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

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

On linking interannual tree ring variability with observations of whole-forest CO2 flux

We used a 10‐year record of the CO₂ flux by an old growth boreal forest in central Manitoba (the Northern Old Black Spruce Site (NOBS)), a ～150‐year‐old Picea mariana [Mill.] stand) to determine whether and how whole‐forest CO₂ flux is related to tree ring width. We compared a 37‐year ring width chronology collected at NOBS to a second chronology that was collected at a nearby Black Spruce stand with a different disturbance history, and also to three measures of annual whole‐forest photosynthesis [gross ecosystem production (GEP)], two measures of annual respiration (R), and one measure of annual carbon balance [net ecosystem production (NEP)]. The year‐to‐year ring width fluctuations were well correlated between the two sites; increasing our confidence in the NOBS chronology and implying that ring width variation is driven and synchronized by the physical environment. Both chronologies exhibited serial correlation, with a fluctuation in ring width that had an apparent periodicity of ～7 years. Neither chronology was correlated with variation in annual precipitation or temperature. Ring width and NEP increased, while R decreased from 1995 to 2004. GEP either remained constant or decreased from 1995 to 2004, depending on which measure was considered. The lack of relationship between ring width and GEP may indicate that ring growth is controlled almost entirely by something other than carbon uptake. Alternative explanations for the ring width chronologies include the possibility that wood production varies as a result of shifts in respiration, or that an unidentified aspect of the environment, rather than the balance between GEP and respiration, controls wood production. The serial correlation in ring width may be related to increases and decreases in carbohydrate pools, or to gradual changes in nutrient availability, pathogens, herbivores, soil frost or soil water table. The cause or causes of serial correlation, and the controls on the allocation of photosynthate to wood production, emerge as critical uncertainties for efforts in predicting the carbon balance of boreal ecosystems and inferring past climate from tree rings.     

青藏高原高寒湿地生态系统CO2通量

依据涡度相关系统连续观测的2005年CO2通量数据,对青藏高原东北隅的高寒湿地生态系统源/汇功能及其部分环境影响因素进行了分析.结果表明,高寒湿地生态系统为明显的碳源,在植物生长季(5～9月份)吸收230.16 gCO2·m-2,非生长季(1～4月份及10～12月份)释放546.18 gCO2·m-2,其中净排放最高在5月份,为181.49 gCO2·m-2,净吸收最高在8月份,为189.69 gCO2·m-2,年释放量为316.02 gCO2·m-2.在平均日变化中,最大吸收值出现在7月份12:00,为(0.45±0.0012) mgCO2·m-2·s-1,最大排放速率出现在8月份0:00,为(0.22±0.0090) mgCO2·m-2·s-1.生长季中6～9月份表现为明显的单峰型日变化,非生长季的变化幅度较小.净生态系统交换量(NEE)和生态系统总初级生产力(GPP)与气温、空气水气饱和亏和地表反射率等环境因素呈现相似的相关性,与地上生物量和群落叶面积指数则为线性负相关,生态系统呼吸(Res)则与上述因子的相关性呈现相反的趋势.     

Diurnal, seasonal and annual variation in net ecosystem CO2 exchange of an alpine shrubland on Qinghai-Tibetan plateau

Thus far, grassland ecosystem research has mainly been focused on low‐lying grassland areas, whereas research on high‐altitude grassland areas, especially on the carbon budget of remote areas like the Qinghai‐Tibetan plateau is insufficient. To address this issue, flux of CO₂ were measured over an alpine shrubland ecosystem (37°36′N, 101°18′E; 325 above sea level [a. s. l.]) on the Qinghai‐Tibetan Plateau, China, for 2 years (2003 and 2004) with the eddy covariance method. The vegetation is dominated by formation Potentilla fruticosa L. The soil is Mol–Cryic Cambisols. To interpret the biotic and abiotic factors that modulate CO₂ flux over the course of a year we decomposed net ecosystem CO₂ exchange (NEE) into its constituent components, and ecosystem respiration (R_eco). Results showed that seasonal trends of annual total biomass and NEE followed closely the change in leaf area index. Integrated NEE were ?58.5 and ?75.5 g C m^?2, respectively, for the 2003 and 2004 years. Carbon uptake was mainly attributed from June, July, August, and September of the growing season. In July, NEE reached seasonal peaks of similar magnitude (4–5 g C m^?2 day^?1) each of the 2 years. Also, the integrated night‐time NEE reached comparable peak values (1.5–2 g C m^?2 day^?1) in the 2 years of study. Despite the large difference in time between carbon uptake and release (carbon uptake time < release time), the alpine shrubland was carbon sink. This is probably because the ecosystem respiration at our site was confined significantly by low temperature and small biomass and large day/night temperature difference and usually soil moisture was not limiting factor for carbon uptake. In general, R_eco was an exponential function of soil temperature, but with season‐dependent values of Q₁₀. The temperature‐dependent respiration model failed immediately after rain events, when large pulses of R_eco were observed. Thus, for this alpine shrubland in Qinghai‐Tibetan plateau, the timing of rain events had more impact than the total amount of precipitation on ecosystem R_eco and NEE.     

Spatial and temporal variability in forest–atmosphere CO2 exchange

Seven years of carbon dioxide flux measurements indicate that a ～90‐year‐old spruce dominated forest in Maine, USA, has been sequestering 174±46 g C m^?2 yr^?1 (mean±1 standard deviation, nocturnal friction velocity (u_*) threshold >0.25 m s^?1). An analysis of monthly flux anomalies showed that above‐average spring and fall temperatures were significantly correlated with greater monthly C uptake while above‐average summer temperatures were correlated with decreased net C uptake. Summer months with significantly drier or wetter soils than normal were also characterized by lower rates of C uptake. Years with above‐average C storage were thus typically characterized by warmer than average spring and fall temperatures and adequate summer soil moisture. Environmental and forest–atmosphere flux data recorded from a second tower surrounded by similar forest, but sufficiently distant that flux source regions (‘footprints’), did not overlap significantly showed almost identical temperature and solar radiation conditions, but some differences in energy partitioning could be seen. Half‐hourly as well as integrated (annual) C exchange values recorded at the separate towers were very similar, with average annual net C uptake differing between the two towers by <6%. Interannual variability in net C exchange was found to be much greater than between tower variability. Simultaneous measurements from two towers were used to estimate flux data uncertainty from a single tower. Carbon‐flux model parameters derived independently from each flux tower data set were not significantly different, demonstrating that flux towers can provide a robust method for establishing C exchange model parameters.     

CO2 flux in alpine wetland ecosystem on the Qinghai-Tibetan Plateau, China

Using the CO₂ flux data measured by the eddy covariance method in the northeast of Qinghai-Tibetan Plateau in 2005, we analyzed the carbon flux dynamics in relation to meteorological and biotic factors. The results showed that the alpine wetland ecosystem was the carbon source, and it emitted 316.02 gCO₂ · m⁻² to atmosphere in 2005 with 230.16 gCO₂ · m⁻² absorbed in the growing season from May to September and 546.18 gCO₂ · m⁻² released in the non-growing season from January to April and from October to December. The maximum of the averaged daily CO₂ uptake rates and release rates was (0.45 ± 0.0012) mgCO₂ · m⁻² · s⁻¹ (Mean ± SE) in July and (0.22 ± 0.0090) mgCO₂ · m⁻² · s⁻¹ in August, respectively. The averaged diurnal variation showed a single-peaked pattern in the growing season, but exhibited very small fluctuation in the non-growing season. Net ecosystem exchange (NEE) and gross primary production (GPP) were all correlated with some meteorological factors, and they showed a negatively linear correlation with aboveground biomass, while a positive correlation existed between the ecosystem respiration (R_es) and those factors.     

Partitioning net ecosystem carbon exchange with isotopic fluxes of CO2

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

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.     

三江源高寒草甸冻融循环期CO2通量变化特征

青藏高原是我国最典型的季节性冻土分布区,近年的气候变化对该区域的土壤冻融及其生态系统碳排放产生了深刻影响。为揭示土壤冻融变化对高寒生态系统呼吸(R_e)的影响,于2016和2017年利用涡度相关和微气象系统对三江源高寒草甸生态系统的碳通量和环境要素进行了观测,重点探讨了季节性冻融循环(Seasonal Freeze-Thaw Cycles, SFTC)对R_e的影响,并分析了昼夜冻融循环(Diurnal Freeze-Thaw Cycles, DFTC)诱导R_e的绝对和相对增量(ΔR₍FTC (p)),R₍FTC (p))/R_min)及对R_e的标准化效应值(lnRR_p)。结果表明,在土壤冻结期R_e维持在低水平,而在春季冻融循环期和融化期冻土逐渐融化,T_s5和SWC的上升促进了冻土有机碳转化为CO₂,使R_e升高。春季冻融循环期的R<...     

Effects of experimental warming on net ecosystem CO2 exchange in Northern Xizang alpine meadow

AimsGlobal warming could have profound effects on ecosystem carbon (C) fluxes in alpine ecosystems. The aim of our study is to examine the effects of gradient warming on net ecosystem carbon exchange (NEE).MethodsIn the Northern Tibetan Grassland Ecosystem Research Station (Nagqu station), Institute of Geographic Sciences and Natural Resources Research, Chinese Academy of Sciences, we conducted various levels of temperature increasing experiments (i.e., 2 °C and 4 °C increments). The warming was achieved using open-top chambers (OTCs). In total, there were three levels of temperature treatments (control, 2 °C and 4 °C increment), and four replicates for each treatment. The ecosystem NEE was monitored every five days during the growing season in 2015.Important findings Our findings highlight the importance of soil moisture in mediating the responses of NEE to climatic warming in alpine meadow ecosystem. The 4 °C warming significantly stimulated NEE,except for July measurements. The 2 °C warming had no effects on NEE during the growing season. Compared to the 2 °C warming, the 4 °C warming significantly stimulated NEE. The results showed that our targeted ecosystem acts as a carbon sink under 2 °C warming, whereas will act as a net carbon source under 4 °C warming in the future. This study provides basic data and theoretical basis for evaluating the alpine ecosystem’s responses to climate change.     

模拟增温对青藏高原高寒草甸根系生物量的影响

在青藏高原高寒草甸布设模拟增温实验样地,采用土钻法于2012—2013年植被生长季获取5个土层的根系生物量,探讨增温处理下根系生物量在生长季不同月份、不同土壤深度的变化趋势及其与相应土层土壤水分、温度的关系。结果表明:(1)根系生物量在2012年随月份呈增加趋势,其中7—9月较大,其平均值在对照、增温处理下分别为3810.88 g/m~2和4468.08 g/m~2;在2013年随月份呈减小趋势,其中5—6月较大,其平均值在对照、增温处理下分别为4175.39 g/m~2和4141.6 g/m~2。增温处理下的总根系生物量高出对照处理293.97 g/m~2,而各月份总根系生物量在处理间的差值均未达到显著水平。表明在增温处理下根系生物量略有增加,但在生长季不同月份其增加的程度不同,致使年际间的增幅出现差异。(2)根系生物量主要分布在0—10 cm深度,所占百分比为50.61%。在增温处理下,0—10 cm深度的根系生物量减少,减幅为8.38%;10—50 cm深度的根系生物量增加,增幅为2.1%。相对于对照处理,增温处理下0—30 cm深度的根系生物量向深层增加,30—50 cm深度的根系生物量增加趋势略有减缓。可见,在增温处理下根系生物量的增幅趋向于土壤深层。(3)根系生物量与土壤水分呈极显著的递减关系,在增温处理下线性关系减弱;与土壤温度呈极显著的递增关系,在增温处理下线性关系增强。表明土壤水分、温度都可极显著影响根系生物量,但在增温处理下土壤温度对根系生物量的影响较土壤水分更为敏感而迅速。     

不同幅度的实验增温对藏北高寒草甸净生态系统碳交换的影响

全球气候变暖将对陆地生态系统(尤其是高寒草甸生态系统)碳循环产生深远影响。该研究依托中国科学院地理科学与资源研究所藏北高原草地生态系统研究站(那曲站), 设置不同增温幅度实验, 模拟未来2 ℃增温和4 ℃增温的情景, 探究不同增温幅度对青藏高原高寒草甸净生态系统碳交换(NEE)的影响。研究结果显示: 1)在2015年生长季(6-9月), 不增温和2 ℃增温处理下NEE小于0, 总体表现为碳汇, 而4 ℃增温处理下NEE大于0, 总体表现为碳源; 2)在生长季的6月、8月及整个生长季, 与不增温相比, 4 ℃增温处理显著提高了NEE, 而2 ℃增温处理没有显著改变NEE; 7月, 2 ℃和4 ℃增温处理均显著提高了NEE; 3)在半干旱的高寒草甸生态系统, 土壤水分是决定NEE的关键因素, 增温通过降低土壤水分而导致高寒草甸生态系统碳汇能力下降。该研究可为青藏高原高寒草甸生态系统应对未来气候变化提供基础数据和理论依据。     

Source components and interannual variability of soil CO2 efflux under experimental warming and clipping in a grassland ecosystem

Partitioning soil CO₂ 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 CO₂ 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 CO₂ efflux and R_H. Warming treatment significantly stimulated soil CO₂ efflux and its components (i.e. R_A and R_H) in most years. In contrast, yearly clipping significantly reduced soil CO₂ efflux only in the last 2 years, although it decreased R_H in every year of the study. Temperature sensitivity (i.e. apparent Q₁₀ values) of soil CO₂ 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 CO₂ efflux with a range from 58% to 73% in the four treatments. Over seasons, the contribution of R_H to soil CO₂ efflux reached a maximum in winter (∼90%) and a minimum in summer (∼35%). Annual soil CO₂ efflux did not vary substantially among years as precipitation did. The interannual variability of soil CO₂ 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 CO₂ 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 CO₂ efflux than the amount of annual precipitation.     

青藏高原高寒湿地生态系统CO2通量

依据涡度相关系统连续观测的2005年CO₂通量数据,对青藏高原东北隅的高寒湿地生态系统源/汇功能及其部分环境影响因素进行了分析。结果表明,高寒湿地生态系统为明显的碳源,在植物生长季（5~9月份）吸收230.16 gCO₂•m^-2,非生长季（1~4月份及10~12月份）释放546.18 gCO₂•m^-2,其中净排放最高在5月份,为181.49 gCO₂•m^-2,净吸收最高在8月份,为189.69 gCO₂•m^-2,年释放量为316.02 gCO₂•m^-2。在平均日变化中,最大吸收值出现在7月份12:00,为（0.45±0.0012） mgCO2•m^-2•s^-1,最大排放速率出现在8月份0:00,为（0.22±0.0090） mgCO₂•m^-2•s^-1。生长季中6~9月份表现为明显的单峰型日变化,非生长季的变化幅度较小。净生态系统交换量（NEE）和生态系统总初级生产力（GPP）与气温、空气水气饱和亏和地表反射率等环境因素呈现相似的相关性,与地上生物量和群落叶面积指数则为线性负相关,生态系统呼吸（Res）则与上述因子的相关性呈现相反的趋势。     

Estimates of CO2 uptake and release among European forests based on eddy covariance data

The net ecosystem exchange (NEE) of forests represents the balance of gross primary productivity (GPP) and respiration (R). Methods to estimate these two components from eddy covariance flux measurements are usually based on a functional relationship between respiration and temperature that is calibrated for night‐time (respiration) fluxes and subsequently extrapolated using daytime temperature measurements. However, respiration fluxes originate from different parts of the ecosystem, each of which experiences its own course of temperature. Moreover, if the temperature–respiration function is fitted to combined data from different stages of biological development or seasons, a spurious temperature effect may be included that will lead to overestimation of the direct effect of temperature and therefore to overestimates of daytime respiration. We used the EUROFLUX eddy covariance data set for 15 European forests and pooled data per site, month and for conditions of low and sufficient soil moisture, respectively. We found that using air temperature (measured above the canopy) rather than soil temperature (measured 5 cm below the surface) yielded the most reliable and consistent exponential (Q₁₀) temperature–respiration relationship. A fundamental difference in air temperature‐based Q₁₀ values for different sites, times of year or soil moisture conditions could not be established; all were in the range 1.6–2.5. However, base respiration (R₀, i.e. respiration rate scaled to 0°C) did vary significantly among sites and over the course of the year, with increased base respiration rates during the growing season. We used the overall mean Q₁₀ of 2.0 to estimate annual GPP and R. Testing suggested that the uncertainty in total GPP and R associated with the method of separation was generally well within 15%. For the sites investigated, we found a positive relationship between GPP and R, indicating that there is a latitudinal trend in NEE because the absolute decrease in GPP towards the pole is greater than in R.     

Cross validation of open-top chamber and eddy covariance measurements of ecosystem CO2 exchange in a Florida scrub-oak ecosystem

Simultaneous measurements of net ecosystem CO₂ exchange (NEE) were made in a Florida scrub‐oak ecosystem in August 1997 and then every month between April 2000 to July 2001, using open top chambers (NEE_O) and eddy covariance (NEE_E). This study provided a cross validation of these two different techniques for measuring NEE. Unique characteristics of the comparison were that the measurements were made simultaneously, in the same stand, with large replicated chambers enclosing a representative portion of the ecosystem (75 m², compared to approximately 1–2 ha measured by the eddy covariance system). The value of the comparison was greatest at night, when the microclimate was minimally affected by the chambers. For six of the 12 measurement periods, night NEE_O was not significantly different to night NEE_E, and for the other periods the maximum difference was 1.1 µ mol m ^{? 2}s ^{? 1}, with an average of 0.72 ± 0.09 µ mol m ^{? 2}s ^{? 1}. The comparison was more difficult during the photoperiod, because of differences between the microclimate inside and outside the chambers. During the photoperiod, air temperature (T_air) and air vapour pressure deficits (VPD) became progressively higher inside the chambers until mid‐afternoon. In the morning NEE_O was higher than NEE_E by about 26%, consistent with increased temperature inside the chambers. Over the mid‐day period and the afternoon, NEE_O was 8% higher that NEE_E, regardless of the large differences in microclimate. This study demonstrates both the uses and difficulties associated with attempting to cross validate NEE measurements made in chambers and using eddy covariance. The exercise was most useful at night when the chamber had a minimal effect on microclimate, and when the measurement of NEE is most difficult.     

Forest and agricultural land-use-dependent CO2 exchange in Thuringia, Germany

Eddy covariance was used to measure the net CO₂ exchange (NEE) over ecosystems differing in land use (forest and agriculture) in Thuringia, Germany. Measurements were carried out at a managed, even‐aged European beech stand (Fagus sylvatica, 70–150 years old), an unmanaged, uneven‐aged mixed beech stand in a late stage of development (F. sylvatica, Fraxinus excelsior, Acer pseudoplantanus, and other hardwood trees, 0–250 years old), a managed young Norway spruce stand (Picea abies, 50 years old), and an agricultural field growing winter wheat in 2001, and potato in 2002. Large contrasts were found in NEE rates between the land uses of the ecosystems. The managed and unmanaged beech sites had very similar net CO₂ uptake rates (～?480 to ?500 g C m^?2 yr^?1). Main differences in seasonal NEE patterns between the beech sites were because of a later leaf emergence and higher maximum leaf area index at the unmanaged beech site, probably as a result of the species mix at the site. In contrast, the spruce stand had a higher CO₂ uptake in spring but substantially lower net CO₂ uptake in summer than the beech stands. This resulted in a near neutral annual NEE (?4 g C m^?2 yr^?1), mainly attributable to an ecosystem respiration rate almost twice as high as that of the beech stands, despite slightly lower temperatures, because of the higher elevation. Crops in the agricultural field had high CO₂ uptake rates, but growing season length was short compared with the forest ecosystems. Therefore, the agricultural land had low‐to‐moderate annual net CO₂ uptake (?34 to ?193 g C m^?2), but with annual harvest taken into account it will be a source of CO₂ (+97 to +386 g C m^?2). The annually changing patchwork of crops will have strong consequences on the regions' seasonal and annual carbon exchange. Thus, not only land use, but also land‐use history and site‐specific management decisions affect the large‐scale carbon balance.     

青藏高原高寒草甸生物量动态变化及与环境因子的关系——基于模拟增温实验

以青藏高原高寒草甸为研究区,设置模拟增温实验样地,于2010年开始持续增温,2012和2013年调查植被地上-地下生物量,探讨气候变暖背景下高寒草甸生物量的动态变化及其与环境因子的关系。结果表明:(1)增温处理下地上-地下生物量与根冠比的中值和平均值大于对照,其中地下生物量(变异系数为0.30)的增加幅度大于地上生物量(变异系数为0.27),根冠比的变异系数(0.33)大于地上-地下生物量,这表明增温可导致高寒草甸植被生物量分配出现差异。(2)地上-地下生物量呈极显著的幂指数函数关系(R~2=0.147,P0.001),表现为异速生长,但在增温处理下异速生长出现减缓(R~2=0.102,P0.05)。(3)地上生物量受深层土壤水分和浅层土壤温度影响较大,地下生物量受深层土壤水分和深层土壤温度影响较大;土壤温度对地上-地下生物量的影响强于土壤水分,表现为20 cm深度土壤温度对地上生物量(R=0.582,P0.01)和根冠比(R=-0.238,P0.05)影响较大,60 cm深度土壤温度对地下生物量影响较大(R=0.388,P0.01),100 cm深度土壤水分对地上生物量(R=0.423,P0.01)和地下生物量(R=0.245,P0.05)影响较大,这说明增温导致浅层土壤温度对生物量分配产生影响,使生物量更多分配到地上部分,而冻土融化致使深层土壤水分对生物量产生影响。     

青藏高原高寒草甸生态系统净二氧化碳交换量特征

高寒草甸是青藏高原广泛分布的植被类型之一,面积约120万km2,地处青藏高原腹地的当雄草原站即位于该类植被的典型分布区。以2003年8~10月中旬在该站用涡度相关法连续观测的CO2通量数据资料为基础,分析了高寒草甸生态系统8~10月份净二氧化碳交换量(NEE)的日变化规律,及其与光合有效辐射、降水、温度等环境因子之间的关系。结果表明,8~10月份的日均NEE有明显的日变化,表现为单峰型,通常在地方时11:00~12:00左右达到碳吸收的最大值,平均为-0.2680mgCO2/(m2·s)(-6.0800μmolCO2/(m2·s))。白天的NEE与光合有效辐射之间符合很好的直角双曲线关系,表观量子产额平均为0.0203μmolCO2/μmolPAR,表观最大光合速率平均为9.7411μmolCO2/(m2·s)。夜晚的NEE与5cm地温有很好的指数函数关系。