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.     

Diurnal and seasonal variation of carbon dioxide exchange from a former true raised bog

Carbon dioxide exchange was measured, using the eddy covariance technique, during a one and a half year period in 1994 and 1995. The measurements took place over a former true raised bog, characterized by a shallow peat layer and a vegetation dominated by Molinia caerulea. The growing season extended from May until late October, with a maximum LAI in August of 1.7. The carbon balance shows a net release of 97 g C m^–2 y^–1 (265 kg C ha^–1 y^–1) from the peat bog ecosystem to the atmosphere. During June, July and August there is net consumption of CO₂, while during the rest of the year there is net production of CO₂. The average daytime assimilation rates ranged between – 0.2 and – 0.5 mg CO₂ m^–2 s^–1 (– 45 and –11.3 μmol CO₂ m^–2 s^–1), in a period where the LAI ranged between 1 and 1.7. A high vapour pressure deficit (> 15 hPa) corresponding with high temperatures was found to reduce the assimilation rate by on average 50%. Apart from these factors, LAI and the soil temperature codetermine the net exchange of CO₂. The total nocturnal respiration during the growing season lies within the same order as the average daytime net assimilation rate. Temperature was found to be the main factor controlling soil respiration, with a Q₁₀ of 4.8.     

Year-round observations of the net ecosystem exchange of carbon dioxide in a native tallgrass prairie

An Ameriflux site was established in mid 1996 to study the exchange of CO₂ in a native tallgrass prairie of north‐central Oklahoma, USA. Approximately the first 20 months of measurements (using eddy covariance) are described here. This prairie, dominated by warm season C4 grasses, is typical of the central Kansas/northern Oklahoma region. During the first three weeks of the measurement period (mid‐July–early August 1996), moisture‐stress conditions prevailed. For the remainder of the period (until March 1998), however, soil moisture was nonlimiting. Mid‐day net ecosystem CO₂ exchange (NEE), under well‐watered conditions, reached a maximum magnitude of 1.4 mg CO₂ m^?2 s^?1 (flux toward the surface is positive) during peak growth (mid‐July 1997), with green leaf area index of 2.8. In contrast, under moisture‐stress conditions in the same growth stage in 1996, mid‐day NEE was reduced to near‐zero. Average night NEE ranged from near‐zero, during winter dormancy, to ? 0.50 mg CO₂ m^?2 s^?1, during peak growth. Most of the variance in average night NEE was explained by changes in soil temperature (0.1 m depth) and green leaf area. The daytime NEE measurements were examined in terms of a rectangular hyperbolic relationship with incident photosynthetically active radiation. The analysis showed that the quantum yield during peak growth was similar to those measured in other prairies and the y‐intercept, so obtained, can be potentially used as an estimate of night‐time CO₂ emissions when eddy covariance data are unavailable. Daily integrated NEE reached its peak magnitude of 30.8 g CO₂ m^?2 d^?1 (8.4 g C m^?2 d^?1) in mid‐July when the green LAI was the largest (about 2.8). In general, the seasonal trend of daily NEE (on relatively clear days) followed that of green LAI. Annually integrated carbon exchange, between prescribed burns in 1997 and 1998, was 268 g C m^?2 y^?1. After incorporating carbon loss during the prescribed burn , the net annual carbon exchange in this prairie was near‐zero in 1998.     

When vegetation change alters ecosystem water availability

The combined effects of vegetation and climate change on biosphere–atmosphere water vapor (H₂O) and carbon dioxide (CO₂) exchanges are expected to vary depending, in part, on how biotic activity is controlled by and alters water availability. This is particularly important when a change in ecosystem composition alters the fractional covers of bare soil, grass, and woody plants so as to influence the accessibility of shallower vs. deeper soil water pools. To study this, we compared 5 years of eddy covariance measurements of H₂O and CO₂ fluxes over a riparian grassland, shrubland, and woodland. In comparison with the surrounding upland region, groundwater access at the riparian sites increased net carbon uptake (NEP) and evapotranspiration (ET), which were sustained over more of the year. Among the sites, the grassland used less of the stable groundwater resource, and increasing woody plant density decoupled NEP and ET from incident precipitation (P), resulting in greater exchange rates that were less variable year to year. Despite similar gross patterns, how groundwater accessibility affected NEP was more complex than ET. The grassland had higher respiration (R_eco) costs. Thus, while it had similar ET and gross carbon uptake (GEP) to the shrubland, grassland NEP was substantially less. Also, grassland carbon fluxes were more variable due to occasional flooding at the site, which both stimulated and inhibited NEP depending upon phenology. Woodland NEP was large, but surprisingly similar to the less mature, sparse shrubland, even while having much greater GEP. Woodland R_eco was greater than the shrubland and responded strongly and positively to P, which resulted in a surprising negative NEP response to P. This is likely due to the large accumulation of carbon aboveground and in the surface soil. These long‐term observations support the strong role that water accessibility can play when determining the consequences of ecosystem vegetation change.     

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.     

Mature semiarid chaparral ecosystems can be a significant sink for atmospheric carbon dioxide

Carbon flux in arid and semiarid area shrublands, especially in old‐growth shrub ecosystems, has been rarely studied using eddy covariance techniques. In this study, eddy covariance measurements over a 100‐year old‐growth chamise‐dominated chaparral shrub ecosystem were conducted for 7 years from 1996 to 2003. A carbon sink, from −96 to −155 g C m⁻² yr⁻¹, was determined under normal weather conditions, while a weak sink of −18 g C m⁻² yr⁻¹ and a strong source of 207 g C m⁻² yr⁻¹ were observed as a consequence of a severe drought. The annual sink strength of carbon in the 7‐year measurement period was −52 g C m⁻² yr⁻¹. The results from our study indicate that, in contrast to previous thought, the old‐growth chaparral shrub ecosystem can be a significant sink of carbon under normal weather conditions and, therefore, be an important component of the global carbon budget.     

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.     

Interannual variability in net CO2 exchange of a native tallgrass prairie

Year‐round eddy covariance flux measurements were made in a native tallgrass prairie in north‐central Oklahoma, USA during 1997–2000 to quantify carbon exchange and its interannual variability. This prairie is dominated by warm season C₄ grasses. The soil is a relatively shallow silty clay loam underlined with a heavy clay layer and a limestone bedrock. During the study period, the prairie was burned in the spring of each year, and was not grazed. In 1997 there was adequate soil moisture through the growing season, but 1998 had two extended periods of substantially low soil moisture (with concurrent high air temperatures and vapor pressure deficits), one early and one later in the growing season. There was also moisture stress in 1999, but it was less severe and occurred later in the season. The annual net ecosystem CO₂ exchange, NEE (before including carbon loss during the burn) was 274, 46 and 124 g C m ^{? 2} yr ^{? 1} in 1997, 1998, and 1999, respectively (flux toward the surface is positive), and the associated variation seemed to mirror the severity of moisture stress. We also examined integrated values of NEE during different periods (e.g. day/night; growing season/senescence). Annually integrated carbon dioxide uptake during the daytime showed the greatest variability from year to year, and was primarily linked to the severity of moisture stress. Carbon loss during nighttime was a significant part of the annual daytime NEE, and was fairly stable from year to year. When carbon loss during the burn (estimated from pre‐ and post‐burn biomass samples) was incorporated in the annual NEE, the prairie was found to be approximately carbon neutral (i.e. net carbon uptake/release was near zero) in years with no moisture stress (1997) or with some stress late in the season (1999). During a year with severe moisture stress early in the season (1998), the prairie was a net source of carbon. It appears that moisture stress (severity as well as timing of occurrence) was a dominating factor regulating the annual carbon exchange of the prairie.     

Carbon dioxide exchange of a Russian boreal forest after disturbance by wind throw

The exchange of carbon dioxide (CO₂) between the atmosphere and a forest after disturbance by wind throw in the western Russian taiga was investigated between July and October 1998 using the eddy covariance technique. The research area was a regenerating forest (400 m × 1000 m), in which all trees of the preceding generation were uplifted during a storm in 1996. All deadwood had remained on site after the storm and had not been extracted for commercial purposes. Because of the heterogeneity of the terrain, several micrometeorological quality tests were applied. In addition to the eddy covariance measurements, carbon pools of decaying wood in a chronosequence of three different wind throw areas were analysed and the decay rate of coarse woody debris was derived. During daytime, the average CO₂ uptake flux was ?3 µmol m^?2s^?1, whereas during night‐time characterised by a well‐mixed atmosphere the rates of release were typically about 6 µmol m^?2s^?1. Suppression of turbulent fluxes was only observed under conditions with very low friction velocity (u* ≤ 0.08 ms^?1). On average, 164 mmol CO₂ m^?2d^?1 was released from the wind throw to the atmosphere, giving a total of 14.9 mol CO₂ m^?2 (180 g CO₂ m^?2) released during the 3‐month study period. The chronosequence of dead woody debris on three different wind throw areas suggested exponential decay with a decay coefficient of ?0.04 yr^?1. From the magnitude of the carbon pools and the decay rate, it is estimated that the decomposition of coarse woody debris accounted for about a third of the total ecosystem respiration at the measurement site. Hence, coarse woody debris had a long‐term influence on the net ecosystem exchange of this wind throw area. From the analysis performed in this work, a conclusion is drawn that it is necessary to include into flux networks the ecosystems that are subject to natural disturbances and that have been widely omitted into considerations of the global carbon budget. The half‐life time of about 17 years for deadwood in the wind throw suggests a fairly long storage of carbon in the ecosystem, and indicates a very different long‐term carbon budget for naturally disturbed vs. commercially managed forests.     

Relations between carbon dioxide fluxes and environmental factors of Kobresia humilis meadows and Potentilla fruticosa meadows

Carbon dioxide fluxes of Kobresia humilis and Potentillafruticosa shrub meadows,two typical ecosystems in the Qinghai-Tibet Plateau,were measured by eddy covariance technology and the data collected in August 2003 were employed to analyze the relations between carbon dioxide fluxes and environmental factors of the ecosystems.August is the time when the two ecosystems reach their peak leaf area indexes and stay stable,and also the period when the net carbon absorptions of Kobresia humilis and Potentilla photo flux densities (PPFD),the carbon dioxide-uptake rate of the Kobresia humilis meadow is higher than that of the Potentilla fruticosa shrub meadow;where the PPFD are rates of the two ecosystems declined as air temperature increased,but the carbon dioxide uptake rate of the Kobresia humilis meadow decreased more quickly (-0.086) than that of the Potentilla fruticosa shrub meadow (-0.016).Soil moistures exert influence on the soil respirations and this varies with the vegetation type.The daily carbon dioxide absorptions of the ecosystems increase with increased diurnal temperature differences and higher diurnal temperature differences result in higher carbon dioxide exchanges.There exists a negative correlation between the vegetation albedos and the carbon dioxide fluxes.     

Strategies for measuring and modelling carbon dioxide and water vapour fluxes over terrestrial ecosystems

Continuous and direct measurements of ecosystem carbon dioxide and water vapour fluxes can improve our ability to close regional and global carbon and hydrological budgets. On this behalf, an international and multidisciplinary group of scientists (micrometeorologists, ecophysiologists and biogeochemists) assembled at La Thuile, Italy to convene a workshop on ‘Strategies for Monitoring and Modelling CO₂ and Water Vapour Fluxes over Terrestrial Ecosystems’. Over the course of the week talks and discussions focused on: (i) the results from recent field studies on the annual cycle of carbon dioxide and water vapour fluxes over terrestrial ecosystems; (ii) the problems and pitfalls associated with making long-term flux measurements; (iii) alternative methods for assessing ecosystem carbon dioxide and water vapour fluxes; (iv) how direct and continuous carbon dioxide and water vapour flux measurements could be used by the ecological and biogeochemical modelling communities; and (v) if, how and where to proceed with establishing a network of long-term flux measurement sites. This report discusses the purpose of the meeting and summarizes the conclusions drawn from the discussions by the attending scientists. There was a consensus that recent advances in instrumentation and software make possible long-term measurements of carbon dioxide and water vapour fluxes over terrestrial ecosystems. At this writing, eight research teams have conducted long-term carbon dioxide and water vapour flux experiments and more long-term studies are anticipated. The participants advocated an experimental design that would make long-term flux measurement valuable to a wider community of modelers, biogeochemists and ecologists. A network of carbon dioxide and water vapour flux measurement stations should include ancillary measurements of meteorological, ecological and biological variables. To assess spatial representativeness of the long term and tower-based flux measurements, periodic aircraft-based flux experiments and satellite-based assessments of land cover were recommended. Occasional cuvette-based measurements of leaf-level carbon dioxide and water vapour fluxes were endorsed to provide information on the biological control of surface fluxes. They can also provide data to parameterize ecophysiological models. Flask sampling of stable carbon isotopes was advocated to extend the flux measurements to the global scale.     

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.     

Linking flux network measurements to continental scale simulations: ecosystem carbon dioxide exchange capacity under non-water-stressed conditions

This paper examines long‐term eddy covariance data from 18 European and 17 North American and Asian forest, wetland, tundra, grassland, and cropland sites under non‐water‐stressed conditions with an empirical rectangular hyperbolic light response model and a single layer two light‐class carboxylase‐based model. Relationships according to ecosystem functional type are demonstrated between empirical and physiological parameters, suggesting linkages between easily estimated parameters and those with greater potential for process interpretation. Relatively sparse documentation of leaf area index dynamics at flux tower sites is found to be a major difficulty in model inversion and flux interpretation. Therefore, a simplification of the physiological model is carried out for a subset of European network sites with extensive ancillary data. The results from these selected sites are used to derive a new parameter and means for comparing empirical and physiologically based methods across all sites, regardless of ancillary data. The results from the European analysis are then compared with results from the other Northern Hemisphere sites and similar relationships for the simplified process‐based parameter were found to hold for European, North American, and Asian temperate and boreal climate zones. This parameter is useful for bridging between flux network observations and continental scale spatial simulations of vegetation/atmosphere carbon dioxide exchange.     

Relations between carbon dioxide fluxes and environmental factors of Kobresia humilis meadows and Potentilla fruticosa meadows

Carbon dioxide fluxes of Kobresia humilis and Potentilla fruticosa shrub meadows, two typical ecosystems in the Qinghai-Tibet Plateau, were measured by eddy covariance technology and the data collected in August 2003 were employed to analyze the relations between carbon dioxide fluxes and environmental factors of the ecosystems. August is the time when the two ecosystems reach their peak leaf area indexes and stay stable, and also the period when the net carbon absorptions of Kobresia humilis and Potentilla fruticosa shrub meadows reach 56.2 g C·m⁻² and 32.6 g C·m⁻², with their highest daily carbon dioxide absorptions standing at 12.7 μmol·m⁻²·s⁻¹ and 9.3 μmol·m⁻²·s⁻¹, and their highest carbon discharges at 5.1 μmol·m⁻²·s⁻¹ and 5.7 μmol·m⁻²·s⁻¹, respectively. At the same photosynthetic photo flux densities (PPFD), the carbon dioxide-uptake rate of the Kobresia humilis meadow is higher than that of the Potentilla fruticosa shrub meadow; where the PPFD are higher than 1,200 μmol·m⁻²·s⁻¹. The carbon dioxide uptake rates of the two ecosystems declined as air temperature increased, but the carbon dioxide uptake rate of the Kobresia humilis meadow decreased more quickly (−0.086) than that of the Potentilla fruticosa shrub meadow (−0.016). Soil moistures exert influence on the soil respirations and this varies with the vegetation type. The daily carbon dioxide absorptions of the ecosystems increase with increased diurnal temperature differences and higher diurnal temperature differences result in higher carbon dioxide exchanges. There exists a negative correlation between the vegetation albedos and the carbon dioxide fluxes. Translated from Acta Bot Boreal—Occident Sin, 2006, 26(1): 133–142 [译自: 西北植物学报]     

辐射变化对中亚热带杉木人工林净CO2交换的影响

太阳总辐射是影响森林生态系统碳交换的重要因子.为认识辐射变化对杉木人工林碳交换的影响,本研究利用开路式涡度相关系统和气象梯度观测系统测得的CO2通量和气象因子长期定位监测数据,用晴空指数(kt)表示太阳辐射情况,分析了kt对中亚热带杉木人工林生长季(4-10月)净C02交换(NEE)的影响.结果 表明:晴天时的太阳总辐...     

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.     

Variations in daytime net carbon and water exchange in a montane shrubland ecosystem in southeast Spain

Carbon and water fluxes in a semiarid shrubland ecosystem located in the southeast of Spain (province of Almería) were measured continuously over one year using the eddy covariance technique. We examined the influence of environmental variables on daytime (photosynthetically active photons, F _P >10 μmol m⁻² s⁻¹) ecosystem gas exchange and tested the ability of an empirical eco-physiological model based on F _P to estimate carbon fluxes over the whole year. The daytime ecosystem fluxes showed strong seasonality. During two solstitial periods, summer with warm temperatures (>15 °C) and sufficient soil moisture (>10 % vol.) and winter with mild temperatures (>5 °C) and high soil moisture contents (>15 % vol.), the photosynthetic rate was higher than the daytime respiration rate and mean daytime CO₂ fluxes were ca. −1.75 and −0.60 μmol m⁻² s⁻¹, respectively. Daytime evapotranspiration fluxes averaged ca. 2.20 and 0.24 mmol m⁻² s⁻¹, respectively. By contrast, in summer and early autumn with warm daytime temperatures (>10 °C) and dry soil (<10 % vol.), and also in mid-winter with near-freezing daytime temperatures the shrubland behaved as a net carbon source (mean daytime CO₂ release of ca. 0.60 and 0.20 μmol m⁻² s⁻¹, respectively). Furthermore, the comparison of water and carbon fluxes over a week in June 2004 and June 2005 suggests that the timing—rather than amount—of spring rainfall may be crucial in determining growing season water and carbon exchange. Due to strongly limiting environmental variables other than F _P, the model applied here failed to describe daytime carbon exchange only as a function of F _P and could not be used over most of the year to fill gaps in the data.     

陆地生态系统碳水通量贡献区评价综述

综述了通量贡献区研究的基本理论、最新进展、研究热点与难点,旨在促进中国区域碳水通量数据空间代表性的定量评价.通量贡献区是通量观测点上风向的空间代表区域,能够反映代表区域对应下垫面的源区内每一点对观测点的通量贡献权重影响,主要受观测高度、空气动力学粗糙度和大气稳定度等因素的影响.通量贡献区通常随着观测高度的增加、空气动力学粗糙度的降低和大气稳定度的增加而变大,反之则变小.通量贡献区的评价模型包括解析模型、拉格朗日随机模型、大涡模拟和闭合模型四类.通量贡献区的评价结果可以广泛应用于通量数据质量评价、实验设计的指导、与遥感技术结合的区域尺度的总初级生产力的估算、城市CO2通量变化的评估以及能量闭合的评价等研究.最新研究表明,对流边界层的通量贡献区存在负的通量贡献区域;有裸地存在的情况下解析模型通常会低估裸地对观测通量的贡献;与水平地面处的通量贡献区相比,山谷处通量贡献区变小而山脊处的通量贡献区变大.通量贡献区模型研究应进一步考虑大气中的平流效应、湍流的非高斯扩散以及建立冠层内部的通量贡献区模型.解决森林冠层内流场的不均匀性、冠层重叠问题、冠层湍流的不稳定性是建立适合冠层内部通量贡献区模型的前提条件.在理想条件的气体释放验证试验的基础上,需要开展复杂条件下的相关试验.     

Measuring and modelling seasonal variation of carbon dioxide and water vapour exchange of a Pinus ponderosa forest subject to soil water deficit

We conducted ecosystem carbon and water vapour exchange studies in an old‐growth Pinus ponderosa forest in the Pacific North‐west region of the United States. The canopy is heterogeneous, with tall multiaged trees and an open, clumped canopy with low leaf area. Carbon assimilation can occur throughout relatively mild winters, although night frosts can temporarily halt the process and physiological factors limit its efficiency. In contrast, carbon assimilation is often limited in the ‘growing season’ by stomatal closure associated with high evaporative demand (D) and soil water deficits. All of these factors present a challenge to effectively modelling ecosystem processes. Our objective was to generate an understanding of the controls on ecosystem processes across seasonal and annual cycles from a combination of fine‐scale process modelling, ecophysiological measurements, and carbon and water vapour fluxes measured by the eddy covariance method. Flux measurements showed that 50% and 70% of the annual carbon uptake occurred outside the ‘growing season’ (defined as bud break to senescence, ～ days 125–275) in 1996 and 1997. On a daily basis in summer, net ecosystem productivity (NEP) was low when D and soil water deficits were large. Whole ecosystem water vapour fluxes (LE) increased from spring to summer (1.0–1.9 mm d?1) as conducting leaf area increased by 30% and as evaporative demand increased, while evaporation from the soil surface became a smaller portion of total LE as soil water deficits increased. The models underestimated soil evaporation, particularly following rain. In the SPA model, varying the temperature optimum for photosynthesis seasonally resulted in overestimation of carbon uptake in winter and spring, showing that in coniferous forests, assumptions about temperature optima are clearly important. Daily estimates of soil surface CO₂ flux from measurements and site meteorological data demonstrated that modelling of soil CO₂ flux based on an Arrhenius‐type equation in CANPOND overestimated CO₂ respired from the soil during drought and when temperatures were low.     

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.