Seasonal and interannual variation in carbon dioxide exchange and carbon balance in a northern temperate grassland

Net ecosystem carbon dioxide (CO₂) exchange (NEE) was measured in a northern temperate grassland near Lethbridge, Alberta, Canada for three growing seasons using the eddy covariance technique. The study objectives were to document how NEE and its major component processes—gross photosynthesis (GPP) and total ecosystem respiration (TER)—vary seasonally and interannually, and to examine how environmental and physiological factors influence the annual C budget. The greatest difference among the three study years was the amount of precipitation received. The annual precipitation for 1998 (481.7 mm) was significantly above the 1971–2000 mean (± SD, 377.9 ± 97.0 mm) for Lethbridge, whereas 1999 (341.3 mm) was close to average, and 2000 (275.5 mm) was significantly below average. The high precipitation and soil moisture in 1998 allowed a much higher GPP and an extended period of net carbon gain relative to 1999 and 2000. In 1998, the peak NEE was a gain of 5 g C m^?2 d^?1 (day 173). Peak NEE was lower and also occurred earlier in the year on days 161 (3.2 g C m^?2 d^?1) and 141 (2.4 g C m^?2 d^?1) in 1999 and 2000, respectively. Change in soil moisture was the most important ecological factor controlling C gain in this grassland ecosystem. Soil moisture content was positively correlated with leaf area index (LAI). Gross photosynthesis was strongly correlated with changes in both LAI and canopy nitrogen (N) content. Maximum GPP (A_max: value calculated from a rectangular hyperbola fitted to the relationship between GPP and incident photosynthetic photon flux density (PPFD)) was 27.5, 12.9 and 8.6 µmol m^?2 s^?1 during 1998, 1999 and 2000, respectively. The apparent quantum yield also differed among years at the time of peak photosynthetic activity, with calculated values of 0.0254, 0.018 and 0.018 during 1998, 1999 and 2000, respectively. The ecosystem accumulated a total of 111.9 g C m^?2 from the time the eddy covariance measurements were initiated in June 1998 until the end of December 2000, with most of that C gained during 1998. There was a net uptake of almost 21 g C m^?2 in 1999, whereas a net loss of 18 g C m^?2 was observed in 2000. The net uptake of C during 1999 was the combined result of slightly higher GPP (287.2 vs. 272.3 g C m^?2 year^?1) and lower TER (266.6 vs. 290.4 g C m^?2 year^?1) than occurred in 2000.     

Seasonal patterns and environmental control of carbon dioxide and water vapour exchange in an ecotonal boreal forest

Carbon dioxide, water vapour, and sensible heat fluxes were measured above and within a spruce dominated forest near the southern ecotone of the boreal forest in Maine, USA. Summer, mid-day carbon dioxide uptake was higher than at other boreal coniferous forests, averaging about – 13 μmol CO₂ m^–2 s^–1. Nocturnal summer ecosystem respiration averaged ≈ 6 μmol CO₂ m^–2 s^–1 at a mean temperature of ≈ 15 °C. Significant ecosystem C uptake began with the thawing of the soil in early April and was abruptly reduced by the first autumn frost in early October. Half-hourly forest CO₂ exchange was regulated mostly by the incident photosynthetically active photon flux density (PPFD). In addition to the threshold effects of freezing temperatures, there were seasonal effects on the inferred photosynthetic parameters of the forest canopy. The functional response of this forest to environmental variation was similar to that of other spruce forests. In contrast to reports of carbon loss from northerly boreal forest sites, in 1996 the Howland forest was a strong carbon sink, storing about 2.1 t C ha^–1.     

Drought controls over conductance and assimilation of a Mediterranean evergreen ecosystem: scaling from leaf to canopy

Drought control over conductance and assimilation was assessed using eddy flux and meteorological data monitored during four summer periods from 1998 to 2001 above a closed canopy of the Mediterranean evergreen oak tree Quercus ilex. Additional discrete measurements of soil water content and predawn leaf water potential were used to characterize the severity of the drought. Canopy conductance was estimated through the big‐leaf approach of Penman–Monteith by inverting latent heat fluxes. The gross primary production ( GPP ) was estimated by adding ecosystem respiration to net ecosystem exchange. Ecosystem respiration was deduced from night flux when friction velocity ( u _*) was greater than 0.35 m s^?1. Empirical equations were identified that related maximal canopy conductance and daily ecosystem GPP to relative soil water content ( RWC) , the ratio of current soil water content to the field capacity, and to the predawn leaf water potential. Both variables showed a strong decline with soil RWC for values lower than 0.7. The sharpest decline was observed for GPP . The curves reached zero for RWC =0.41 and 0.45 for conductance and GPP , respectively. When the predawn leaf water potential was used as a surrogate for soil water potential, both variables showed a hyperbolic decline with decreasing water potential. These results were compared with already published literature values obtained at leaf level from the same tree species. Scaling up from the leaf to ecosystem highlighted the limitation of two big‐leaf representations: Penman–Monteith and Sellers' Π factor. Neither held completely for comparing leaf and canopy fluxes. Tower measurements integrate fluxes from foliage elements clumped at several levels of organization: branch, tree, and ecosystem. The Q. ilex canopy exhibited non‐random distribution of foliage, emphasizing the need to take into account a clumping index, the factor necessary to apply the Lambert–Beer law to natural forests. Our results showed that drought is an important determinant in water losses and CO₂ fluxes in water‐limited ecosystems. In spite of the limitations inherent to the big‐leaf representation of the canopy, the equations are useful for predicting the influence of environmental factors in Mediterranean woodlands and for interpreting ecosystem exchange measurements.     

Transpiration and stomatal behaviour of Quercus ilex plants during the summer in a Mediterranean carbon dioxide spring

Variations in the water relations and stomatal response of Quercus ilex were analysed under field conditions by comparing trees at two locations in a Mediterranean environment during two consecutive summers (1993 and 1994). We used the heat-pulse velocity technique to estimate transpirational water use of trees during a 5 month period from June to November 1994. At the end of sap flow measurements, the trees were harvested, and the foliage and sapwood area measured. A distinct environmental gradient exists between the two sites with higher atmospheric CO₂ concentrations in the proximity of a natural CO₂ spring. Trees at the spring site have been growing for generations in elevated atmospheric CO₂ concentrations. At both sites, maximum leaf conductance was related to predawn shoot water potential. The effects of water deficits on water relations and whole-plant transpiration during the summer drought were severe. Leaf conductance and water potential recovered after major rainfall in September to predrought values. Sap flow, leaf conductance and predawn water potential decreased in parallel with increases in hydraulic resistance, reaching a minimum in mid-summer. These relationships are in agreement with the hypothesis of the stomatal control of transpiration to prevent desiccation damage but also to avoid ‘runaway embolism’. Trees at the CO₂ spring underwent less reduction in hydraulic resistance for a given value of predawn water potential. The decrease in leaf conductance caused by elevated CO₂ was limited and tended to be less at high than at low atmospheric vapour pressure deficit. Mean (and diurnal) sap flux were consistently higher in the control site trees than in the CO₂ spring trees. The degree of reduction in water use between the two sites varied among the summer periods. The control site trees had consistently higher sap flow at corresponding values of either sapwood cross-sectional area or foliage area. Larger trees displayed smaller differences than smaller trees, between the control and the CO₂ spring trees. A strong association between foliage area and sapwood cross-sectional area was found in both the control and the CO₂ spring trees, the latter supporting a smaller foliage area at the corresponding sapwood stem cross-sectional area. The specific leaf area (SLA) of the foliage was not influenced by site. The results are discussed in terms of the effects of elevated CO₂ on plant water use at the organ and whole-tree scale.     

Seasonal net carbon dioxide exchange of a beech forest with the atmosphere

The seasonal carbon dioxide exchange of a beech forest of Central Italy was studied by means of the eddy covariance technique. Additional measurements of biomass respiration with cuvettes and relationship of carbon dioxide exchanges with temperature and light were used to interpolate missing data during the dormant and part of the growing season. The net ecosystem production of the forest equals 472 g C m^?2 y^?1 while the gross ecosystem production 1016 g C m^?2 y^?1 and respiration 544 g C m^?2 y^?1. These estimates are compared with the net primary production determined by direct biomass sampling which amounts to 802 g C m^?2 y^?1.     

Increasing drought decreases phosphorus availability in an evergreen Mediterranean forest

Mediterranean ecosystems are water-limited and frequently also nutrient-limited. We aimed to investigate the effects of increasing drought, as predicted by GCM and eco-physiological models for the next decades, on the P cycle and P plant availability in a Mediterranean forest. We conducted a field experiment in a mature evergreen oak forest, establishing four drought-treatment plots and four control plots (150 m² each). After three years, the runoff and rainfall exclusion reduced an overall 22% the soil moisture, and the runoff exclusion alone reduced it 10%. The reduction of 22% in soil moisture produced a decrease of 40% of the accumulated aboveground plant P content, above all because there was a smaller increase in aerial biomass. The plant leaf P content increased by 100 ± 40 mg m⁻² in the control plots, whereas it decreased by 40 ± 40 mg m⁻² in the drought plots. The soil Po-NaHCO₃ (organic labile-P fraction) increased by 25% in consonance with the increase in litterfall, while the inorganic labile-P fraction decreased in relation to the organic labile-P fraction up to 48%, indicating a decrease in microbial activity. Thus, after just three years of slight drought, a clear trend towards an accumulation of P in the soil and towards a decrease of P in the stand biomass was observed. The P accumulation in the soil in the drought plots was mainly in forms that were not directly available to plants. These indirect effects of drought including the decrease in plant P availability, may become a serious constraint for plant growth and therefore may have a serious effect on ecosystem performance.     

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.     

Seasonal variation in net carbon exchange and evapotranspiration in a Brazilian rain forest: a modelling analysis

Tropical rain forests account for a significant fraction of global net primary productivity, and are important latent energy (LE) sources, affecting extra-tropical atmospheric circulation. The influence of environmental factors on these fluxes has until recently been poorly understood, largely due to a paucity of data, but in recent years the amount of available data has been increased greatly by use of eddy covariance techniques. In this paper we examine the factors that control daily and seasonal carbon (C) and LE fluxes, by comparing a detailed model of the soil–plant–atmosphere continuum against a unique long-term data-set collected using eddy covariance at an undisturbed rain forest site north of Manaus, Brazil. Our initial application of the model was parametrized with simple measurements of canopy structure, and driven with local meteorological data. It made effective predictions of C and LE exchange during the wet season, but dry season predictions were overestimates in both cases. Sensitivity analyses indicated that the best explanation for this behaviour was a seasonal change in soil and root hydraulic resistances ( R _b). An optimization routine was then used to estimate the increase in R _b during the dry season that would be required to explain the reduced dry season fluxes. The local soil, a clay latosol, is typical of much of Amazônia, having very low available water and low hydraulic conductivity. We conclude that an increase in soil–root hydraulic resistance in the dry season introduces a significant seasonal cycle to carbon and water fluxes from this tropical forest. Furthermore, our model structure appears to be an effective tool for regional and temporal scaling of C and LE fluxes, with primary data requirements being regional and temporal information on meteorology, leaf area index (LAI), foliar N, critical leaf water potentials, and plant and soil hydraulic characteristics.     

Seasonal controls on interannual variability in carbon dioxide exchange of a near-end-of rotation Douglas-fir stand in the Pacific Northwest, 1997–2006

This study analyzes 9 years of eddy‐covariance (EC) data carried out in a Pacific Northwest Douglas‐fir (Pseudotsuga menzesii) forest (58‐year old in 2007) on the east coast of Vancouver Island, Canada, and characterizes the seasonal and interannual variability in net ecosystem productivity (NEP), gross primary productivity (GPP), and ecosystem respiration (R_e) and primary climatic controls on these fluxes. The annual values (± SD) of NEP, GPP and R_e were 357 ± 51, 2124 ± 125, and 1767 ± 146 g C m^?2 yr^?1, respectively, with ranges of 267–410, 1592–2338, and 1642–2071 g C m^?2 yr^?1, respectively. Spring to early summer (March–June) accounted for more than 80% of annual NEP while late spring to early autumn (May–August) was mainly responsible for its interannual variability (～80%). The major drivers of interannual variability in annual carbon (C) fluxes were annual and spring mean air temperatures (T_a) and water deficiency during late summer and autumn (July–October) when this Douglas‐fir forest growth was often water‐limited. Photosynthetically active radiation (Q), and the combination of Q and soil water content (θ) explained 85% and 91% of the variance of monthly GPP, respectively; and 91% and 96% of the variance of monthly R_e was explained by T_a and the combination of T_a and θ, respectively. Annual net C sequestration was high during optimally warm and normal precipitation years, but low in unusually warm or severely dry years. Excluding 1998 and 1999, the 2 years strongly affected by an El Niño/La Niña cycle, annual NEP significantly decreased with increasing annual mean T_a. Annual NEP will likely decrease whereas both annual GPP and R_e will likely increase if the future climate at the site follows a trend similar to that of the past 40 years.     

An initial intercomparison of micrometeorological and ecological inventory estimates of carbon exchange in a mid-latitude deciduous forest

The role of mid‐latitude forests in the sequestration of carbon (C) is of interest to an increasing number of scientists and policy‐makers alike. Net CO₂ exchange can be estimated on an annual basis, using eddy‐covariance techniques or from ecological inventories of C fluxes to and from a forest. Here we present an intercomparison of annual estimates of C exchange in a mixed hardwood forest in the Morgan‐Monroe State Forest, Indiana, USA for two years, 1998 and 1999. Based on eddy‐covariance measurements made at 1.8 times canopy height from a tower, C uptake by the forest was 237 and 287 g C m^?2 y^?1 for 1998 and 1999, respectively. For the same time period, biometric and ecophysiological measures and modelled estimates of all significant carbon fluxes within deciduous forests were made, including: change in living biomass, aboveground and belowground detritus production, foliage consumption, and forest floor and soil respiration. Using this ecological inventory method for these same two time periods, C uptake was estimated to be 271 and 377 g C m^?2 y^?1, which are 14.3% and 31.4% larger, respectively, than the tower‐based values. The relative change between this method's annual estimates is consistent with that of the eddy‐covariance based values. Our results indicate that the difference in annual C exchange rates was due to reduced heterotrophic soil respiration in 1999.     

Assessment of annual carbon exchange in a water-stressed Pinus radiata plantation: an analysis based on eddy covariance measurements and an integrated biophysical model

We used a combination of eddy flux, chamber and environmental measurements with an integrated suite of models to analyse the seasonality of net ecosystem carbon uptake (F_CO2) in an 8-year-old, closed canopy Pinus radiata D.Don plantation in New Zealand (42°52′ S, 172°45′ E). The analyses utilized a biochemically based, big-leaf model of tree canopy photosynthesis (A_c), coupled to multiplicative environmental-constraint functions of canopy stomatal conductance (G_c) via environmental measurements, a temperature-dependent model of ecosystem respiration (R_eco), and a soil water balance model. Available root zone water storage capacity at the measurement site is limited to about 50 mm for the very stony soil, and annual precipitation is only 660 mm, distributed evenly throughout the year. Accordingly the site is prone to soil moisture deficit throughout the summer. G _c and A_c obtained maximum rates early in the growing season when plentiful soil water supply was associated with sufficient quantum irradiance (Q_abs), and moderate air saturation deficit (D) and temperature (T). From late spring onwards, soil water deficit and D confined G_c and A_c congruously, which together with the solely temperature dependency of R_eco resulted in the pronounced seasonality in F_CO2. Reflecting a light-limitation of A_c in the closed canopy, modelled annual carbon (C) uptake was most sensitive to changes in Q_abs. However, Q_abs did not vary significantly between years, and changes in annual F_CO2 were mostly due to variability in summer rainfall and D. Annual C-uptake of the forest was 717 g C m^–2 in a near-average rainfall year, exceeding by one third the net uptake in a year with 20% less than average rainfall (515 g C m^–2).     

Diversity and specificity of ectomycorrhizal fungi retrieved from an old-growth Mediterranean forest dominated by Quercus ilex

We analysed the ectomycorrhizal (ECM) fungal diversity in a Mediterranean old-growth Quercus ilex forest stand from Corsica (France), where Arbutus unedo was the only other ECM host. On a 6400 m2 stand, we investigated whether oak age and host species shaped below-ground ECM diversity. Ectomycorrhizas were collected under Q. ilex individuals of various ages (1 yr seedlings; 3-10 yr saplings; old trees) and A. unedo. They were typed by ITS-RFLP analysis and identified by match to RFLP patterns of fruitbodies, or by sequencing. A diversity of 140 taxa was found among 558 ectomycorrhizas, with many rare taxa. Cenococcum geophilum dominated (35% of ECMs), as well as Russulaceae, Cortinariaceae and Thelephoraceae. Fungal species richness was comparable above and below ground, but the two levels exhibited < 20% overlap in fungal species composition. Quercus ilex age did not strongly shape ECM diversity. The two ECM hosts, A. unedo and Q. ilex, tended to share few ECM species (< 15% of the ECM diversity). Implications for oak forest dynamics are discussed.     

Seasonal changes of root hydraulic conductance (KRL) in four forest trees: an ecological interpretation

This paper deals with the possibility of relating root hydraulic parameters to an ecological index describing the continentality/oceanicity of four forest trees. Root hydraulic conductance ($K_R$) of seedlings of Fagus sylvatica L., Quercus ilex L., Quercus suber L. and Quercus pubescens Willd. was measured in May, August and November 1996. $K_R$ was calculated in terms of the relation of the water flow through intact root systems in situ measured with the pressure chamber, and the pressure driving the flow. The sufficiency of the root system to supply the foliage was estimated by dividing $K_R$ by the seedlings leaf surface area ($A_L$) thus obtaining $K_RL$. In the spring, $K_RL$ was largest in F. sylvatica and smallest in Q. pubescens with intermediate values recorded in Q. ilex and Q. suber. All the species studied showed a large decline in $K_RL$ just prior to the winter rest except for Q. suber which mantained $K_RL$ approximately constant through the period of study. In most cases, $K_RL$ changed in accordance with analogous changes in the flow. When the total seedlings' leaf surface area ($A_L$) was plotted versus $K_RL$, it appeared that $K_RL$ of Q. pubescens increased with $A_L$, proportionally, while $K_RL$ of F. sylvatica was inversely related to $A_L$. This, together with the largest $K_RL$ recorded in the summer in Q. pubescens, was interpreted as advantageous to this species (which is adapted to semi-arid environments) in that: (a) roots could supply water to foliage efficiently even during the adverse season and (b) the foliage growth could be sustained even in summer.No statistically significant relation of $K_RL$ to the continentality index calculated for the four species studied on the basis of their European distribution, was found to exist. Nonetheless, our data appear to be encouraging for future research aimed at better interpreting the typical distribution areas of plant species.     

Seasonal and interannual variations in carbon dioxide exchange over a cropland in the North China Plain

In China, croplands account for a relatively large form of vegetation cover. Quantifying carbon dioxide exchange and understanding the environmental controls on carbon fluxes over croplands are critical in understanding regional carbon budgets and ecosystem behaviors. In this study, the net ecosystem exchange (NEE) at a winter wheat/summer maize rotation cropping site, representative of the main cropping system in the North China Plain, was continuously measured using the eddy covariance technique from 2005 to 2009. In order to interpret the abiotic factors regulating NEE, NEE was partitioned into gross primary production (GPP) and ecosystem respiration (R_eco). Daytime R_eco was extrapolated from the relationship between nighttime NEE and soil temperature under high turbulent conditions. GPP was then estimated by subtracting daytime NEE from the daytime estimates of R_eco. Results show that the seasonal patterns of the temperature responses of R_eco and light‐response parameters are closely related to the crop phenology. Daily R_eco was highly dependent on both daily GPP and air temperature. Interannual variability showed that GPP and R_eco were mainly controlled by temperature. Water availability also exerted a limit on R_eco. The annual NEE was ?585 and ?533 g C m^?2 for two seasons of 2006–2007 and 2007–2008, respectively, and the wheat field absorbed more carbon than the maize field. Thus, we concluded that this cropland was a strong carbon sink. However, when the grain harvest was taken into account, the wheat field was diminished into a weak carbon sink, whereas the maize field was converted into a weak carbon source. The observations showed that severe drought occurring during winter did not reduce wheat yield (or integrated NEE) when sufficient irrigation was carried out during spring.     

Estimation of the carbon sequestration by a heterogeneous forest: night flux corrections, heterogeneity of the site and inter-annual variability

Continuous measurements of the net CO₂ flux exchanged in a mixed forest with the atmosphere were performed over 5 years at the Vielsalm experimental site. The carbon sequestration at the site was deduced by a summation of the measurements. Problems associated with this summation procedure were discussed. The carbon sequestration in the ecosystem was presented and its interannual variability was discussed. An estimation of the night flux correction was given. The correction was applied by replacing measurements made during quiet nights by a parameterization. The impact of the correction was shown to vary between 10 and 20% of the uncorrected flux, according to the year. The need to include the storage flux during turbulent periods was emphasized: its neglect leads to an error which will be greater than the one it tries to correct. It was also shown that the heterogeneity of the site made it necessary to split the data into separate series corresponding to the different vegetation patches and to fill the data gaps by using an algorithm that takes account of the weather conditions. Two series were defined, one corresponding to a beech subplot, the other to a conifer subplot. The uncertainty owing to the data split and the data gap‐filling was about 15–20% annually. The carbon sequestration was then analysed in both the subplots. The length of the growing season was about 210 days in the beech and 240 days in the conifer. The carbon sequestration over 5 years was 2.28 kg C m^2?2 in the beech and 3.58 kg C m^2?2 in the conifer. The main difference between the species appeared in spring, between March and May, when the beeches were leafless. Significant interannual variations were observed in both the subplots. They appeared mainly in summer and were primarily because of the variations in the radiation and air humidity regimes. In addition, an impact of the interannual variation of the vegetation area index (VAI) and of the leaf initiation date was observed in the beech. Finally, a decline of the carbon sequestration efficiency of the ecosystem during the season was observed in both the subplots. It was because of neither the variation in any climatic variables nor VAI variation.     

Monoterpene emission and monoterpene synthase activities in the Mediterranean evergreen oak Quercus ilex L. grown at elevated CO2 concentrations

Monoterpene emissions, monoterpene synthase activities, photosynthesis, fluorescence yield in the dark and drought stress indicators (stomatal conductance and mid‐day water potential) were concurrently measured under similar temperature and illumination in current‐year leaves of Quercus ilex L. of plants grown in open‐top chambers at ambient (350 ppm) and elevated (700 ppm) CO₂. The study was undertaken to understand the effect of CO₂ on monoterpene biosynthesis, and to predict and parameterize the biogenic emissions at growing CO₂ concentrations. The results of the 1998 and 1999 studies show that at elevated CO₂, and in the absence of persistent environmental stresses, photosynthesis was stimulated with respect to ambient CO₂, but that the emission of the three most abundantly emitted monoterpenes (α‐pinene, sabinene and β‐pinene) was inhibited by approximately 68%. The enzyme activities of the monoterpene synthases catalysing the formation of the three monoterpenes were also inhibited at elevated CO₂ and an excellent relationship was found between monoterpene emission and activity of the corresponding enzyme both at ambient and elevated CO₂. Interestingly, however, limonene emission was enhanced in conditions of elevated CO₂ as it was also the corresponding synthase. The ratio between enzyme activity and emission of the three main monoterpenes was high (above 20) at ambient CO₂ but it was below 10 at elevated CO₂ and, for limonene, on both treatments. Our results indicate that the overall emission of monoterpenes at elevated CO₂ will be inhibited because of a concurrent, strong down‐regulation of monoterpene synthase activities. When the enzyme activity does not change, as for limonene, the high photosynthetic carbon availability at elevated CO₂ conditions may even stimulate emission. The results of the 1997 study show that severe and persistent drought, as commonly occurs in the Mediterranean, may inhibit both photosynthesis and monoterpene (α‐pinene) emission, particularly at ambient CO₂. Thus, emission is probably limited by photosynthetic carbon availability; the effect of elevated CO₂per se is not apparent if drought, and perhaps other environmental stresses, are also present.     

The growth respiration component in eddy CO2 flux from a Quercus ilex mediterranean forest

Ecosystem respiration, arising from soil decomposition as well as from plant maintenance and growth, has been shown to be the most important component of carbon exchange in most terrestrial ecosystems. The goal of this study was to estimate the growth component of whole‐ecosystem respiration in a Mediterranean evergreen oak (Quercus ilex) forest over the course of 3 years. Ecosystem respiration (R_eco) was determined from night‐time carbon dioxide flux (F_c) using eddy correlation when friction velocity (u_*) was greater than 0.35 m s^?1 We postulated that growth respiration could be evaluated as a residual after removing modeled base R_eco from whole‐ecosystem R_eco during periods when growth was most likely occurring. We observed that the model deviated from the night‐time F_c‐based R_eco during the period from early February to early July with the largest discrepancies occurring at the end of May, coinciding with budburst when active aboveground growth and radial growth increment are greatest. The highest growth respiration rates were observed in 2001 with daily fluxes reaching up to 4 g C m^?2. The cumulative growth respiration for the entire growth period gave total carbon losses of 170, 208, and 142 g C m^?2 for 1999, 2001, and 2002, respectively. Biochemical analysis of soluble carbohydrates, starch, cellulose, hemicellulose, proteins, lignin, and lipids for leaves and stems allowed calculation of the total construction costs of the different growth components, which yielded values of 154, 200, and 150 g C for 3 years, respectively, corresponding well to estimated growth respiration. Estimates of both leaf and stem growth showed very large interannual variation, although average growth respiration coefficients and average yield of growth processes were fairly constant over the 3 years and close to literature values. The time course of the growth respiration may be explained by the growth pattern of leaves and stems and by cambial activity. This approach has potential applications for interpreting the effects of climate variation, disturbances, and management practices on growth and ecosystem respiration.     

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     

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

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