Interannual variability in carbon dioxide fluxes and flux–climate relationships on grazed and ungrazed northern mixed-grass prairie

The annual carbon (C) budget of grasslands is highly dynamic, dependent on grazing history and on effects of interannual variability (IAV) in climate on carbon dioxide (CO₂) fluxes. Variability in climatic drivers may directly affect fluxes, but also may indirectly affect fluxes by altering the response of the biota to the environment, an effect termed ‘functional change’. We measured net ecosystem exchange of CO₂ (NEE) and its diurnal components, daytime ecosystem CO₂ exchange (P_D) and night‐time respiration (R_E), on grazed and ungrazed mixed‐grass prairie in North Dakota, USA, for five growing seasons. Our primary objective was to determine how climatic anomalies influence variability in CO₂ exchange. We used regression analysis to distinguish direct effects of IAV in climate on fluxes from functional change. Functional change was quantified as the improvement in regression on fitting a model in which slopes of flux–climate relationships vary among years rather than remain invariant. Functional change and direct effects of climatic variation together explained about 20% of variance in weekly means of NEE, P_D, and R_E. Functional change accounted for more than twice the variance in fluxes of direct effects of climatic variability. Grazing did not consistently influence the contribution of functional change to flux variability, but altered which environmental variable best explained year‐to‐year differences in flux–climate slopes, reduced IAV in seasonal means of fluxes, lessened the strength of flux–climate correlations, and increased NEE by reducing R_E relatively more than P_D. Most of these trends are consistent with the interpretation that grazing reduced the influence of plants on ecosystem fluxes. Because relationships between weekly values of fluxes and climatic regulators changed annually, year‐to‐year differences in the C balance of these ecosystems cannot be predicted from knowledge of IAV in climate alone.     

Environmental variation is directly responsible for short- but not long-term variation in forest-atmosphere carbon exchange

Tower‐based eddy covariance measurements of forest‐atmosphere carbon dioxide (CO₂) exchange from many sites around the world indicate that there is considerable year‐to‐year variation in net ecosystem exchange (NEE). Here, we use a statistical modeling approach to partition the interannual variability in NEE (and its component fluxes, ecosystem respiration, R_eco, and gross photosynthesis, P_gross) into two main effects: variation in environmental drivers (air and soil temperature, solar radiation, vapor pressure deficit, and soil water content) and variation in the biotic response to this environmental forcing (as characterized by the model parameters). The model is applied to a 9‐year data set from the Howland AmeriFlux site, a spruce‐dominated forest in Maine, USA. Gap‐filled flux measurements at this site indicate that the forest has been sequestering, on average, 190 g C m⁻² yr⁻¹, with a range from 130 to 270 g C m⁻² yr⁻¹. Our fitted model predicts somewhat more uptake (mean 270 g C m⁻² yr⁻¹), but interannual variation is similar, and wavelet variance analyses indicate good agreement between tower measurements and model predictions across a wide range of timescales (hours to years). Associated with the interannual variation in NEE are clear differences among years in model parameters for both R_eco and P_gross. Analysis of model predictions suggests that, at the annual time step, about 40% of the variance in modeled NEE can be attributed to variation in environmental drivers, and 55% to variation in the biotic response to this forcing. As model predictions are aggregated at longer timescales (from individual days to months to calendar year), variation in environmental drivers becomes progressively less important, and variation in the biotic response becomes progressively more important, in determining the modeled flux. There is a strong negative correlation between modeled annual P_gross and R_eco (r=−0.93, P≤0.001); two possible explanations for this correlation are discussed. The correlation promotes homeostasis of NEE: the interannual variation in modeled NEE is substantially less than that for either P_gross or R_eco     

Modelled net carbon gain responses to climate change in boreal trees: Impacts of photosynthetic parameter selection and acclimation

Boreal forests are crucial in regulating global vegetation‐atmosphere feedbacks, but the impact of climate change on boreal tree carbon fluxes is still unclear. Given the sensitivity of global vegetation models to photosynthetic and respiration parameters, we determined how predictions of net carbon gain (C‐gain) respond to variation in these parameters using a stand‐level model (MAESTRA). We also modelled how thermal acclimation of photosynthetic and respiratory temperature sensitivity alters predicted net C‐gain responses to climate change. We modelled net C‐gain of seven common boreal tree species under eight climate scenarios across a latitudinal gradient to capture a range of seasonal temperature conditions. Physiological parameter values were taken from the literature together with different approaches for thermally acclimating photosynthesis and respiration. At high latitudes, net C‐gain was stimulated up to 400% by elevated temperatures and CO₂ in the autumn but suppressed at the lowest latitudes during midsummer under climate scenarios that included warming. Modelled net C‐gain was more sensitive to photosynthetic capacity parameters (V_cmax, J_max, Arrhenius temperature response parameters, and the ratio of J_max to V_cmax) than stomatal conductance or respiration parameters. The effect of photosynthetic thermal acclimation depended on the temperatures where it was applied: acclimation reduced net C‐gain by 10%–15% within the temperature range where the equations were derived but decreased net C‐gain by 175% at temperatures outside this range. Thermal acclimation of respiration had small, but positive, impacts on net C‐gain. We show that model simulations are highly sensitive to variation in photosynthetic parameters and highlight the need to better understand the mechanisms and drivers underlying this variability (e.g., whether variability is environmentally and/or biologically driven) for further model improvement.     

Partitioning net ecosystem carbon exchange with isotopic fluxes of CO2

Because biological and physical processes alter the stable isotopic composition of atmospheric CO₂, variations in isotopic content can be used to investigate those processes. Isotopic flux measurements of ¹³CO₂ above terrestrial ecosystems can potentially be used to separate net ecosystem CO₂ exchange (NEE) into its component fluxes, net photosynthetic assimilation (F_A) and ecosystem respiration (F_R). In this paper theory is developed to partition measured NEE into F_A and F_R, using measurements of fluxes of CO₂ and ¹³CO₂, and isotopic composition of respired CO₂ and forest air. The theory is then applied to fluxes measured (or estimated, for ¹³CO₂) in a temperate deciduous forest in eastern Tennessee (Walker Branch Watershed). It appears that there is indeed enough additional information in ¹³CO₂ fluxes to partition NEE into its photosynthetic and respiratory components. Diurnal patterns in F_A and F_R were obtained, which are consistent in magnitude and shape with patterns obtained from NEE measurements and an exponential regression between night‐time NEE and temperature (a standard technique which provides alternate estimates of F_R and F_A). The light response curve for photosynthesis (F_A vs. PAR) was weakly nonlinear, indicating potential for saturation at high light intensities. Assimilation‐weighted discrimination against ¹³CO₂ for this forest during July 1999 was 16.8–17.1‰, depending on canopy conductance. The greatest uncertainties in this approach lie in the evaluation of canopy conductance and its effect on whole‐canopy photosynthetic discrimination, and thus the indirect methods used to estimate isotopic fluxes. Direct eddy covariance measurements of ¹³CO₂ flux are needed to assess the validity of the assumptions used and provide defensible isotope‐based estimates of the component fluxes of net ecosystem exchange.     

Temperature controls ecosystem CO2 exchange of an alpine meadow on the northeastern Tibetan Plateau

Alpine ecosystems are extremely vulnerable to climate change. To address the potential variability of the responses of alpine ecosystems to climate change, we examined daily CO₂ exchange in relation to major environmental variables. A dataset was obtained from an alpine meadow on the Qinghai‐Tibetan Plateau from eddy covariance measurements taken over 3 years (2002–2004). Path analysis showed that soil temperature at 5 cm depth (T_s5) had the greatest effect on daily variation in ecosystem CO₂ exchange all year around, whereas photosynthetic photon flux density (PPFD) had a high direct effect on daily variation in CO₂ flux during the growing season. The combined effects of temperature and light regimes on net ecosystem CO₂ exchange (NEE) could be clearly categorized into three areas depending on the change in T_s5: (1) almost no NEE change irrespective of variations in light and temperature when T_s5 was below 0 °C; (2) an NEE increase (i.e. CO₂ released from the ecosystem) with increasing T_s5, but little response to variation in light regime when 0 °C≤T_s5≤8 °C; and (3) an NEE decrease with increase in T_s5 and PPFD when T_s5 was approximately >8 °C. The highest daily net ecosystem CO₂ uptake was observed under the conditions of daily mean T_s5 of about 15 °C and daily mean PPFD of about 50 mol m⁻² day⁻¹. The results suggested that temperature is the most critical determinant of CO₂ exchange in this alpine meadow ecosystem and may play an important role in the ecosystem carbon budget under future global warming conditions.     

Growing season net ecosystem CO2 exchange of two desert ecosystems with alkaline soils in Kazakhstan

Central Asia is covered by vast desert ecosystems, and the majority of these ecosystems have alkaline soils. Their contribution to global net ecosystem CO₂ exchange (NEE) is of significance simply because of their immense spatial extent. Some of the latest research reported considerable abiotic CO₂ absorption by alkaline soil, but the rate of CO₂ absorption has been questioned by peer communities. To investigate the issue of carbon cycle in Central Asian desert ecosystems with alkaline soils, we have measured the NEE using eddy covariance (EC) method at two alkaline sites during growing season in Kazakhstan. The diurnal course of mean monthly NEE followed a clear sinusoidal pattern during growing season at both sites. Both sites showed significant net carbon uptake during daytime on sunny days with high photosynthetically active radiation (PAR) but net carbon loss at nighttime and on cloudy and rainy days. NEE has strong dependency on PAR and the response of NEE to precipitation resulted in an initial and significant carbon release to the atmosphere, similar to other ecosystems. These findings indicate that biotic processes dominated the carbon processes, and the contribution of abiotic carbon process to net ecosystem CO₂ exchange may be trivial in alkaline soil desert ecosystems over Central Asia.     

Spatial variability and major controlling factors of CO2 sink strength in Asian terrestrial ecosystems: evidence from eddy covariance data

Asian terrestrial ecosystems cover an extensive area characterized by a large variety in climates and ecosystem properties. The observations of ecosystem CO₂ flux in this area are increasing both in duration and spatial density, but no synthesis has yet been conducted. We surveyed CO₂ flux observation data obtained by eddy covariance methods at 49 sites in terrestrial Asia. The measurements at most sites (44 of 49) began after 2000. The net ecosystem uptake of CO₂ (NEE) varied greatly among sites and years and averaged −132.6±73.7, −250.1±206.1, and −180.1±361.7 g C m⁻² yr⁻¹, in boreal, temperate, and tropical Asia, respectively, and the coefficient of variation among sites increased from boreal to tropical Asia. The site-averaged annual NEE was correlated linearly with the mean annual temperature (T_air) and also correlated logarithmically with the precipitation. Multiple regression analysis and stepwise analysis indicated that photosynthetic active radiation (PAR) and T_air were the most significant predictors of the annual NEE. The study results suggest that Asian terrestrial ecosystems are currently significant net CO₂ sinks and that the sink strength is largely controlled by temperature, moisture, and light conditions.     

Temperature and precipitation controls over leaf‐ and ecosystem‐level CO2 flux along a woody plant encroachment gradient

Conversion of grasslands to woodlands may alter the sensitivity of CO₂ exchange of individual plants and entire ecosystems to air temperature and precipitation. We combined leaf‐level gas exchange and ecosystem‐level eddy covariance measurements to quantify the effects of plant temperature sensitivity and ecosystem temperature responses within a grassland and mesquite woodland across seasonal precipitation periods. In so doing, we were able to estimate the role of moisture availability on ecosystem temperature sensitivity under large‐scale vegetative shifts. Optimum temperatures (T_opt) for net photosynthetic assimilation (A) and net ecosystem productivity (NEP) were estimated from a function fitted to A and NEP plotted against air temperature. The convexities of these temperature responses were quantified by the range of temperatures over which a leaf or an ecosystem assimilated 50% of maximum NEP (Ω₅₀). Under dry pre‐ and postmonsoon conditions, leaf‐level Ω₅₀ in C₃ shrubs were two‐to‐three times that of C₄ grasses, but under moist monsoon conditions, leaf‐level Ω₅₀ was similar between growth forms. At the ecosystems‐scale, grassland NEP was more sensitive to precipitation, as evidenced by a 104% increase in maximum NEP at monsoon onset, compared to a 57% increase in the woodland. Also, woodland NEP was greater across all temperatures experienced by both ecosystems in all seasons. By maintaining physiological function across a wider temperature range during water‐limited periods, woody plants assimilated larger amounts of carbon. This higher carbon‐assimilation capacity may have significant implications for ecosystem responses to projected climate change scenarios of higher temperatures and more variable precipitation, particularly as semiarid regions experience conversions from C₄ grasses to C₃ shrubs. As regional carbon models, CLM 4.0, are now able to incorporate functional type and photosynthetic pathway differences, this work highlights the need for a better integration of the interactive effects of growth form/functional type and photosynthetic pathway on water resource acquisition and temperature sensitivity.     

Ecophysiological traits of plant functional groups in forest and pasture ecosystems from eastern Amazônia,Brazil

The plant functional group approach has the potential to clarify ecological patterns and is of particular importance in simplifying the application of ecological models in high biodiversity ecosystems. Six functional groups (pasture grass, pasture sapling, top-canopy tree, top-canopy liana, mid canopy tree, and understory tree) were established a priori based on ecosystem inhabited, life form, and position within the forest canopy profile on eastern Amazonian region. Ecophysiological traits related to photosynthetic gas exchange were then used to characterize such groups. The ecophysiological traits evaluated showed considerable variations among groups. The pasture grass functional group (a C₄ photosynthetic pathway species) showed high instantaneous water use efficiency (A _max/g _s@A _max), high photosynthetic nitrogen use efficiency (A _max/N _area), and high ratio of A _max to dark respiration (A _max/R _d). Among the species with the C₃ photosynthetic pathway, the top-canopy liana group showed the highest mean of A _max/g _s@A _max, statistically distinct from the lowest average presented by the understory tree group. Furthermore, the pasture sapling group showed the lowest average of A _max/R _d, statistically distinct from the high average observed for the understory tree group. Welch-ANOVAs followed by Games–Howell post hoc tests applied to ecophysiological traits produced reasonable distinctions among functional groups, although no significant distinction was detected between the groups top-canopy tree and pasture sapling. Species distribution within the functional groups was accurately reproduced by discriminant analyses based on species averages of ecophysiological traits. The present work convincingly shows that the functional groups identified have distinct ecophysiological characteristics, with the potential to respond differently to environmental factors. Such information is of great importance in modeling efforts that evaluate the effects of dynamic changes in tropical plant communities over ecosystem primary productivity.     

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.     

Differential responses of production and respiration to temperature and moisture drive the carbon balance across a climatic gradient in New Mexico

Southwestern North America faces an imminent transition to a warmer, more arid climate, and it is critical to understand how these changes will affect the carbon balance of southwest ecosystems. In order to test our hypothesis that differential responses of production and respiration to temperature and moisture shape the carbon balance across a range of spatio‐temporal scales, we quantified net ecosystem exchange (NEE) of CO₂ and carbon storage across the New Mexico Elevational Gradient, which consists of six eddy‐covariance sites representing biomes ranging from desert to subalpine conifer forest. Within sites, hotter and drier conditions were associated with an increasing advantage of respiration relative to production such that daily carbon uptake peaked at intermediate temperatures – with carbon release often occurring on the hottest days – and increased with soil moisture. Across sites, biotic adaptations modified but did not override the dominant effects of climate. Carbon uptake increased with decreasing temperature and increasing precipitation across the elevational gradient; NEE ranged from a source of ～30 g C m^?2 yr^?1 in the desert grassland to a sink of ～350 g C m^?2 yr^?1 in the subalpine conifer forest. Total aboveground carbon storage increased dramatically with elevation, ranging from 186 g C m^?2 in the desert grassland to 26 600 g C m^?2 in the subalpine conifer forest. These results make sense in the context of global patterns in NEE and biomass storage, and support that increasing temperature and decreasing moisture shift the carbon balance of ecosystems in favor of respiration, such that the potential for ecosystems to sequester and store carbon is reduced under hot and/or dry conditions. This implies that projected climate change will trigger a substantial net release of carbon in these New Mexico ecosystems (～3 Gt CO₂ statewide by the end of the century), thereby acting as a positive feedback to climate change.     

Disentangling the contributions of ontogeny and water stress to photosynthetic limitations in almond trees

Very few studies have attempted to disentangle the respective role of ontogeny and water stress on leaf photosynthetic attributes. The relative significance of both effects on photosynthetic attributes has been investigated in leaves of field‐grown almond trees [Prunus dulcis (Mill.) D. A. Webb] during four growth cycles. Leaf ontogeny resulted in enhanced leaf dry weight per unit area (W_a), greater leaf dry‐to‐fresh weight ratio and lower N content per unit of leaf dry weight (N_w). Concomitantly, area‐based maximum carboxylation rate (V_cmax), maximum electron transport rate (J_max), mesophyll conductance to CO₂ diffusion (gm)′ and light‐saturated net photosynthesis (A_max) declined in both well‐watered and water‐stressed almond leaves. Although g_m and stomatal conductance (g_s) seemed to be co‐ordinated, a much stronger coordination in response to ontogeny and prolonged water stress was observed between g_m and the leaf photosynthetic capacity. Under unrestricted water supply, the leaf age‐related decline of A_max was equally driven by diffusional and biochemical limitations. Under restricted soil water availability, A_max was mainly limited by g_s and, to a lesser extent, by photosynthetic capacity and g_m. When both ontogeny and water stress effects were combined, diffusional limitations was the main determinant of photosynthesis limitation, while stomatal and biochemical limitations contributed similarly.     

Partitioning net carbon dioxide fluxes into photosynthesis and respiration using neural networks

The eddy covariance (EC) technique is used to measure the net ecosystem exchange (NEE) of CO₂ between ecosystems and the atmosphere, offering a unique opportunity to study ecosystem responses to climate change. NEE is the difference between the total CO₂ release due to all respiration processes (RECO), and the gross carbon uptake by photosynthesis (GPP). These two gross CO₂ fluxes are derived from EC measurements by applying partitioning methods that rely on physiologically based functional relationships with a limited number of environmental drivers. However, the partitioning methods applied in the global FLUXNET network of EC observations do not account for the multiple co‐acting factors that modulate GPP and RECO flux dynamics. To overcome this limitation, we developed a hybrid data‐driven approach based on combined neural networks (NN_C‐part). NN_C‐part incorporates process knowledge by introducing a photosynthetic response based on the light‐use efficiency (LUE) concept, and uses a comprehensive dataset of soil and micrometeorological variables as fluxes drivers. We applied the method to 36 sites from the FLUXNET2015 dataset and found a high consistency in the results with those derived from other standard partitioning methods for both GPP (R² > .94) and RECO (R² > .8). High consistency was also found for (a) the diurnal and seasonal patterns of fluxes and (b) the ecosystem functional responses. NN_C‐part performed more realistic than the traditional methods for predicting additional patterns of gross CO₂ fluxes, such as: (a) the GPP response to VPD, (b) direct effects of air temperature on GPP dynamics, (c) hysteresis in the diel cycle of gross CO₂ fluxes, (d) the sensitivity of LUE to the diffuse to direct radiation ratio, and (e) the post rain respiration pulse after a long dry period. In conclusion, NN_C‐part is a valid data‐driven approach to provide GPP and RECO estimates and complementary to the existing partitioning methods.     

Carbon sequestration and ecosystem respiration for 4 years in a Scots pine forest

The net exchange of CO₂ (NEE) between a Scots pine (Pinus sylvestris L.) forest ecosystem in eastern Finland and the atmosphere was measured continuously by the eddy covariance (EC) technique over 4 years (1999–2002). The annual temperature coefficient (Q₁₀) of ecosystem respiration (R) for these years, respectively, was 2.32, 2.66, 2.73 and 2.69. The light‐saturated rate of photosynthesis (A_max) was highest in July or August, with an annual average A_max of 10.9, 14.6, 15.3 and 17.1 μmol m^?2 s^?1 in the 4 years, respectively. There was obvious seasonality in NEE, R and gross primary production (GPP), exhibiting a similar pattern to photosynthetically active radiation (PAR) and air temperature. The integrated daily NEE ranged from 2.59 to ?4.97 g C m^?2 day^?1 in 1999, from 2.70 to ?4.72 in 2000, from 2.61 to ?4.71 in 2001 and from 5.27 to ?4.88 in 2002. The maximum net C uptake occurred in July, with the exception of 2000, when it was in June. The interannual variation in ecosystem C flux was pronounced. The length of the growing season, based on net C uptake, was 179, 170, 175 and 176 days in 1999–2002, respectively, and annual net C sequestration was 152, 101, 172 and 205 g C m^?2 yr^?1. It is estimated that ecosystem respiration contributed 615, 591, 752 and 879 g C m^?2 yr^?1 to the NEE in these years, leading to an annual GPP of ?768, ?692, ?924 and ?1084 g C m^?2 yr^?1. It is concluded that temperature and PAR were the main determinants of the ecosystem CO₂ flux. Interannual variations in net C sequestration are predominantly controlled by average air temperature and integrated radiation in spring and summer. Four years of EC data indicate that boreal Scots pine forest ecosystem in eastern Finland acts as a relatively powerful carbon sink. Carbon sequestration may benefit from warmer climatic conditions.     

Coupling between carbon cycling and climate in a high-elevation,subalpine forest: a model-data fusion analysis

Fundamental questions exist about the effects of climate on terrestrial net ecosystem CO₂ exchange (NEE), despite a rapidly growing body of flux observations. One strategy to clarify ecosystem climate–carbon interactions is to partition NEE into its component fluxes, gross ecosystem CO₂ exchange (GEE) and ecosystem respiration (R _E), and evaluate the responses to climate of each component flux. We separated observed NEE into optimized estimates of GEE and R _E using an ecosystem process model combined with 6 years of continuous flux data from the Niwot Ridge AmeriFlux site. In order to gain further insight into the processes underlying NEE, we partitioned R _E into its components: heterotrophic (R _H) and autotrophic (R _A) respiration. We were successful in separating GEE and R _E, but less successful in accurately partitioning R _E into R _A and R _H. Our failure in the latter was due to a lack of adequate contrasts in the assimilated data set to distinguish between R _A and R _H. We performed most model runs at a twice-daily time step. Optimizing on daily-aggregated data severely degraded the model’s ability to separate GEE and R _E. However, we gained little benefit from using a half-hourly time step. The model-data fusion showed that most of the interannual variability in NEE was due to variability in GEE, and not R _E. In contrast to several previous studies in other ecosystems, we found that longer growing seasons at Niwot Ridge were correlated with less net CO₂ uptake, due to a decrease of available snow-melt water during the late springtime photosynthetic period. Warmer springtime temperatures resulted in increased net CO₂ uptake only if adequate moisture was available; when warmer springtime conditions led into mid-summer drought, the annual net uptake declined.     

Photosynthesis drives anomalies in net carbon-exchange of pine forests at different latitudes

The growth rate of atmospheric CO₂ exhibits large temporal variation that is largely determined by year‐to‐year fluctuations in land–atmosphere CO₂ fluxes. This land–atmosphere CO₂‐flux is driven by large‐scale biomass burning and variation in net ecosystem exchange (NEE). Between‐ and within years, NEE varies due to fluctuations in climate. Studies on climatic influences on inter‐ and intra‐annual variability in gross photosynthesis (GPP) and net carbon uptake in terrestrial ecosystems have shown conflicting results. These conflicts are in part related to differences in methodology and in part to the limited duration of some studies. Here, we introduce an observation‐driven methodology that provides insight into the dependence of anomalies in CO₂ fluxes on climatic conditions. The methodology was applied on fluxes from a boreal and two temperate pine forests. Annual anomalies in NEE were dominated by anomalies in GPP, which in turn were correlated with incident radiation and vapor pressure deficit (VPD). At all three sites positive anomalies in NEE (a reduced uptake or a stronger source than the daily sites specific long‐term average) were observed on summer days characterized by low incident radiation, low VPD and high precipitation. Negative anomalies in NEE occurred mainly on summer days characterized by blue skies and mild temperatures. Our study clearly highlighted the need to use weather patterns rather than single climatic variables to understand anomalous CO₂ fluxes. Temperature generally showed little direct effect on anomalies in NEE but became important when the mean daily air temperature exceeded 23 °C. On such days GPP decreased likely because VPD exceeded 2.0 kPa, inhibiting photosynthetic uptake. However, while GPP decreased, the high temperature stimulated respiration, resulting in positive anomalies in NEE. Climatic extremes in summer were more frequent and severe in the South than in the North, and had larger effects in the South because the criteria to inhibit photosynthesis are more often met.     

Evaluating the carbon balance estimate from an automated ground‐level flux chamber system in artificial grass mesocosms

Measuring and modeling carbon (C) stock changes in terrestrial ecosystems are pivotal in addressing global C‐cycling model uncertainties. Difficulties in detecting small short‐term changes in relatively large C stocks require the development of robust sensitive flux measurement techniques. Net ecosystem exchange (NEE) ground‐level chambers are increasingly used to assess C dynamics in low vegetation ecosystems but, to date, have lacked formal rigorous field validation against measured C stock changes. We developed and deployed an automated and multiplexed C‐flux chamber system in grassland mesocosms in order rigorously to compare ecosystem total C budget obtained using hourly C‐flux measurements versus destructive net C balance. The system combines transparent NEE and opaque respiration chambers enabling partitioning of photosynthetic and respiratory fluxes. The C‐balance comparison showed good agreement between the two methods, but only after NEE fluxes were corrected for light reductions due to chamber presence. The dark chamber fluxes allowed assessing temperature sensitivity of ecosystem respiration (R_eco) components (i.e., heterotrophic vs. autotrophic) at different growth stages. We propose that such automated flux chamber systems can provide an accurate C balance, also enabling pivotal partitioning of the different C‐flux components (e.g., photosynthesis and respiration) suitable for model evaluation and developments.     

Photosynthetic limitation of several representative subalpine species in the Catalan Pyrenees in summer

Information on the photosynthetic process and its limitations is essential in order to predict both the capacity of species to adapt to conditions associated with climate change and the likely changes in plant communities. Considering that high‐mountain species are especially sensitive, three species representative of subalpine forests of the Central Catalan Pyrenees: mountain pine (Pinus uncinata Mill.), birch (Betula pendula Roth) and rhododendron (Rhododendron ferrugineum L.) were studied under conditions associated with climate change, such as low precipitation, elevated atmospheric [CO₂] and high solar irradiation incident at Earth's surface, in order to detect any photosynthetic limitations. Short‐term high [CO₂] increased photosynthesis rates (A) and water use efficiency (WUE), especially in birch and mountain pine, whereas stomatal conductance (g_s) was not altered in either species. Birch showed photosynthesis limitation through stomatal closure related to low rainfall, which induced photoinhibition and early foliar senescence. Rhododendron was especially affected by high irradiance, showing early photosynthetic saturation in low light, highest chlorophyll content, lowest gas exchange rates and least photoprotection. Mountain pine had the highest A, photosynthetic capacity (A_max) and light‐saturated rates of net CO₂ assimilation (A_sat), which were maintained under reduced precipitation. Furthermore, maximum quantum yield (F_v/F_m), thermal energy dissipation, PRI and SIPI radiometric index, and ascorbate content indicated improved photoprotection with respect to the other two species. However, maximum velocity of carboxylation of RuBisco (V_cmax) indicated that N availability would be the main photosynthetic limitation in this species.     

Temperature responses of photosynthetic capacity parameters were not affected by foliar nitrogen content in mature Pinus sylvestris

A key weakness in current Earth System Models is the representation of thermal acclimation of photosynthesis in response to changes in growth temperatures. Previous studies in boreal and temperate ecosystems have shown leaf‐scale photosynthetic capacity parameters, the maximum rates of carboxylation (V_cmax) and electron transport (J_max), to be positively correlated with foliar nitrogen (N) content at a given reference temperature. It is also known that V_cmax and J_max exhibit temperature optima that are affected by various environmental factors and, further, that N partitioning among the foliar photosynthetic pools is affected by N availability. However, despite the strong recent anthropogenic influence on atmospheric temperatures and N deposition to forests, little is known about the role of foliar N contents in controlling the photosynthetic temperature responses. In this study, we investigated the temperature dependencies of V_cmax and J_max in 1‐year‐old needles of mature boreal Pinus sylvestris (Scots pine) trees growing under low and high N availabilities in northern Sweden. We found that needle N status did not significantly affect the temperature responses of V_cmax or J_max when the responses were fitted to a peaked function. If such N insensitivity is a common tree trait it will simplify the interpretation of the results from gradient and multi‐species studies, which commonly use sites with differing N availabilities, on temperature acclimation of photosynthetic capacity. Moreover, it will simplify modeling efforts aimed at understanding future carbon uptake by precluding the need to adjust the shape of the temperature response curves to variation in N availability.     

Photosynthetic regulation of C4 desert plant Haloxylon ammodendron under drought stress

About 20-year-old desert plants of C₄ species, Haloxylon ammodendron, growing at the southern edge of the Badain Jaran Desert in China, were selected to study the photosynthetic characteristics and changes in chlorophyll fluorescence when plants were subject to a normal arid environment (AE), moist atmospheric conditions during post-rain (PR), and the artificial supplement of soil water (SW). Results showed that under high radiation, in the AE, the species down-regulated its net assimilation rate (A) and maximum photochemical efficiency of PS II (Fv/Fm), indicating photoinhibition. However, under the PR and SW environments, A was up-regulated, with a unimodal diurnal course of A and a small diurnal change in Fv/Fm, suggesting no photoinhibition. When the air humidity or SW content was increased, the light compensation points were reduced; light saturation points were enhanced; while light saturated rate of CO₂ assimilation (A _max) and apparent quantum yield of CO₂ assimilation (Φ_C) increased. Φ_C was higher while the A _max was reduced under PR relative to the SW treatment. It was concluded that under high-radiation conditions drought stress causes photoinhibition of H. ammodendron. Increasing air humidity or soil moisture content can reduce photoinhibition and increase the efficiency of solar energy use.