually sequestered 2.6 kg and 1.2 kg, respectively. The above-mentioned carbon storage equaled the amount of carbon emitted from the gasoline consumption of about 42 L for <italic>Q. myrsinaefolia</italic>, 20 L for <italic>C. japonica</italic>, and 14 L for <italic>L. indica</italic>. A tree with the diameter size of 10 cm annually offset carbon emissions from gasoline use of approximately 8 L for <italic>Q. myrsinaefolia</italic>, 5 L for <italic>C. japonica</italic>, and 2 L for <italic>L. indica</italic>. The study pioneers in quantifying biomass and carbon reduction for the landscape tree species in the southern region despite difficulties in direct cutting and root digging of the planted trees.

Regional Wind Variability Modulates the Southern Ocean Carbon Sink

The Southern Ocean south of 35°S accounts for approximately half of the annual oceanic carbon uptake, thereby substantially mitigating the effects of anthropogenic carbon dioxide (CO2) emissions. The intensity of this important carbon sink varies considerably on inter-annual to decadal timescales. However, the drivers of this variability are still debated, challenging our ability to accurately predict the future role of the Southern Ocean in absorbing atmospheric carbon. Analysing mapped sea-air CO2 fluxes, estimated from upscaled surface ocean CO2 measurements, we find that the overall Southern Ocean carbon sink has weakened since ~2011, reversing the trend of the reinvigoration period of the 2000s. Although we find significant regional positive and negative responses of the Southern Ocean carbon uptake to changes in the Southern Annular Mode (SAM) over the past 35 years, the net effect of the SAM on the Southern Ocean carbon sink variability is approximately zero, due to the opposing effects of enhanced outgassing in upwelling regions and enhanced carbon uptake elsewhere. Instead, regional shifts in sea level pressure, linked to zonal wavenumber 3 (ZW3) and related changes in surface winds substantially contribute to the inter-annual to decadal variability of the Southern Ocean carbon sink.

Carbon reduction and planning strategies for urban parks in Seoul

This study quantified carbon storage and uptake for urban parks in Seoul, the capital of the Republic of Korea. A total of 38 study parks were selected using a systematic random sampling method and all the trees in the parks were field-inventoried. Carbon storage and uptake by the park trees were estimated applying a quantitative model for urban open-grown trees of each species. Mean carbon storage per unit of park area, basal area, and crown cover by the trees was 38.5 ± 3.0 t/ha, 27.3 ± 0.8 kg/100 cm2, and 7.4 ± 0.4 kg/m2, respectively. Annual carbon uptake per unit area and cover by the trees averaged 3.5 ± 0.2 t/ha/yr, 2.5 ± 0.1 kg/100 cm2/yr, and 0.7 ± 0.0 kg/m2/yr, respectively. The major determinants of the levels of carbon storage and uptake were species, density, sizes, and layering structures of the planted trees. The trees across all urban parks in Seoul were estimated to store 222.3 kt of carbon and to annually sequester 20.2 kt of carbon. The trees in these parks played an important role in annually offsetting carbon emissions from gasoline consumption by approximately 2.3% of the total population of the city. The economic value of the annual carbon uptake, which was $7.1million/yr, equaled 15.1% of the annual maintenance budget of the parks in the city. However, the role of study parks as a source of carbon uptake was limited due to the distribution of large grass and impervious areas, the single-layered structures, and the dominance of small trees. Planning strategies were explored to enhance carbon reduction effects of the parks. They included the expansion of tree planting spaces through the minimization of unnecessary grass and paving areas, the active tree planting in the potential planting spaces, the multi-layered planting grouped with larger trees, and the planting of tree species having satisfactory growth rates. This study puts an emphasis on finding out the present carbon offset levels of urban parks on which information is limited and suggesting a future direction of park planning based on a detailed actual survey.

Time shift between net and gross CO2 uptake and growth derived from tree rings in pine and spruce

Key messageA 6–9 month backward time shift of the carbon uptake gave the highest correlation between annual biomass increment and carbon uptake in this old even aged forest.Plants’ carbon uptake and allocation to different biomass compartments is an important process for both wood production and climate mitigation. Measurements of the net ecosystem carbon dioxide exchange between ecosystems and the atmosphere provide insights into the processes of photosynthesis, respiration and accumulation of carbon over time, and the increase in woody biomass can be assessed by allometric functions based on stem diameter measurements. The fraction of carbon allocated to radial stem growth varies over time, and a lag between carbon uptake and growth can be expected. The dynamics of non-structural carbohydrates and autotrophic and heterotrophic respiration are key mechanisms for understanding this lag effect. In this study, a 9-year record of carbon flux and tree-ring data from Norunda, Sweden was used to investigate the relationship between net and gross carbon uptake and carbon allocated to growth. The flux data were aggregated to monthly sums. When full 12-month periods of accumulated carbon exchange were successively shifted backwards in time, the highest correlation was found with a 6–9 month shift, showing that a large part of the previous growing season was important for explaining the biomass increment of the following year.

Carbon flux phenology and net ecosystem productivity simulated by a bioclimatic index in an alpine steppe-meadow on the Tibetan Plateau

Plant phenology is one of the main controls of variation in net ecosystem productivity (NEP). Accurate representation of vegetation phenology is important for predicting ecosystem carbon budget. Although using satellite observation to determine vegetation phenology is becoming a mature option, there are still uncertainties in its application at site scales. Our purpose is to establish a more robust phenological index to accurately predict carbon uptake phenology, which detailed results can complement the shortcomings of MODIS NDVI-derived phenology. Here we used a growing season index (GSI) phenology model to simulate carbon flux phenology (CFP) including the start of carbon uptake (CUstart), the end of carbon uptake (CUend) and the length of carbon uptake period (CUP) in an alpine meadow ecosystem on the Tibetan Plateau and to compare the results with those modeled from MODIS NDVI. We also further analyzed the main environmental factors in controlling CFP. The results indicated that the GSI model made substantially more precise prediction for CUstart, CUend and CUP (with higher correlation R2>0.90) than that of the MODIS derived phenology. The GSI model was also superior to NDVI in predicting both seasonal and annual variations of net ecosystem productivity (NEP). Moreover, CUP played an important role in regulating ecosystem carbon balance in the study site because NEP was significantly positive correlated with the period of annual carbon uptake. NEP would increase by 1.63 g C m−2 year-1 if one CUP-day was extended. Further, CUP was influenced by variation in CUstart. Previously overlooked water variability (soil water content and VPD) played a significant role in controlling CUP and CUstart. In addition, temperature could enhance water stress to delay CUstart and shorten CUP. It is indicated that decrease in carbon uptake could be induced by accelerative water stress in the face of global warming in the alpine meadow. These results suggest that CFP is more sensitive to not only temperature but also water condition, and a combination of soil water and temperature could be a useful way to enhance the estimation of CFP in future ecosystem model.

Assessing the dynamics of vegetation productivity in circumpolar regions with different satellite indicators of greenness and photosynthesis

Abstract. High-latitude treeless ecosystems represent spatially highly heterogeneous landscapes with small net carbon fluxes and a short growing season. Reliable observations and process understanding are critical for projections of the carbon balance of the climate-sensitive tundra. Space-borne remote sensing is the only tool to obtain spatially continuous and temporally resolved information on vegetation greenness and activity in remote circumpolar areas. However, confounding effects from persistent clouds, low sun elevation angles, numerous lakes, widespread surface inundation, and the sparseness of the vegetation render it highly challenging. Here, we conduct an extensive analysis of the timing of peak vegetation productivity as shown by satellite observations of complementary indicators of plant greenness and photosynthesis. We choose to focus on productivity during the peak of the growing season, as it importantly affects the total annual carbon uptake. The suite of indicators are as follows: (1) MODIS-based vegetation indices (VIs) as proxies for the fraction of incident photosynthetically active radiation (PAR) that is absorbed (fPAR), (2) VIs combined with estimates of PAR as a proxy of the total absorbed radiation (APAR), (3) sun-induced chlorophyll fluorescence (SIF) serving as a proxy for photosynthesis, (4) vegetation optical depth (VOD), indicative of total water content and (5) empirically upscaled modelled gross primary productivity (GPP). Averaged over the pan-Arctic we find a clear order of the annual peak as APAR ≦ GPP&lt;SIF&lt;VIs/VOD. SIF as an indicator of photosynthesis is maximised around the time of highest annual temperatures. The modelled GPP peaks at a similar time to APAR. The time lag of the annual peak between APAR and instantaneous SIF fluxes indicates that the SIF data do contain information on light-use efficiency of tundra vegetation, but further detailed studies are necessary to verify this. Delayed peak greenness compared to peak photosynthesis is consistently found across years and land-cover classes. A particularly late peak of the normalised difference vegetation index (NDVI) in regions with very small seasonality in greenness and a high amount of lakes probably originates from artefacts. Given the very short growing season in circumpolar areas, the average time difference in maximum annual photosynthetic activity and greenness or growth of 3 to 25 days (depending on the data sets chosen) is important and needs to be considered when using satellite observations as drivers in vegetation models.

Study of the Carbon Budget of a Temperate-Climate Vineyard: Inter-Annual Variability of CO2 Flux

Agriculture can improve the global carbon budget by reducing emissions and increasing soil carbon sequestration. Woody crops like vineyards present biological, structural, and management peculiarities, such as perennial structure, abundant pruning debris, and interrow cover cropping, which could potentially sequester a significant amount of CO2. However, such ecosystems are still poorly understood. Here, we present results from a long-term study from northeastern Italy on the carbon budget of a vineyard (cv. Sauvignon blanc, Guyot pruned, vertical shoot-positioned trellised, with grass-covered alleys). We used the eddy covariance technique to continuously monitor the net ecosystem CO2 exchange (NEE), and here we present the first three years of data (May 2014 to April 2017). The vineyard acted as a moderate carbon sink, with an average annual net carbon uptake of 134 g C/ m2. However, the inter-annual variability of NEE was high, and environmental conditions during the growing season had a strong impact. The annual carbon uptake was greatest in 2014 (207 g C/m2), when there was abundant rain in summer. In contrast, in 2015, several heat waves decreased the annual carbon uptake to 69 g C/m2. Elevated air temperature and low soil moisture in summer significantly reduced net carbon flux during times when the vineyard activity was at maximum. Nevertheless, we show that a vineyard can behave as a carbon sink in the medium-to-long term, which should be factored into agricultural and environmental policies, complementing the standard life cycle assessment of the wine production chain.

The surface-atmosphere exchange of carbon dioxide in tropical rainforests: Sensitivity to environmental drivers and flux measurement methodology

Tropical rainforests play a central role in the Earth system by regulating climate, maintaining biodiversity, and sequestering carbon. They are under threat by direct anthropogenic impacts like deforestation and the indirect anthropogenic impacts of climate change. A synthesis of the factors that determine the net ecosystem exchange of carbon dioxide (NEE) at the site scale across different forests in the tropical rainforest biome has not been undertaken to date. Here, we study NEE and its components, gross ecosystem productivity (GEP) and ecosystem respiration (RE), across thirteen natural and managed forests within the tropical rainforest biome with 63 total site-years of eddy covariance data. Our results reveal that the five ecosystems with the largest annual gross carbon uptake by photosynthesis (i.e. GEP > 3000 g C m−2 y-1) have the lowest net carbon uptake – or even carbon losses – versus other study ecosystems because RE is of a similar magnitude. Sites that provided subcanopy CO2 storage observations had higher average magnitudes of GEP and RE and lower average magnitudes of NEE, highlighting the importance of measurement methodology for understanding carbon dynamics in ecosystems with characteristically tall and dense vegetation. A path analysis revealed that vapor pressure deficit (VPD) played a greater role than soil moisture or air temperature in constraining GEP under light saturated conditions across most study sites, but to differing degrees from -0.31 to -0.87 μmol CO2 m−2 s-1 hPa-1. Climate projections from 13 general circulation models (CMIP5) under the representative concentration pathway that generates 8.5 W m−2 of radiative forcing suggest that many current tropical rainforest sites on the lower end of the current temperature range are likely to reach a climate space similar to present-day warmer sites by the year 2050, warmer sites will reach a climate not currently experienced, and all forests are likely to experience higher VPD. Results demonstrate the need to quantify if and how mature tropical trees acclimate to heat and water stress, and to further develop flux-partitioning and gap-filling algorithms for defensible estimates of carbon exchange in tropical rainforests.

Carbon sink and source dynamics of a eutrophic deep lake using multiple flux observations over multiple years

AbstractRecent research has shown lakes play an outsized role in carbon cycling, but long‐term continuous observations and analysis of carbon dynamics are rare, limiting our understanding of interannual variation, important timescales of variability, and drivers of efflux. Therefore, we examined lake‐atmosphere carbon fluxes with the goal of quantifying annual trends and patterns in lake carbon efflux and identifying important timescales. To do so, this study integrated 6 yr of eddy‐covariance flux tower observations of lake‐atmosphere fluxes with high‐frequency observations of in‐lake temperature, dissolved oxygen, and partial pressure of CO2, for a eutrophic lake in Wisconsin, U.S.A. While growing season fluxes are variable and switch between source and sink, annual net carbon fluxes show the lake acts as an annual sink of carbon, with the magnitude depending on climate, along with the timing and strength of fall turnover, with half of the total annual carbon uptake happening in October and November.