EVIDENCE from ice cores 1 indicates that concentrations of atmospheric carbon dioxide were lower by about 75 p.p.m. during the Last Glacial Maximum (LGM; 6518,000 years ago) than during the present interglacial (10,000 years ago to the present). The causes of such large changes in atmospheric CO 2 remain uncertain. Using a climate model, Prentice and Fung 2 have estimated that there was approximately the same amount of carbon in vegetation and soils during the LGM as there was during the present (pre-industrial) interglacial. In contrast, we present here results based on palynological, pedological and sedimentological evidence which indicate that in fact the amount of carbon in vegetation, soils and peatlands may have been smaller during the LGM by 651.3x 10 12 tonnes. Thus, organic carbon in vegetation and soils has more than doubled (from 0.96 to 2.3 x 10 12 tonnes) since the LGM. Oceanic CO 2 reservoirs seem to be the only possible source of this large quantity of carbon that has entered the terrestrial biosphere since the LGM (in addition to that which has entered the atmosphere to give the higher interglacial CO 2 levels).

Researchers have a poor understanding of the mechanisms that allow freshwater marshes to achieve rates of net primary production (NPP) that are higher than those reported for most other types of ecosystems. We used an 8-year record of the gross primary production (GPP) and NPP at the San Joaquin Freshwater Marsh (SJFM) in Southern California to determine the relative importance of GPP and carbon use efficiency (CUE; the ratio of total NPP to GPP calculated as NPP GPP 1) in determining marsh NPP. GPP was calculated from continuous eddy covariance measurements and NPP was calculated from annual harvests. The NPP at the SJFM was typical of highly productive freshwater marshes, while the GPP was similar to that reported for other ecosystem types, including some with comparatively low NPPs. NPP was weakly related to GPP in the same year, and was better correlated with the GPP summed from late in the previous year's growing season to early in the current growing season. This lag was attributed to carbohydrate reserves, which supplement carbon for new leaf growth in the early growing season of the current year. The CUE at the SJFM for the 8-year period was 0.61 卤 0.05. This CUE is larger than that reported for tropical, temperate, and boreal ecosystems, and indicates that high marsh NPP is attributable to a high CUE and not a high GPP. This study underscores the importance of autotrophic respiration and carbon allocation in determining marsh NPP.

根据冬小麦田间试验结果,用通径分析的方法,把不同灌溉决策指标对干物质和叶片水分利用效率(WUE)的影响进行了分析。通过土壤水分含量、叶面蒸腾、光合有效辐射、光合作用速率、气孔导度和叶温对作物干物质产量的通径分析发现,影响干物质产量最大的是叶面蒸腾,其次是光合有效辐射和叶温。通过充分灌溉和干旱水分处理相应时刻太阳辐射、光合有效辐射、空气水汽压差、气孔导度和叶温对作物叶片WUE的通径分析发现,对于适宜水分处理来说,主要影响WUE因子不是上述5个指标,而是土壤水分状况起到决定因素;对于干旱处理来说,主要影响因子是光合有效辐射和叶面温度。通过对干物质通径分析自变量指标的增减对结果的影响分析,发现,在考虑干物质生产和作物水分利用效率最优的目标时,灌溉决策指标要注意叶面蒸腾、光合有效辐射、叶面温度和土壤水分的变化。

根据冬小麦田间试验结果,用通径分析的方法,把不同灌溉决策指标对干物质和叶片水分利用效率(WUE)的影响进行了分析。通过土壤水分含量、叶面蒸腾、光合有效辐射、光合作用速率、气孔导度和叶温对作物干物质产量的通径分析发现,影响干物质产量最大的是叶面蒸腾,其次是光合有效辐射和叶温。通过充分灌溉和干旱水分处理相应时刻太阳辐射、光合有效辐射、空气水汽压差、气孔导度和叶温对作物叶片WUE的通径分析发现,对于适宜水分处理来说,主要影响WUE因子不是上述5个指标,而是土壤水分状况起到决定因素;对于干旱处理来说,主要影响因子是光合有效辐射和叶面温度。通过对干物质通径分析自变量指标的增减对结果的影响分析,发现,在考虑干物质生产和作物水分利用效率最优的目标时,灌溉决策指标要注意叶面蒸腾、光合有效辐射、叶面温度和土壤水分的变化。

Inter-annual variability in total precipitation can lead to significant changes in carbon flux.In this study,we used the eddy covariance(EC) technique to measure the net CO_2 ecosystem exchange(NEE) of an alpine meadow in the northern Tibetan Plateau.In 2005 the meadow had precipitation of 489.9 mm and in 2006 precipitation of 241.1 mm,which,respectively,represent normal and dry years as compared to the mean annual precipitation of 476 mm.The EC measured NEE was 87.70 g C m~(-2) yr~(-1) in 2006 and-2.35 g C m~(-2) yr~(-1) in 2005.Therefore,the grassland was carbon neutral to the atmosphere in the normal year,while it was a carbon source in the dry year,indicating this ecosystem will become a CO_2 source if climate warming results in more drought conditions.The drought conditions in the dry year limited gross ecosystem CO_2 exchange(GEE),leaf area index(LAI) and the duration of ecosystem carbon uptake.During the peak of growing season the maximum daily rate of NEE and Pmax and a were approximately 30%-50% of those of the normal year.GEE and NEE were strongly related to photosynthetically active radiation(PAR) on half-hourly scale,but this relationship was confounded by air temperature(Ta),soil water content(SWC) and vapor pressure deficit(VPD).The absolute values of NEE declined with higher Ta,higher VPD and lower SWC conditions.Beyond the appropriate range of PAR,high solar radiation exacerbated soil water conditions and thus reduced daytime NEE.Optimal T_a and VPD for maximum daytime NEE were 12.7鈩 and 0.42 KPa respectively,and the absolute values of NEE increased with SWC.Variation in LAI explained around 77% of the change in GEE and NEE.Variations in R_e were mainly controlled by soil temperature(T_s),whereas soil water content regulated the responses of R_e to T_s.

Abstract Plant functional traits control a variety of terrestrial ecosystem processes, including soil carbon storage which is a key component of the global carbon cycle. Plant traits regulate net soil carbon storage by controlling carbon assimilation, its transfer and storage in belowground biomass, and its release from soil through respiration, fire and leaching. However, our mechanistic understanding of these processes is incomplete. Here, we present a mechanistic framework, based on the plant traits that drive soil carbon inputs and outputs, for understanding how alteration of vegetation composition will affect soil carbon sequestration under global changes. First, we show direct and indirect plant trait effects on soil carbon input and output through autotrophs and heterotrophs, and through modification of abiotic conditions, which need to be considered to determine the local carbon sequestration potential. Second, we explore how the composition of key plant traits and soil biota related to carbon input, release and storage prevail in different biomes across the globe, and address the biome-specific mechanisms by which plant trait composition may impact on soil carbon sequestration. We propose that a trait-based approach will help to develop strategies to preserve and promote carbon sequestration.

Based on eddy covariance measurement over a degraded grassland and a cropland with maize (Zea mays) ecosystems from 2003 to 2009, carbon exchange processes and their responses to environmental factors in different temporal scales were analyzed in semiarid of China. Accounting for carbon export and import, NBP (net biome production) of cropland with maize ranged from 54.3 to 100.6g Cm612yr611. NBP remained positive indicating a carbon net loss from this ecosystem although NEE (net ecosystem exchange) was negative in most of years. Due to negligible carbon import and export, NBP of degraded grassland ecosystem was equal to NEE, with an average value of 138.4g Cm612yr611. The grassland ecosystem behaved as carbon source during the whole period. PPFD (incident photosynthetic active radiation) was the main driver for diurnal variation of NEE during growing season in most years. NDVI (normal difference vegetation index) was in accordance with seasonal patterns of NEE especially for cropland with maize ecosystem. Soil temperature at a depth of 5cm was also a main driver for seasonal variation of NEE at the degraded grassland ecosystem in normal precipitation years (2003 and 2005). Annual peak NDVI (NDVImax) was significantly correlated with annual NEE and GPP (gross primary productivity). The amount of growing season precipitation was more responsible for annual variation of NEE. The increasing number of precipitation event (>1mmday611) was associated with increasing annual carbon uptake. Drought in the early growing period is more critical to carbon dynamics of degraded grassland ecosystem while drought in the middle of growing season was more critical for cropland with maize ecosystem.

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 mumol 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

In order to assess the capacity of the boreal forest ecosystem to intercept atmospheric carbon over a period of years, a climate-driven growth model (FinnFor, process-based) was applied to calculate the seasonal and inter-annual variability of net ecosystem CO 2 exchange (NEE) and component carbon fluxes (gross primary production – GPP and total ecosystem respiration – TER) against a 10-year (1999–2008) period of eddy covariance (EC) measurements in a Scots pine ( Pinus sylvestris L.) stand in Eastern Finland. Furthermore, the role of climatic factors, leaf area index ( LAI) and physiological responses of trees regarding the ecosystem carbon fixation processes were evaluated. An hourly time-step was used to simulate the carbon exchange based on measured tree/stand characteristics and meteorological input for the experimental site, and the dynamic LAI was used throughout the 10-year simulations. The model predicted well the annual course of NEE compared to the measured values for most of the years, with the development of LAI (2.4–3.3 m 2 m 612, as simulated). The simulated NEE over the study period shows that, on average, 62% of the variation refers to daily and 88% to monthly measured NEE. Both modeled and measured daily NEE showed similar responses to the temperature, photosynthetically active radiation and vapor pressure deficit during the growing seasons. In the simulation, the annual amount of GPP varied from 720.8 to 910.4 g C m 612 with a mean value of 825.3 g C m 612, and the annual mean TER/GPP ratio was 0.79, close to the measured value. Carbon efflux from the forest floor was the dominant contributor to the forest ecosystem respiration. The inter-annual variation of GPP mostly corresponded to the development of LAI, temperature sum and total incoming radiation over the 10-year simulation period. It was suggested that the process-based model could be applied to study the carbon processes for natural and management-induced dynamics of Scots pine forest ecosystem over longer periods across a wider climate gradient in the boreal zone.

Tower CO 2 flux measurements from 20 European grasslands in the EUROGRASSFLUX data set covering a wide range of environmental and management conditions were analyzed with respect to their ecophysiological characteristics and CO 2 exchange (gross primary production, P g , and ecosystem respiration, R e ) using light-response function analysis. Photosynthetically active radiation ( Q ) and top-soil temperature ( T s ) were identified as key factors controlling CO 2 exchange between grasslands and the atmosphere at the 30-min scale. A nonrectangular hyperbolic light-response model P ( Q ) and modified nonrectangular hyperbolic light–temperature-response model P ( Q , T s ) proved to be flexible tools for modeling CO 2 exchange in the light. At night, it was not possible to establish robust instantaneous relationships between CO 2 evolution rate r n and environmental drivers, though under certain conditions, a significant relationship r n =r 0 65ek T T s r n = r 0 65 e k T T s mathContainer Loading Mathjax was found using observation windows 7–14 days wide. Principal light-response parameters—apparent quantum yield, saturated gross photosynthesis, daytime ecosystem respiration, and gross ecological light-use efficiency, 07 02=02 P g / Q , display patterns of seasonal dynamics which can be formalized and used for modeling. Maximums of these parameters were found in intensively managed grasslands of Atlantic climate. Extensively used semi-natural grasslands of southern and central Europe have much lower production, respiration, and light-use efficiency, while temperate and mountain grasslands of central Europe ranged between these two extremes. Parameters from light–temperature-response analysis of tower data are in agreement with values obtained using closed chambers and free-air CO 2 enrichment. Correlations between light-response and productivity parameters provides the possibility to use the easier to measure parameters to estimate the parameters that are more difficult to measure. Gross primary production ( P g ) of European grasslands ranges from 170002g02CO 2 02m 612 02year 611 in dry semi-natural pastures to 690002g02CO 2 02m 612 02year 611 in intensively managed Atlantic grasslands. Ecosystem respiration ( R e ) is in the range 180002240002g02CO 2 02m 612 02year 611 ) to significant release (<6160002g02CO 2 02m 612 02year 611 ), though in 15 out of 19 cases grasslands performed as net CO 2 sinks. The carbon source was associated with organic rich soils, grazing, and heat stress. Comparison of P g , R e , and NEE for tower sites with the same characteristics from previously published papers obtained with other methods did not reveal significant discrepancies. Preliminary results indicate relationships of grassland P g and R e to macroclimatic factors (precipitation and temperature), but these relationships cannot be reduced to simple monofactorial models.

Abstract The measured net ecosystem exchange (NEE) of CO 2 between the ecosystem and the atmosphere reflects the balance between gross CO 2 assimilation [gross primary production (GPP)] and ecosystem respiration (R eco ). For understanding the mechanistic responses of ecosystem processes to environmental change it is important to separate these two flux components. Two approaches are conventionally used: (1) respiration measurements made at night are extrapolated to the daytime or (2) light–response curves are fit to daytime NEE measurements and respiration is estimated from the intercept of the ordinate, which avoids the use of potentially problematic nighttime data. We demonstrate that this approach is subject to biases if the effect of vapor pressure deficit (VPD) modifying the light response is not included. We introduce an algorithm for NEE partitioning that uses a hyperbolic light response curve fit to daytime NEE, modified to account for the temperature sensitivity of respiration and the VPD limitation of photosynthesis. Including the VPD dependency strongly improved the model's ability to reproduce the asymmetric diurnal cycle during periods with high VPD, and enhances the reliability of R eco estimates given that the reduction of GPP by VPD may be otherwise incorrectly attributed to higher R eco . Results from this improved algorithm are compared against estimates based on the conventional nighttime approach. The comparison demonstrates that the uncertainty arising from systematic errors dominates the overall uncertainty of annual sums (median absolute deviation of GPP: 47gCm 612 yr 611 ), while errors arising from the random error (median absolute deviation: 652gCm 612 yr 611 ) are negligible. Despite site-specific differences between the methods, overall patterns remain robust, adding confidence to statistical studies based on the FLUXNET database. In particular, we show that the strong correlation between GPP and R eco is not spurious but holds true when quasi-independent, i.e. daytime and nighttime based estimates are compared.

[1] The alpine meadow ecosystem on the Qinghai-Tibetan Plateau may play a significant role in the regional carbon cycle. To assess the CO2 flux and its relationship to environmental controls in the ecosystem, eddy covariance of CO2, H2O, and energy fluxes was measured with an open-path system in an alpine meadow on the plateau at an elevation of 3,250 m. Net ecosystem CO2 influx (Fc) averaged 8.8 g m0908082 day0908081 during the period from August 9 to 31, 2001, with a maximum of 15.9 g m0908082 day0908081 and a minimum of 2.3 g m0908082 day0908081. Daytime Fc averaged 16.7 g m0908082 day0908081 and ranged from 10.4 g m0908082 day0908081 to 21.7 g m0908082 day0908081 during the study period. For the same photosynthetic photon flux density (PPFD), gross CO2 uptake (Gc) was significantly higher on cloudy days than on clear days. However, mean daily Gc was higher on clear days than on cloudy days. With high PPFD, Fc decreased as air temperature increased from 1000°C to 2300°C. The greater the difference between daytime and nighttime air temperatures, the more the sink was strengthened. Daytime average water use efficiency of the ecosystem (WUEe) was 8.7 mg (CO2)(g H2O)0908081; WUEe values ranged from 5.8 to 15.3 mg (CO2)(g H2O)0908081. WUEe increased with the decrease in vapor pressure deficit. Daily albedo averaged 0.20, ranging from 0.19 to 0.22 during the study period, and was negatively correlated with daily Fc. Our measurements provided some of the first evidence on CO2 exchange for a temperate alpine meadow ecosystem on the Qinghai-Tibetan Plateau, which is necessary for assessing the carbon budget and carbon cycle processes for temperate grassland ecosystems.

Sulfur group analyses of reinforced MBTS accelerated vulcanizates reveal chemical differences which can be related to the surface chemistry of the carbon blacks. The analytical procedure is based on the treatment of the vulcanizate with lithium aluminum hydride followed by potentiometric titration of sulfide and mercaptan sulfur. The polysulfide crosslink density and the values of x in the RSSxSR polysulfides are dependent upon the quinone content of the carbon black. Both the quinone content and the surface area of a carbon black appear to determine its influence on the disappearance of free sulfur and the apparent crosslinking of the vulcanizate.

First page of article

The understanding of the controlling factors determining interannual variability (IAV) of carbon dioxide (CO 2 ) exchange between different ecosystems is crucial when assessing present and future responses to climate variability and climate change. Six years of eddy covariance (EC) data from three neighboring sites (agriculture, forest, and meadow) subjected to management in variable degree were evaluated to determine typical CO 2 budgets and controlling factors of IAV. In terms of average annual net ecosystem exchange (NEE) the agricultural and wet meadow site showed identical rates of 61156 (±110 and ±116, respectively) g C m 612 02y 611 , with large IAV and individual years even showing near zero net uptake. In contrast, the forest was a substantial and persistent sink of CO 2 (avg.02±02s.d. 6169102±0214302g02C02m 612 02y 611 ), but had a higher absolute IAV. A homogeneity-of-slopes (HOS) model was utilized to partition sources of IAV of CO 2 fluxes between direct climatic effects and indirect effects (functional changes). This analysis showed that NEE at the forest (through both GPP and RE) was most prone to interannual functional changes. The wet meadow showed moderate functional changes with respect to RE and thus NEE, whereas the cropland did not show any statistically significant functional changes. We argue that the delicate interplay between climate forcing, land use specific traits, management practices and intensities, and functional changes has to be taken into account when predicting the atmospheric CO 2 sink/source strengths of land ecosystems for longer timescales.

To date the implications of greater intra-annual variability and extremes in precipitation on ecosystem functioning have received little attention. This study presents results on soil and vegetation carbon and water fluxes in the understorey of a Mediterranean oak woodland in response to increasing precipitation variability, with an extension of the dry period between precipitation events from 3 to 602weeks, without altering total annual precipitation inputs. With prolonged dry periods soil moisture did breach the stress thresholds for ecosystem processes, which led to short-term treatment differences in photosynthesis, but not in system carbon losses, with subsequent short-term decreases in net ecosystem exchange. Independent of treatment, irrigation events rapidly increased carbon and water fluxes. However, contradicting the predictions drawn from the ‘bucket model’, over the course of the growing season no all-over treatment differences were found in system assimilation and respiration, nor in evapotranspiration and ecosystem water use efficiency. This lack of responsiveness is attributed to the ecosystem’s resilience to low soil moisture during the growing season of the herbaceous understorey, with temperature rather than soil moisture controlling key ecosystem processes. Moreover, severe nitrogen limitation of the studied ecosystem may explain the lack of moisture effects on net system carbon dynamics. Thus, although the bucket model predicts changes in soil water dynamics with increasing precipitation variability, ecosystem responses to more extreme precipitation regimes may be influenced by additional factors, such as inter-annual variability in nutrient availability.

[1] We measured the net ecosystem CO2 exchange (NEE) in an alpine meadow ecosystem (latitude 3700°2909000509000945090005N, longitude 10100°1209000509000923090005E, 3250 m above sea level) on the Qinghai-Tibetan Plateau throughout 2002 by the eddy covariance method to examine the carbon dynamics and budget on this unique plateau. Diurnal changes in gross primary production (GPP) and ecosystem respiration (Re) showed that an afternoon increase of NEE was highly associated with an increase of Re. Seasonal changes in GPP corresponded well to changes in the leaf area index and daily photosynthetic photon flux density. The ratio of GPP/Re was high and reached about 2.0 during the peak growing season, which indicates that mainly autotrophic respiration controlled the carbon dynamics of the ecosystem. Seasonal changes in mean GPP and Re showed compensatory behavior as reported for temperate and Mediterranean ecosystems, but those of GPPmax and Remax were poorly synchronized. The alpine ecosystem exhibited lower GPP (575 g C m0908082 y0908081) than, but net ecosystem production (78.5 g C m0908082 y0908081) similar to, that of subalpine forest ecosystems. The results suggest that the alpine meadow behaved as a CO2 sink during the 1-year measurement period but apparently sequestered a rather small amount of C in comparison with similar alpine ecosystems.

We examined the CO 2 exchange of a Kobresia meadow ecosystem on the Qinghai–Tibetan plateau using a chamber system. CO 2 efflux from the ecosystem was strongly dependence on soil surface temperature. The CO 2 efflux–temperature relationship was identical under both light and dark conditions, indicating that no photosynthesis could be detected under light conditions during the measurement period. The temperature sensitivity ( Q 10) of the CO 2 efflux showed a marked transition around 611.0 °C; Q 10 was 2.14 at soil surface temperatures above and equal to 611.0 °C but was 15.3 at temperatures below 611.0 °C. Our findings suggest that soil surface temperature was the major factor controlling winter CO 2 flux for the alpine meadow ecosystem and that freeze–thaw cycles at the soil surface layer play an important role in the temperature dependence of winter CO 2 flux.

We used the eddy covariance method to measure the CO 2 exchange between the atmosphere and an alpine meadow ecosystem (37°29–45′N, 101°12–23′E, 3250 m a.s.l.) on the Qinghai–Tibetan Plateau, China in the 2001 and 2002 growing seasons. The maximum rates of CO 2 uptake and release derived from the diurnal course of CO 2 flux ( F CO 2) were 6110.8 and 4.4 μmol m 612 s 611, respectively, indicating a relatively high net carbon sequestration potential as compared to subalpine coniferous forest at similar elevation and latitude. The largest daily CO 2 uptake was 3.9 g C m 612 per day on 7 July 2002, which is less than half of those reported for lowland grassland and forest at similar latitudes. The daily CO 2 uptake during the measurement period indicated that the alpine ecosystem might behave as a sink of atmospheric CO 2 during the growing season if the carbon lost due to grazing is not significant. The daytime CO 2 uptake was linearly correlated with the daily photosynthetic photon flux density each month. The nighttime averaged F CO 2 showed a positive exponential correlation with the soil temperature, but apparently negative correlation with the soil water content.

Abstract Ecosystem responses to increased variability in rainfall, a prediction of general circulation models, were assessed in native grassland by reducing storm frequency and increasing rainfall quantity per storm during a 4-year experiment. More extreme rainfall patterns, without concurrent changes in total rainfall quantity, increased temporal variability in soil moisture and plant species diversity. However, carbon cycling processes such as soil CO2 flux, CO2 uptake by the dominant grasses, and aboveground net primary productivity (ANPP) were reduced, and ANPP was more responsive to soil moisture variability than to mean soil water content. Our results show that projected increases in rainfall variability can rapidly alter key carbon cycling processes and plant community composition, independent of changes in total precipitation.

Abstract The measured net ecosystem exchange (NEE) of CO 2 between the ecosystem and the atmosphere reflects the balance between gross CO 2 assimilation [gross primary production (GPP)] and ecosystem respiration (R eco ). For understanding the mechanistic responses of ecosystem processes to environmental change it is important to separate these two flux components. Two approaches are conventionally used: (1) respiration measurements made at night are extrapolated to the daytime or (2) light–response curves are fit to daytime NEE measurements and respiration is estimated from the intercept of the ordinate, which avoids the use of potentially problematic nighttime data. We demonstrate that this approach is subject to biases if the effect of vapor pressure deficit (VPD) modifying the light response is not included. We introduce an algorithm for NEE partitioning that uses a hyperbolic light response curve fit to daytime NEE, modified to account for the temperature sensitivity of respiration and the VPD limitation of photosynthesis. Including the VPD dependency strongly improved the model's ability to reproduce the asymmetric diurnal cycle during periods with high VPD, and enhances the reliability of R eco estimates given that the reduction of GPP by VPD may be otherwise incorrectly attributed to higher R eco . Results from this improved algorithm are compared against estimates based on the conventional nighttime approach. The comparison demonstrates that the uncertainty arising from systematic errors dominates the overall uncertainty of annual sums (median absolute deviation of GPP: 47gCm 612 yr 611 ), while errors arising from the random error (median absolute deviation: 652gCm 612 yr 611 ) are negligible. Despite site-specific differences between the methods, overall patterns remain robust, adding confidence to statistical studies based on the FLUXNET database. In particular, we show that the strong correlation between GPP and R eco is not spurious but holds true when quasi-independent, i.e. daytime and nighttime based estimates are compared.

以金露梅(Potentillafruticosa)灌丛草甸生态系统为对象,应用静态密闭箱-气相色谱法对高寒灌丛(GG)、丛内草甸(GC)和裸地(GL)的CO2释放进行了初步研究。结果表明:GG、GC和GLCO2的释放速率均呈明显的单峰型日变化进程,最大释放速率出现在15∶00～17∶00之间,最小值在7∶00前后出现,白天释放速率大于夜晚;CO2释放速率具有明显的季节性变化特征,生长期CO2释放速率明显高于枯黄期,且均表现为正排放,8月为CO2释放高峰期,释放速率GGGCGL(P0.01);2003年6月30日至2004年2月28日,高寒灌丛植被-土壤系统CO2释放量为3088.458±287.02g/m2,丛内草甸植被-土壤系统CO2释放量为2239.685±183.68g/m2,其中基础土壤呼吸CO2的释放量约为1346.748±176.24g/m2,分别占GG和GC释放量的43.61%和60.13%;CO2释放速率的日变化主要受地表和5cm地温制约,而季节动态与5cm地温呈显著正相关关系(P0.01)。

以金露梅(Potentillafruticosa)灌丛草甸生态系统为对象,应用静态密闭箱-气相色谱法对高寒灌丛(GG)、丛内草甸(GC)和裸地(GL)的CO2释放进行了初步研究。结果表明:GG、GC和GLCO2的释放速率均呈明显的单峰型日变化进程,最大释放速率出现在15∶00～17∶00之间,最小值在7∶00前后出现,白天释放速率大于夜晚;CO2释放速率具有明显的季节性变化特征,生长期CO2释放速率明显高于枯黄期,且均表现为正排放,8月为CO2释放高峰期,释放速率GGGCGL(P0.01);2003年6月30日至2004年2月28日,高寒灌丛植被-土壤系统CO2释放量为3088.458±287.02g/m2,丛内草甸植被-土壤系统CO2释放量为2239.685±183.68g/m2,其中基础土壤呼吸CO2的释放量约为1346.748±176.24g/m2,分别占GG和GC释放量的43.61%和60.13%;CO2释放速率的日变化主要受地表和5cm地温制约,而季节动态与5cm地温呈显著正相关关系(P0.01)。

Alpine ecosystems play an important role in the global carbon cycle, yet the long-term response ofin situground-based observations of carbon fluxes to climate change remains not fully understood. Here, we analyzed the continuous net ecosystem CO2exchange (NEE) measured with the eddy covariance technique over an alpinePotentilla fruticosashrubland on the northeastern Qinghai-Tibetan Plateau from 2003 to 2012. The shrubland acted as a net CO2sink with a negative NEE (6174.4±12.7gCm612year611, Mean±S.E.). The mean annual gross primary productivity (GPP) and annual ecosystem respiration (RES) were 511.8±11.3 and 437.4±17.8gCm612year611, respectively. The classification and regression trees (CART) analysis showed that aggregated growing season degree days (GDD) was the predominant determinant on variations in monthly NEE and monthly GPP, including its effect on leaf area index (LAI, satellite-retrieved data). However, variations in monthly RES were determined much more strongly by LAI. Non-growing season soil temperature (Ts) and growing season length (GSL) accounted for 59% and 42% of variations in annual GPP and annual NEE, respectively. Growing season soil water content (SWC) exerted a positive linear influence on variations in annual RES (r2=0.40,p=0.03). The thermal conditions and soil water status during the onset of the growing season are crucial for inter-annual variations of carbon fluxes. Our results suggested that an extended growing season and warmer non-growing season would enhance carbon assimilation capacity in the alpine shrubland.

<FONT face=Verdana>采用涡度相关法,并结合小气候观测,对荒漠生态系统净二氧化碳通量进行了连续3个生长季的观测（2004—2006年）,并据此分析了荒漠生态系统净二氧化碳通量及其主要成分GPP和Reco的季节和年际间变化特征。结果表明,在生长季尺度上,各个阶段二氧化碳吸收量的大小分别为:生长旺盛期&gt;生长初期&gt;生长末期,这可能与植物叶面积的大小以及光合有效辐射,大气温度等环境要素有关系。在年际尺度上,3个生长季同阶段的二氧化碳吸收量存在明显差异,生长季初期5月,2004年碳吸收最强,2006年次之,2005年最小。对于生长旺盛期,降水量最大的2004年碳吸收能力最强,正午最大值可以达到-0.12 mg·m-2·s-1,2005年次之,最大值达-0.06 mg·m-2·s-1,仅仅是2004年最大值的1/2,2006年最小,正午吸收的最大值为-0.02 mg·m-2·s-1,生长季末期,3个生长季的月均日变化非常相似,其在正午的最大吸收值也没有显著差异,正午最大吸收量为-0.01 mg·m-2·s-1左右,其他时段均在0值附近。即使在降水量相近的两个年份里（2005—2006年）,其NEE的最大值出现时间也不一致,2005年NEE的最大月累计量出现在8月和9月,而2006年则出现在6月和7月,这可能与年内降水量分布格局有关。3个生长季荒漠生态系统均表现为净二氧化碳吸收,其吸收量分别为:-236.18 g·m-2,-63.07 g·m-2和-91.97 g·m-2。年际差异的形成原因是降水差异造成的一年生草本植物数量变化,不利用降水的建群种应该对此没有贡献。</FONT>

<FONT face=Verdana>采用涡度相关法,并结合小气候观测,对荒漠生态系统净二氧化碳通量进行了连续3个生长季的观测（2004—2006年）,并据此分析了荒漠生态系统净二氧化碳通量及其主要成分GPP和Reco的季节和年际间变化特征。结果表明,在生长季尺度上,各个阶段二氧化碳吸收量的大小分别为:生长旺盛期&gt;生长初期&gt;生长末期,这可能与植物叶面积的大小以及光合有效辐射,大气温度等环境要素有关系。在年际尺度上,3个生长季同阶段的二氧化碳吸收量存在明显差异,生长季初期5月,2004年碳吸收最强,2006年次之,2005年最小。对于生长旺盛期,降水量最大的2004年碳吸收能力最强,正午最大值可以达到-0.12 mg·m-2·s-1,2005年次之,最大值达-0.06 mg·m-2·s-1,仅仅是2004年最大值的1/2,2006年最小,正午吸收的最大值为-0.02 mg·m-2·s-1,生长季末期,3个生长季的月均日变化非常相似,其在正午的最大吸收值也没有显著差异,正午最大吸收量为-0.01 mg·m-2·s-1左右,其他时段均在0值附近。即使在降水量相近的两个年份里（2005—2006年）,其NEE的最大值出现时间也不一致,2005年NEE的最大月累计量出现在8月和9月,而2006年则出现在6月和7月,这可能与年内降水量分布格局有关。3个生长季荒漠生态系统均表现为净二氧化碳吸收,其吸收量分别为:-236.18 g·m-2,-63.07 g·m-2和-91.97 g·m-2。年际差异的形成原因是降水差异造成的一年生草本植物数量变化,不利用降水的建群种应该对此没有贡献。</FONT>

Eddy covariance measurements of water and carbon (C) fluxes were carried out in a desert halophyte community in western China, during two years differing greatly in precipitation (2006 and 2007). The first year was dry, with annual precipitation 22% below the long-term mean (163mm) and the second year was wet (42% above the long-term mean). The main goal of this study was to develop an understanding of how ecological and hydrological processes and vegetation composition respond to precipitation variability in halophyte desert ecosystems. On an annual basis, the desert halophyte community was a weak sink or source in the dry year (615±12gCm612year611), but a strong sink in the wet year (6140±12gCm612year611). Groundwater was a stable water source for evaporation and transpiration, supplying average of 14mm in both the dry and the wet years for each part. However, water supply for plant transpiration from precipitation differed remarkably between the two years: 17 and 48mm for the dry and wet years, respectively. Connecting water use and C gain, ecosystem water use efficiency was markedly different for the dry and wet years, with values of 0.03 and 0.15gC per kg H2O, respectively; however, plant water use efficiency was differed only slightly (3.58 and 3.51gC per kg H2O). Vegetation community surveys and root investigations revealed that more shallow-rooted herbaceous plants occurred in the wet year compared to the dry year. Thus the inter-annual variation of water and C fluxes may have resulted from adjustment of community structure to precipitation with more annuals or ephemeral plants in the wet year. These shallow-rooted plants use the extra water input in the wet year, and consequently the community productivity increases. Water use efficiency at the ecosystem level increased in the wet year at this desert, contrary to findings in more humid environments where water use efficiency increases in dry years.

Abstract Wetlands play an important role in regulating the atmospheric carbon dioxide (CO 2 ) concentrations and thus affecting the climate. However, there is still lack of quantitative evaluation of such a role across different wetland types, especially at the global scale. Here, we conducted a meta-analysis to compare ecosystem CO 2 fluxes among various types of wetlands using a global database compiled from the literature. This database consists of 143 site-years of eddy covariance data from 22 inland wetland and 21 coastal wetland sites across the globe. Coastal wetlands had higher annual gross primary productivity (GPP), ecosystem respiration (R e ), and net ecosystem productivity (NEP) than inland wetlands. On a per unit area basis, coastal wetlands provided large CO 2 sinks, while inland wetlands provided small CO 2 sinks or were nearly CO 2 neutral. The annual CO 2 sink strength was 93.15 and 208.370002g0002C0002m -2 for inland and coastal wetlands, respectively. Annual CO 2 fluxes were mainly regulated by mean annual temperature (MAT) and mean annual precipitation (MAP). For coastal and inland wetlands combined, MAT and MAP explained 71%, 54%, and 57% of the variations in GPP, R e , and NEP, respectively. The CO 2 fluxes of wetlands were also related to leaf area index (LAI). The CO 2 fluxes also varied with water table depth (WTD), although the effects of WTD were not statistically significant. NEP was jointly determined by GPP and R e for both inland and coastal wetlands. However, the NEP/R e and NEP/GPP ratios exhibited little variability for inland wetlands and decreased for coastal wetlands with increasing latitude. The contrasting of CO 2 fluxes between inland and coastal wetlands globally can improve our understanding of the roles of wetlands in the global C cycle. Our results also have implications for informing wetland management and climate change policymaking, for example, the efforts being made by international organizations and enterprises to restore coastal wetlands for enhancing blue carbon sinks. 0008 2016 John Wiley & Sons Ltd.

During the summer period (day 150–240), total evapotranspiration for non-drought years ranged from 224 to 27302mm with a mean and standard error of ja:math . The mean and standard error of the net ecosystem exchange (NEE) rate of carbon dioxide for the same summer period was ja:math . In a year with severe drought (1998) total evapotranspiration for the summertime period was 14502mm. The lack of precipitation during this time resulted in total losses to the atmosphere of 15502g C/m 2 from soil respiration.

A vegetation index (VI) model for predicting evapotranspiration (ET) from data from the Moderate Resolution Imaging Spectrometer (MODIS) on the EOS-1 Terra satellite and ground meteorological data was developed for riparian vegetation along the Middle Rio Grande River in New Mexico. Ground ET measurements obtained from eddy covariance towers at four riparian sites were correlated with MODIS VIs, MODIS land surface temperatures (LSTs), and ground micrometeorological data over four years. Sites included two saltcedar ( Tamarix ramosissima ) and two Rio Grande cottonwood ( Populus deltoides ssp. Wislizennii) dominated stands. The Enhanced Vegetation Index (EVI) was more closely correlated ( r =0.76) with ET than the Normalized Difference Vegetation Index (NDVI; r =0.68) for ET data combined over sites and species. Air temperature ( T a ) measured over the canopy from towers was the meteorological variable that was most closely correlated with ET ( r =0.82). MODIS LST data at 1- and 5-km resolutions were too coarse to accurately measure the radiant surface temperature within the narrow riparian corridor; hence, energy balance methods for estimating ET using MODIS LSTs were not successful. On the other hand, a multivariate regression equation for predicting ET from EVI and T a had an r 2 =0.82 across sites, species, and years. The equation was similar to VI T models developed for crop species. The finding that ET predictions did not require species-specific equations is significant, inasmuch as these are mixed vegetation zones that cannot be easily mapped at the species level.

Variability and future alterations in regional and global climate patterns may exert a strong control on the carbon dioxide (CO 2 ) exchange of grassland ecosystems. We used 6years of eddy-covariance measurements to evaluate the impacts of seasonal and inter-annual variations in environmental conditions on the net ecosystem CO 2 exchange (NEE), gross ecosystem production (GEP), and ecosystem respiration (ER) of an intensively managed grassland in the humid temperate climate of southern Ireland. In all the years of the study period, considerable uptake of atmospheric CO 2 occurred in this grassland with a narrow range in the annual NEE from 61245 to 61284gCm 612 y 611 , with the exception of 2008 in which the NEE reached 61352gCm 612 y 611 . None of the measured environmental variables (air temperature (Ta), soil moisture, photosynthetically active radiation, vapor pressure deficit (VPD), precipitation (PPT), and so on) correlated with NEE on a seasonal or annual scale because of the equal responses from the component fluxes GEP and ER to variances in these variables. Pronounced reduction of summer PPT in two out of the six studied years correlated with decreases in both GEP and ER, but not with NEE. Thus, the stable annual NEE was primarily achieved through a strong coupling of ER and GEP on seasonal and annual scales. Limited inter-annual variations in Ta (±0.5°C) and generally sufficient soil moisture availability may have further favored a stable annual NEE. Monthly ecosystem carbon use efficiency (CUE; as the ratio of NEE:GEP) during the main growing season (April 1–September 30) was negatively correlated with temperature and VPD, but positively correlated with soil moisture, whereas the annual CUE correlated negatively with annual NEE. Thus, although drier and warmer summers may mildly reduce the uptake potential, the annual uptake of atmospheric CO 2 , in this intensively managed grassland, may be expected to continue even under predicted future climatic changes in the humid temperate climate region.

78 We investigated changes in NPP and NEP of Qinghai–Tibetan grasslands from 1961 to 2009. 78 A systematically calibrated process-based ecosystem model called ORCHIDEE was applied. 78 Qinghai–Tibetan grassland NPP significantly increased with a rate of 1.9Tg Cyr612 since 1961. 78 NEP increased from a net carbon source of 610.5Tg Cyr611 in the 1960s to a net carbon sink of 21.8Tg Cyr611 in the 2000s.

For most ecosystems, net ecosystem exchange of CO2 (NEE) varies within and among years in response to environmental change. We analyzed measurements of CO2 exchange from eight native rangeland ecosystems in the western United States (58 site-years of data) in order to determine the contributions of photosynthetic and respiratory (physiological) components of CO2 exchange to environmentally caused variation in NEE. Rangelands included Great Plains grasslands, desert shrubland, desert grasslands, and sagebrush steppe. We predicted that (1) week-to-week change in NEE and among-year variation in the response of NEE to temperature, net radiation, and other environmental drivers would be better explained by change in maximum rates of ecosystem photosynthesis (Amax) than by change in apparent light-use efficiency ( ) or ecosystem respiration at 10 C (R10) and (2) among-year variation in the responses of NEE, Amax, and to environmental drivers would be explained by changes in leaf area index (LAI). As predicted, NEE was better correlated with Amax than or R10 for six of the eight rangelands. Week-to-week variation in NEE and physiological parameters correlated mainly with time-lagged indices of precipitation and water-related environmental variables, like potential evapotranspiration, for desert sites and with net radiation and temperature for Great Plains grasslands. For most rangelands, the response of NEE to a given change in temperature, net radiation, or evaporative demand differed among years because the response of photosynthetic parameters (Amax, ) to environmental drivers differed among years. Differences in photosynthetic responses were not explained by variation in LAI alone. A better understanding of controls on canopy photosynthesis will be required to predict variation in NEE of rangeland ecosystems.

Abstract Respiration (carbon efflux) by terrestrial ecosystems is a major component of the global carbon (C) cycle, but the response of C efflux to atmospheric CO 2 enrichment remains uncertain. Respiration may respond directly to an increase in the availability of C substrates at high CO 2 , but also may be affected indirectly by a CO 2 -mediated alteration in the amount by which respiration changes per unit of change in temperature or C uptake (sensitivity of respiration to temperature or C uptake). We measured CO 2 fluxes continuously during the final 2 years of a 4-year experiment on C 3 /C 4 grassland that was exposed to a 200 560 molmol 1 CO 2 gradient. Flux measurements were used to determine whether CO 2 treatment affected nighttime respiration rates and the response of ecosystem respiration to seasonal changes in net C uptake and air temperature. Increasing CO 2 from subambient to elevated concentrations stimulated grassland respiration at night by increasing the net amount of C fixed during daylight and by increasing either the sensitivity of C efflux to daily changes in C fixation or the respiration rate in the absence of C uptake (basal ecosystem respiration rate). These latter two changes contributed to a 30鈥47% increase in the ratio of nighttime respiration to daytime net C influx as CO 2 increased from subamient to elevated concentrations. Daily changes in net C uptake were highly correlated with variation in temperature, meaning that the shared contribution of C uptake and temperature in explaining variance in respiration rates was large. Statistically controlling for collinearity between temperature and C uptake reduced the effect of a given change in C influx on respiration. Conversely, CO 2 treatment did not affect the response of grassland respiration to seasonal variation in temperature. Elevating CO 2 concentration increased grassland respiration rates by increasing both net C input and respiration per unit of C input. A better understanding of how C efflux varies with substrate supply thus may be required to accurately assess the C balance of terrestrial ecosystems.

Abstract Terrestrial plant and soil respiration, or ecosystem respiration (Reco), represents a major CO2 flux in the global carbon cycle. However, there is disagreement in how Reco will respond to future global changes, such as elevated atmosphere CO2 and warming. To address this, we synthesized six years (2007–2012) of Reco data from the Prairie Heating And CO2 Enrichment (PHACE) experiment. We applied a semi-mechanistic temperature–response model to simultaneously evaluate the response of Reco to three treatment factors (elevated CO2, warming, and soil water manipulation) and their interactions with antecedent soil conditions [e.g., past soil water content (SWC) and temperature (SoilT)] and aboveground factors (e.g., vapor pressure deficit, photosynthetically active radiation, vegetation greenness). The model fits the observed Reco well ( R 202=020.77). We applied the model to estimate annual (March–October) Reco, which was stimulated under elevated CO2 in most years, likely due to the indirect effect of elevated CO2 on SWC. When aggregated from 2007 to 2012, total six-year Reco was stimulated by elevated CO2 singly (24%) or in combination with warming (28%). Warming had little effect on annual Reco under ambient CO2, but stimulated it under elevated CO2 (32% across all years) when precipitation was high (e.g., 44% in 2009, a ‘wet’ year). Treatment-level differences in Reco can be partly attributed to the effects of antecedent SoilT and vegetation greenness on the apparent temperature sensitivity of Reco and to the effects of antecedent and current SWC and vegetation activity (greenness modulated by VPD) on Reco base rates. Thus, this study indicates that the incorporation of both antecedent environmental conditions and aboveground vegetation activity are critical to predicting Reco at multiple timescales (subdaily to annual) and under a future climate of elevated CO2 and warming.

Abstract 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 CO 2 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 5cm depth ( T s5 ) had the greatest effect on daily variation in ecosystem CO 2 exchange all year around, whereas photosynthetic photon flux density (PPFD) had a high direct effect on daily variation in CO 2 flux during the growing season. The combined effects of temperature and light regimes on net ecosystem CO 2 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. CO 2 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 CO 2 uptake was observed under the conditions of daily mean T s5 of about 15°C and daily mean PPFD of about 50molm 612 day 611 . The results suggested that temperature is the most critical determinant of CO 2 exchange in this alpine meadow ecosystem and may play an important role in the ecosystem carbon budget under future global warming conditions.

Summary 1 The arctic environment is highly heterogeneous in terms of plant distribution and productivity. If we are to make regional scale predictions of carbon exchange it is necessary to find robust relationships that can simplify this variability. One such potential relationship is that of leaf area to photosynthetic CO 2 flux at the canopy scale. 2 In this paper we assess the effectiveness of canopy leaf area in explaining variation in gross primary productivity (GPP): (i) across different vegetation types; (ii) at various stages of leaf development; and (iii) under enhanced nutrient availability. To do this we measure net CO 2 flux light response curves with a 1×1m chamber, and calculate GPP at a photosynthetic photon flux density (PPFD) of 60008molm 612 s 611 . 3 At a subarctic site in Sweden, we report 10-fold variation in GPP among natural vegetation types with leaf area index (LAI) values of 0.05–2.31m 2 m 612 . At a site of similar latitude in Alaska we document substantially elevated rates of GPP in fertilized vegetation. 4 We can explain 80% of the observed variation in GPP in natural vegetation (including vegetation measured before deciduous leaf bud burst) by leaf area alone, when leaf area is predicted from measurements of normalized difference vegetation index (NDVI). 5 In fertilized vegetation the relative increase in leaf area between control and fertilized treatments exceeds the relative increase in GPP. This suggests that higher leaf area causes increased self-shading, or that lower leaf nitrogen per unit leaf area causes a reduction in the rate of photosynthesis. 6 The results of this study indicate that canopy leaf area is an excellent predictor of GPP in diverse low arctic tundra, across a wide range of plant functional types.

A mechanistic understanding of the carbon (C) cycle-climate change feedback is essential for projecting future states of climate and ecosystems. Here we report a novel field mechanism and evidence supporting the hypothesis that nocturnal warming in a temperate steppe ecosystem in northern China can result in a minor C sink instead of a C source as models have predicted. Nocturnal warming increased leaf respiration of two dominant grass species by 36.3%, enhanced consumption of carbohydrates in the leaves (72.2% and 60.5% for sugar and starch, respectively), and consequently stimulated plant photosynthesis by 19.8% in the subsequent days. Our experimental findings confirm previous observations of nocturnal warming stimulating plant photosynthesis through increased draw-down of leaf carbohydrates at night. The enhancement of plant photosynthesis overcompensated the increased C loss via plant respiration under nocturnal warming and shifted the steppe ecosystem from a minor C source (1.87 g C·m6305·yr6301) to a C sink (21.72 g C·m6305·yr6301) across the three growing seasons from 2006 to 2008. Given greater increases in daily minimum than maximum temperature in many regions, plant photosynthetic overcompensation may partially serve as a negative feedback mechanism for terrestrial biosphere to climate warming.

The CO 2 flux was measured by the eddy covariance method on a temperate Leymus chinensis steppe over a period of 1702months spanning two consecutive growing seasons. The amount of precipitation was nearly normal, but it was low in the early and high in the late growing period in 2006. In the 2007 growing season, the amount of precipitation was about 45% less than the multi-year average and more evenly distributed. Comparisons were made between a moderately grazed site and a 28-year-old fenced site. The maximum instantaneous CO 2 release and uptake rates were 0.12 (May) and 61020.1102mg CO 2 02m 612 02s 611 (July) at the fenced site, and 0.11 and 61020.1602mg CO 2 02m 612 02s 611 (both in July) at the grazed site. In both growing seasons, the grazed site always had a higher daily uptake rate or lower release rate than the fenced site. The grazed site was a CO 2 sink during the growing season of 2007 and a CO 2 source in the growing season of 2006, whereas the fenced site was a CO 2 source in both seasons. Lower precipitation decreased CO 2 loss during the growing season more in the grazed site than in the fenced site, mainly because of depression of total ecosystem respiration ( R e ) in the former and stimulation in the latter. During the dormant season (from October to April), the fenced and grazed sites released 60.0 and 32.402g of C per m 2 , respectively. Path analysis showed that temperature had the greatest effect on daily variation of ecosystem CO 2 exchange during the growing seasons at the two study sites. The results suggest that decrease of precipitation and/or increase of temperature will likely promote C loss from L. chinensis steppes, whether fenced or grazed, and that a grazed site is more sensitive.

基于多变量统计方法同时研究自然系统内多个因子之间的相互关系,是阐释复杂的自然系统的一个重要手段.相比传统的多变量统计法,结构方程模型基于研究者的先验知识预先设定系统内因子间的依赖关系,不仅能够判别各因子之间的关系强度(路径系数),还能对整体模型进行拟合和判断,从而能更全面地了解自然系统.由于结构方程模型只在近年才被应用到生态学的数据分析中,因此该文试图对其作一简略介绍,包括结构方程模型的定义和变量类型,结合事例研究展现结构方程模型分析的一般步骤、在生态学中的应用以及相关软件的介绍等.望能为相关研究人员提供直观的认识,加强结构方程模型在生态学数据分析中的应用.

基于多变量统计方法同时研究自然系统内多个因子之间的相互关系,是阐释复杂的自然系统的一个重要手段.相比传统的多变量统计法,结构方程模型基于研究者的先验知识预先设定系统内因子间的依赖关系,不仅能够判别各因子之间的关系强度(路径系数),还能对整体模型进行拟合和判断,从而能更全面地了解自然系统.由于结构方程模型只在近年才被应用到生态学的数据分析中,因此该文试图对其作一简略介绍,包括结构方程模型的定义和变量类型,结合事例研究展现结构方程模型分析的一般步骤、在生态学中的应用以及相关软件的介绍等.望能为相关研究人员提供直观的认识,加强结构方程模型在生态学数据分析中的应用.

Pasture and afforestation are land-use types of major importance in the tropics, yet, most flux tower studies have been conducted in mature tropical forests. As deforestation in the tropics is expected to continue, it is critical to improve our understanding of alternative land-use types, and the impact of interactions between land use and climate on ecosystem carbon dynamics. Thus, we measured net ecosystem CO 2 fluxes of a pasture and an adjacent tropical afforestation (native tree species plantation) in Sardinilla, Panama from 2007 to 2009. The objectives of our paired site study were: (1) to assess seasonal and inter-annual variations in net ecosystem CO 2 exchange (NEE) of pasture and afforestation, (2) to identify the environmental controls of net ecosystem CO 2 fluxes, and (3) to constrain eddy covariance derived total ecosystem respiration (TER) with chamber-based soil respiration ( R Soil ) measurements. We observed distinct seasonal variations in NEE that were more pronounced in the pasture compared to the afforestation, reflecting changes in plant and microbial activities. The land conversion from pasture to afforestation increased the potential for carbon uptake by trees vs. grasses throughout most of the year. R Soil contributed about 50% to TER, with only small differences between ecosystems or seasons. Radiation and soil moisture were the main environmental controls of CO 2 fluxes while temperature had no effect on NEE. The pasture ecosystem was more strongly affected by soil water limitations during the dry season, probably due to the shallower root system of grasses compared to trees. Thus, it seems likely that predicted increases in precipitation variability will impact seasonal variations of CO 2 fluxes in Central Panama, in particular of pasture ecosystems.

Integrated values of GPP, R eco , and net ecosystem exchange (NEE) were 867, 735, and 6113202g02C02m 612 , respectively, for the 2000–2001 season, and 729, 758, and 2902g02C02m 612 for the 2001–2002 season. Thus, the grassland was a moderate carbon sink during the first season and a weak carbon source during the second season. In contrast to a well-accepted view that annual production of grass is linearly correlated to precipitation, the large difference in GPP between the two seasons were not caused by the annual precipitation. Instead, a shorter growing season, due to late start of the rainy season, was mainly responsible for the lower GPP in the second season. Furthermore, relatively higher R eco during the non-growing season occurred after a late spring rain. Thus, for this Mediterranean grassland, the timing of rain events had more impact than the total amount of precipitation on ecosystem GPP and NEE. This is because its growing season is in the cool and wet season when carbon uptake and respiration are usually limited by low temperature and sometimes frost, not by soil moisture.

Abstract Soil respiration is recognized to be influenced by temperature, moisture, and ecosystem production. However, little is known about how plant community structure regulates responses of soil respiration to climate change. Here, we used a 13-year field warming experiment to explore the mechanisms underlying plant community regulation on feedbacks of soil respiration to climate change in a tallgrass prairie in Oklahoma, USA. Infrared heaters were used to elevate temperature about 202°C since November 1999. Annual clipping was used to mimic hay harvest. Our results showed that experimental warming significantly increased soil respiration approximately from 10% in the first 702years (2000–2006) to 30% in the next 602years (2007–2012). The two-stage warming stimulation of soil respiration was closely related to warming-induced increases in ecosystem production over the years. Moreover, we found that across the 1302years, warming-induced increases in soil respiration were positively affected by the proportion of aboveground net primary production (ANPP) contributed by C3 forbs. Functional composition of the plant community regulated warming-induced increases in soil respiration through the quantity and quality of organic matter inputs to soil and the amount of photosynthetic carbon (C) allocated belowground. Clipping, the interaction of clipping with warming, and warming-induced changes in soil temperature and moisture all had little effect on soil respiration over the years (all P > 0.05). Our results suggest that climate warming may drive an increase in soil respiration through altering composition of plant communities in grassland ecosystems.

Measurements of net ecosystem carbon dioxide (CO 2) exchange (NEE) were made, using eddy covariance, to investigate the biophysical regulation of a temperate desert steppe characterized drought in Inner Mongolia, China during 2008. The half-hourly maximum and minimum NEE were 613.07 and 0.85 μmol CO 2 m 612 s 611 (negative values denoting net carbon uptake). The maximum daily NEE was 616.0 g CO 2 m 612 day 611. On an annual basis, integrated NEE was 617.2 g C m 612 y 611, indicating a weak carbon sink. The light response curves of NEE showed a rather low apparent quantum yield ( α) and saturation value of NEE (NEE sat). Moreover, α and NEE sat varied with canopy development, soil water content (SWC), air temperature ( T a), and vapor pressure deficit (VPD). Piecewise regression results suggested that the optimal SWC, T a, and VPD for half-hourly daytime NEE were 12.6%, 24.3 °C, and 1.7 kPa, respectively. The apparent temperature sensitivity of ecosystem respiration was 1.6 for the entire growing season, and it was significantly controlled by soil moisture. During the growing season, leaf area index explained about 26% of the variation in daily NEE. Overall, NEE was strongly suppressed by water stress and this was the dominant biophysical regulator in the desert steppe.

Understanding the dynamics and underlying mechanism of carbon exchange between terrestrial ecosystems and the atmosphere is one of the key issues in global change research. In this study, we quantified the carbon fluxes in different terrestrial ecosystems in China, and analyzed their spatial variation and environmental drivers based on the long-term observation data of ChinaFLUX sites and the published data from other flux sites in China. The results indicate that gross ecosystem productivity (GEP), ecosystem respiration (ER), and net ecosystem productivity (NEP) of terrestrial ecosystems in China showed a significantly latitudinal pattern, declining linearly with the increase of latitude. However, GEP, ER, and NEP did not present a clear longitudinal pattern. The carbon sink functional areas of terrestrial ecosystems in China were mainly located in the subtropical and temperate forests, coastal wetlands in eastern China, the temperate meadow steppe in the northeast China, and the alpine meadow in eastern edge of Qinghai-Tibetan Plateau. The forest ecosystems had stronger carbon sink than grassland ecosystems. The spatial patterns of GEP and ER in China were mainly determined by mean annual precipitation (MAP) and mean annual temperature (MAT), whereas the spatial variation in NEP was largely explained by MAT. The combined effects of MAT and MAP explained 79%, 62%, and 66% of the spatial variations in GEP, ER, and NEP, respectively. The GEP, ER, and NEP in different ecosystems in China exhibited ositive coupling correlation in their spatial patterns. Both ER and NEP were significantly correlated with GEP, with 68% of the per-unit GEP contributed to ER and 29% to NEP. MAT and MAP affected the spatial patterns of ER and NEP mainly by their direct effects on the spatial pattern of GEP.

Over the last two and half decades, strong evidence showed that the terrestrial ecosystems are acting as a net sink for atmospheric carbon. However the spatial and temporal patterns of variation in the sink are not well known. In this study, we examined latitudinal patterns of interannual variability (IAV) in net ecosystem exchange (NEE) of CO2 based on 163 site-years of eddy covariance data, from 39 northern-hemisphere research sites located at latitudes ranging from 29 N to 64 N. We computed the standard deviation of annual NEE integrals at individual sites to represent absolute interannual variability (AIAV), and the corresponding coefficient of variation as a measure of relative interannual variability (RIAV). Our results showed decreased trends of annual NEE with increasing latitude for both deciduous broadleaf forests and evergreen needleleaf forests. Gross primary production (GPP) explained a significant proportion of the spatial variation of NEE across evergreen needleleaf forests, whereas, across deciduous broadleaf forests, it is ecosystem respiration (Re). In addition, AIAV in GPP and Re increased significantly with latitude in deciduous broadleaf forests, but AIAV in GPP decreased significantly with latitude in evergreen needleleaf forests. Furthermore, RIAV in NEE, GPP, and Re appeared to increase significantly with latitude in deciduous broadleaf forests, but not in evergreen needleleaf forests. Correlation analyses showed air temperature was the primary environmental factor that determined RIAV of NEE in deciduous broadleaf forest across the North American sites, and none of the chosen climatic factors could explain RIAV of NEE in evergreen needleleaf forests. Mean annual NEE significantly increased with latitude in grasslands. Precipitation was dominant environmental factor for the spatial variation of magnitude and IAV in GPP and Re in grasslands.

Vegetation phenology is a sensitive indicator of global warming,especially on the Tibetan Plateau.However,whether climate warming has enhanced the advance of grassland phenology since 2000 remains debated and little is known about the warming effect on semiarid grassland phenology and interactions with early growing season precipitation.In this study,we extracted phenological changes from average NDVI in the growing season(GNDVI) to analyze the relationship between changes in NDVI,phenology and climate in the Northern Tibetan Damxung grassland from 2000 to 2014.The GNDVI of the grassland declined.Interannual variation of GNDVI was mainly affected by mean temperature from late May to July and precipitation from April to August.The length of the growing season was significantly shortened due to a delay in the beginning of the growing season and no advancement of the end of the growing season,largely caused by climate warming and enhanced by decreasing precipitation in spring.Water availability was the major determinant of grass growth in the study area.Warming increased demand for water when the growth limitation of temperature to grass was exceeded in the growing season.Decreased precipitation likely further exacerbated the effect of warming on vegetation phenology in recent decades due to increasing evapotranspiration and water limitations.The comprehensive effects of global warming and decreasing precipitation may delay the phenological responses of semiarid alpine grasslands.

利用涡度相关技术观测了青藏高原两个典型的生态系统即矮嵩草(K obresia hum ilis)草甸和金露梅(P oten-tilla f ruticosa)灌丛草甸的CO2通量,并就2003年8月份的数据,分析了生态系统通量变化与环境因子的关系.8月份是这两个生态系统的叶面积指数达到最高也是相对稳定的时期,在此期间矮嵩草草甸和金露梅灌丛草甸净碳吸收量分别达56.2和32.6 g C.m-2,日CO2吸收量最大值分别为12.7μm o l.m-2.-s 1和9.3μm o l.m-2.-s 1,排放量最大值分别为5.1μm o l.m-2.-s 1和5.7μm o l.m-2.-s 1.在相同光合有效光量子通量密度(PPFD)条件下,矮嵩草草甸CO2吸收速度大于金露梅灌丛草甸;在PPFD高于1 200μm o l.m-2.s-1的条件下,随气温增加,两生态系统的CO2吸收速度都下降,但矮嵩草草甸的下降速度(-0.086)比金露梅灌丛草甸(-0.016)快.土壤水分影响土壤呼吸,并且影响差异因植被类型不同而不同.生态系统日CO2吸收量随昼夜温差增加而增大;较大的昼夜温差导致较高的净CO2交换量;植物反射率与CO2通量之间存在负相关关系.

利用涡度相关技术观测了青藏高原两个典型的生态系统即矮嵩草(K obresia hum ilis)草甸和金露梅(P oten-tilla f ruticosa)灌丛草甸的CO2通量,并就2003年8月份的数据,分析了生态系统通量变化与环境因子的关系.8月份是这两个生态系统的叶面积指数达到最高也是相对稳定的时期,在此期间矮嵩草草甸和金露梅灌丛草甸净碳吸收量分别达56.2和32.6 g C.m-2,日CO2吸收量最大值分别为12.7μm o l.m-2.-s 1和9.3μm o l.m-2.-s 1,排放量最大值分别为5.1μm o l.m-2.-s 1和5.7μm o l.m-2.-s 1.在相同光合有效光量子通量密度(PPFD)条件下,矮嵩草草甸CO2吸收速度大于金露梅灌丛草甸;在PPFD高于1 200μm o l.m-2.s-1的条件下,随气温增加,两生态系统的CO2吸收速度都下降,但矮嵩草草甸的下降速度(-0.086)比金露梅灌丛草甸(-0.016)快.土壤水分影响土壤呼吸,并且影响差异因植被类型不同而不同.生态系统日CO2吸收量随昼夜温差增加而增大;较大的昼夜温差导致较高的净CO2交换量;植物反射率与CO2通量之间存在负相关关系.

Abstract Abstract: In the present study, we used the eddy covariance method to measure CO 2 exchange between the atmosphere and an alpine shrubland meadow ecosystem (37°36’N, 101°18’E; 3 250 m a.s.l.) on the Qinghai-Tibetan Plateau, China, during the growing season in 2003, from 20 April to 30 September. This meadow is dominated by formations of Potentilla fruticosa L. The soil is Mol-Cryic Cambisols. During the study period, the meadow was not grazed. The maximum rates of CO 2 uptake and release derived from the diurnal course of CO 2 flux were -9.38 and 5.02 μmol·m -2 ·s -1 , respectively. The largest daily CO 2 uptake was 1.7 g C·m -2 ·d -1 on 14 July, which is less than half that of an alpine Kobresia meadow ecosystem at similar latitudes. Daily CO 2 uptake during the measurement period indicated that the alpine shrubland meadow ecosystem may behave as a sink of atmospheric CO 2 during the growing season. The daytime CO 2 uptake was correlated exponentially or linearly with the daily photo synthetic photon flux density each month. The daytime average water use efficiency of the ecosystem was 6.47 mg CO 2 /g H 2 O. The efficiency of the ecosystem increased with a decrease in vapor pressure deficit. (Managing editor: Ya-Qin HAN)

Partitioning soil CO 2 efflux into autotrophic ( R A ) and heterotrophic ( R H ) components is crucial for understanding their differential responses to climate change. We conducted a long-term experiment (2000–2005) to investigate effects of warming 2°C and yearly clipping on soil CO 2 efflux and its components (i.e. R A and R H ) in a tallgrass prairie ecosystem. Interannual variability of these fluxes was also examined. Deep collars (70 cm) were inserted into soil to measure R H . R A was quantified as the difference between soil CO 2 efflux and R H . Warming treatment significantly stimulated soil CO 2 efflux and its components (i.e. R A and R H ) in most years. In contrast, yearly clipping significantly reduced soil CO 2 efflux only in the last 2 years, although it decreased R H in every year of the study. Temperature sensitivity (i.e. apparent Q 10 values) of soil CO 2 efflux was slightly lower under warming ( P >0.05) and reduced considerably by clipping ( P <0.05) compared with that in the control. On average over the 4 years, R H accounted for approximately 65% of soil CO 2 efflux with a range from 58% to 73% in the four treatments. Over seasons, the contribution of R H to soil CO 2 efflux reached a maximum in winter (6590%) and a minimum in summer (6535%). Annual soil CO 2 efflux did not vary substantially among years as precipitation did. The interannual variability of soil CO 2 efflux may be mainly caused by precipitation distribution and summer severe drought. Our results suggest that the effects of warming and yearly clipping on soil CO 2 efflux and its components did not result in significant changes in R H or R A contribution, and rainfall timing may be more important in determining interannual variability of soil CO 2 efflux than the amount of annual precipitation.