中国亚热带-暖温带过渡区锐齿栎林净生态系统碳交换特征

在2017年1月1日-2017年12月31日期间,采用涡度相关法对位于亚热带-暖温带气候过渡区的河南宝天曼国家级自然保护区的65年生锐齿栎（Quercus aliena）天然次生林的碳通量进行了连续观测。结果表明：在观测期间,该森林生态系统在生长季5-10月份为碳汇,非生长季各月为碳源,净碳吸收量与释放量分别在7月和4月达到最大。净生态系统生产力为569.4 g C m^-2a^-1,生态系统呼吸为529.9 g C m^-2a^-1,总生态系统生产力为1099.3 g C m^-2a^-1。30min尺度上夜间净生态系统碳交换量与5cm深度土壤温度的关系可用指数方程表示（R²=0.21,P < 0.001）,其温度敏感性系数（Temperature sensitivity coefficient,Q₁₀）为2.2。如果排除夜间通量观测的误差,处在海拔较高地区的夜间低温和非生长季的低温抑制了生态系统呼吸排放,可能导致全年生态系统呼吸量较低。在生长季5-10月份,各月的白天净生态系统碳交换量对光合有效辐射的响应符合直角双曲线模型,初始光能利用效率、平均最大光合速率和白天平均生态系统呼吸强度呈明显的季节变化,范围分别是0.06-0.12 μmol CO₂ μmol^-1 photon、0.44-1.47 mg CO₂ m^-2s^-1和0.07-0.19 mg CO₂ m^-2s^-1。夏季7、8月份,较高的饱和水汽压差对白天锐齿栎林的碳吸收有明显的抑制作用;生长季末期9月份较高的土壤含水量对白天锐齿栎林的碳吸收也产生了抑制作用,表明生长末期降水过多影响森林的碳吸收。     

基于NPP分配的生产和生态功能协同提升模式——以西藏拉萨河谷半农半牧村为例

在人类占用净初级生产力（HANPP）等研究基础上,提出了净初级生产力（NPP）权衡假设,即生态系统供给、调节服务的权衡受到HANPP各组分分配比例的影响;基于NPP权衡假设构建了生产生态协同提升模式效果的分析框架。以西藏拉萨河流域白朗村为例,基于实地采样、监测数据,分析了生态修复前、协同提升及河谷种草模式下,白朗村的HANPP组分、空间格局,以及生产、生态功能（牲畜养殖数量、碳固定服务和空气净化服务）。结果表明：在实施生态修复前,白朗村共有牲畜15990羊单位,HANPP为35.0 g C/m²,占潜在NPP的13.8%,其中收获导致的HANPP_harv占40.0%;生态系统空气净化服务为12.0 g SO₂ m^-2 a^-1,碳固定服务为6245.4 g C m^-2 a^-1。协同提升后,HANPP总量基本保持不变,但结构发生了变化,其中HANPP_harv提高了4.2%,土地利用导致的HANPP_luc降低了1.8%。同时,养殖牲畜数量增加6.3%,生态系统调节服务基本保持同一水平。在河谷种草模式下,HANPP总量相比实施生态修复前降低了67.0%,结构变化更加剧烈。HANPP_harv增加了84.2%,为25.8 g C/m²;HANPP_luc降低了167.0%,为-14.2 g C/m²。牲畜数量大幅增加了2.2倍（35195.0羊单位）;而空气净化服务也提高了15.1%（13.8 g SO₂ m^-2 a^-1）,碳固定服务提高了5.0%（6560.1 g C m^-2 a^-1）。研究表明,NPP权衡假设可以为定量分析区域尺度生态系统服务权衡提供一定的理论支持,促进生产生态功能协同提升的生态修复模式优化。     

青海湖北岸高寒草甸草原生态系统CO2通量特征及其驱动因子

 草甸草原是青藏高原的重要植被类型, 与其他植被类型相比, 其碳交换过程和驱动机理的研究仍较薄弱。利用青海湖东北岸草甸草原的涡度相关系统观测的连续数据(2010年7月1日–2011年6月30日), 分析了草甸草原CO₂通量特征及其驱动因子。结果表明: 草甸草原净生态系统CO₂交换量(NEE)在植物生长季的5–9月, 其日变化主要受控于光合光量子通量密度(PPFD); 而非生长季(10月21日–4月19日)和生长季初(4月下旬)、末期(10月中上旬) NEE的日变化主要受气温(T_a)的影响。CO₂
日最大吸收值和释放值分别出现在7月1日(11.37 g CO₂·m^–2·d^–1)和10月21日(4.04 g CO₂·m^–2·d^–1)。逐日NEE主要受控于T_a, 两者关系可用指数线性(explinear)方程表示(R² = 0.54, p < 0.01)。叶面积指数(LAI)和增强型植被指数(EVI)对逐日NEE的影响表现为渐近饱和型, LAI和T_a交互作用明显(p < 0.05), EVI的主效应强烈(p < 0.001)。生态系统的呼吸熵(Q₁₀)为2.42, 总呼吸(R_eco)约占总初级生产力(GPP)的74%。生长季适度的昼夜温差(<14.8 ℃)有利于系统的碳蓄积。研究时段该草甸草原作为碳汇从大气吸收271.31 g CO₂· m^–2。     

1961-2010年中国区域氮沉降时空格局模拟研究

由于人类活动的干扰,近年来,通过沉降和施肥形式进入陆地生态系统的氮素持续增加,众多研究表明,中国已经成为继欧洲和北美之后的第三大氮沉降区。氮与陆地生态系统生物地球化学循环的一系列过程都相互联系,碳循环及其格局也受到氮的影响,因此大气氮沉降的变化受到广泛关注,探明区域大气氮沉降的时空格局对评估氮沉降对陆地生态系统碳循环的影响具有重要意义。构建了一个基于降水、能源消费和施肥数据的氮沉降时空格局模拟方法,通过与观测数据的比较说明该模拟方法能够较好地模拟氮沉降的时空变化,在此基础上,利用该方法模拟了1961-2010年中国区域氮沉降的时空格局。结果表明:(1)1961-2010年中国区域年平均氮沉降速率为0.81 g N m^-2 a^-1,由20世纪60年代的0.31 g N m^-2 a^-1增加到21世纪初的1.71 g N m^-2 a^-1,年增长率为0.04 g N m^-2 a^-1。总氮沉降量由20世纪60年代的2.85 TgN/a增加至15.68 TgN/a。(2)NH_x-N的沉降速率大约是NO_y-N的4倍,是主要的氮沉降形式。1961-2010年我国湿沉降平均速率为0.63 g N m^-2 a^-1,是干沉降速率(0.17 g N m^-2 a^-1)的3.63倍,是氮素进入陆地生态系统的重要途径。(3)在空间上,我国的大气氮沉降速率呈现出由东南向西北梯度递减的格局,华北、华中和东北的农田是氮沉降速率最大的区域,同时也是氮沉降速率增长最快的区域。     

改进指数模型对紫茉莉光合-光响应及CO2响应适用性研究

五种模型分别运用于紫茉莉的光合—光响应及CO₂响应曲线的拟合,研究其光合效率参数的变化,探讨紫茉莉光合—光响应及CO₂响应的最适模型。结果表明:(1)紫茉莉的光合—光响应及CO₂响应改进指数模型拟合R²均为0.999,拟合效果优于非直角双曲线、直角双曲线和直角双曲线修正模型。其饱和光强和最大净光合速率分别为797.299和7.879 μmolCO₂·m^-2·s^-1,饱和CO₂浓度和最大光合能力分别为1 264.447和16.783 μmol CO₂·m^-2·s^-1,均与实测值最接近;(2)五个模型拟合和预测的均方误差(MSE)、平均绝对误差(MAE),都是改进指数模型小于其他模型。改进指数模型为紫茉莉光合—光响应及CO₂响应曲线的最佳模型,实验结果可为紫茉莉的生理生态应用研究提供参考。     

西双版纳热带人工雨林生物量及净第一性生产力的研究

通过标准木法和收获法研究分析了西双版纳热带人工模拟雨林的生物量及净第一性生产力。结果表明,林分总生物量约为390.4t·hm^-2,其中乔木层生物量达362.5t·hm^-2,占总生物量的92.8%,灌木层生物量为19.3t·hm^-2,占4.9%,层间植物9包括附生植物）的生物量为3.6t·hm^-2,草本层生物量为5.0t·hm^-2,分别占1.3%和0.9%。林分净第一性生产力为2227.3g.m^-2.年^-1,其中乔木层的净生产力为1553.5g·m^-2.年^-1,占整个林分净生产力的69.7%,灌木层、草本层及层间植物分别仅占26.9%、2.4%和1.0%,其器官分配比例以茎最高,0达42.0%;其次为叶,占30.2%;枝占13.5%。叶面积指数为7.061。同时建立了林分优势种及乔木层各器官生物量的优化回归模型。     

修剪节位及修剪时期对四季蜜芒枝条生长及反季节开花的综合效应

五种模型分别运用于紫茉莉的光合—光响应及CO₂响应曲线的拟合,研究其光合效率参数的变化,探讨紫茉莉光合—光响应及CO₂响应的最适模型。结果表明:(1)紫茉莉的光合—光响应及CO₂响应改进指数模型拟合R²均为0.999,拟合效果优于非直角双曲线、直角双曲线和直角双曲线修正模型。其饱和光强和最大净光合速率分别为797.299和7.879 μmolCO₂·m^-2·s^-1,饱和CO₂浓度和最大光合能力分别为1 264.447和16.783 μmol CO₂·m^-2·s^-1,均与实测值最接近;(2)五个模型拟合和预测的均方误差(MSE)、平均绝对误差(MAE),都是改进指数模型小于其他模型。改进指数模型为紫茉莉光合—光响应及CO₂响应曲线的最佳模型,实验结果可为紫茉莉的生理生态应用研究提供参考。     

六种温带森林类型凋落物量长期动态及其环境驱动

阐明凋落物动态及其环境控制机制，可以为森林生态系统生产力及碳汇功能的维持提供重要的数据支持和理论依据。以长白山系余脉张广才岭西坡林龄相近但立地条件不同的4种天然次生林（即硬阔叶林、杨桦林、杂木林和蒙古栎林）和2种人工林（落叶松人工林和红松人工林）为研究对象，对其地上凋落物产量及其组分以及相关环境因子进行了14年（2008-2021年）的连续测定，旨在揭示森林凋落物量及其组分的时空变化（林型间和年际变异）及其环境驱动机制。结果表明：6种森林类型的凋落总量（TL）无显著差异，波动范围为500.5-556.1 g m^-2 a^-1；但其叶凋落量（LL）、繁殖组织凋落量（RT）和其他组织凋落量（OT）均存在显著差异，波动范围依次分别为333.9-391.8 g m^-2 a^-1、8.43-69.93 g m^-2 a^-1和93.4-185.9 g m^-2 a^-1。6种森林类型的TL均存在显著的年际变化；其中LL和OT年际变化的显著性因森林类型而不同，而RT的年际变化不显著。除落叶松人工林外，其余5种森林类型的LL与生长季平均气温、日最低气温均值、土壤10 cm深度处的平均温度、最低温度（Ts_min）和土壤5 cm含水量（Ms）均呈显著正相关。杂木林、硬阔叶林和红松人工林的RT与Ms呈显著负相关；杂木林、杨桦林和硬阔叶林的OT与Ts_min呈显著负相关。样地水平的LL与土壤10 cm处含水量存在显著的正相关关系，而RT和OT则与其呈现显著负相关关系。这些结果表明林龄相似的温带森林地上凋落物总量有趋同趋势，但其通过改变组分分配格局来适应立地条件的变化；土壤湿度和温度变化会引起凋落物量的年际变化，但不同森林类型的凋落物量对环境波动的敏感性不同。     

大气氮沉降对中亚草地生态系统净初级生产力的影响

氮沉降作为除气候变化、CO₂浓度升高以及土地利用变化之外的第四大主要影响陆地生态系统结构和功能的因素,其对碳循环过程的影响研究相对薄弱,同时也是不确定性最大的环节之一。近年来,由于长期高强度的放牧导致草地生态系统的生产力降低,氮成为典型草地植物生长和生态系统净初级生产力的主要限制因子。据研究,亚洲的氮沉降平均增速极有可能高于全球氮沉降平均增速,成为未来氮沉降增加最快的区域。在此背景下,研究大气氮沉降对于中亚草地生态系统的影响具有重要的意义。利用反硝化-分解模型（DNDC）分析1979-2014年中亚地区草地生态系统净初级生产力（NPP）的时空分异,探讨氮沉降对草地NPP影响。结果表明：（1）1979-2014年间,中亚地区平均草地NPP约为（173.10±31.80） g C m^-2 a^-1,草地NPP时空分异明显,各草地类型的NPP从大到小依次为森林草甸、温带草原和荒漠草原,并且草地NPP以（2.67±1.30） g C m^-2 a^-1的速度逐年增长;（2）当前氮沉降情景总体上促进了中亚地区草地NPP的增长,1979-2014年氮沉降使得中亚草地NPP增加了0.42 Pg C。     

闽江河口芦苇沼泽和短叶茳芏沼泽生态系统含碳温室气体的年收支

河口感潮沼泽是全球重要的蓝碳生态系统,具有很强的固碳能力。碳收支研究是量化生态系统碳源/汇过程及固碳规模的基础。本研究运用透明箱和不同遮光率布遮盖+红外气体分析仪/气相色谱相结合的方法,模拟不同光照条件,测定闽江河口鳝鱼滩半咸水芦苇沼泽和短叶茳芏沼泽的瞬时净生态系统二氧化碳（CO₂）交换量（net ecosystem exchange,NEE）、生态系统呼吸（ecosystem respiration,ER）以及甲烷（CH₄）排放通量,并通过对总光合吸收量（gross ecosystem exchange,GEE）与光合有效辐射的拟合以及ER与气温的拟合,外推2个沼泽生态系统CO₂气体在月、年尺度上的NEE和ER,评估其年固碳量。2个沼泽生态系统的NEE和ER均具有明显的季节变化,春夏秋季为大气中CO₂的汇,而冬季则转化为大气中CO₂的源,芦苇沼泽年尺度固碳能力显著高于短叶茳芏沼泽。芦苇沼泽与短叶茳芏沼泽CH₄排放通量差异不显著。综合考虑CH₄排放,闽江河口鳝鱼滩半咸水芦苇沼泽、短叶茳芏沼泽生态系统年固碳量分别为（5371.52±336.97） g CO₂-eq/m²和（2730.32±503.67） g CO₂-eq/m²。研究表明：闽江河口半咸水沼泽湿地在年尺度上是一个较强的碳汇,在缓解全球变暖方面发挥着重要的角色。     

Seasonal variability in net ecosystem carbon dioxide exchange over a young Switchgrass stand

Understanding carbon dynamics of switchgrass ecosystems is crucial as switchgrass (Panicum virgatum L.) acreage is expanding for cellulosic biofuels. We used eddy covariance system and examined seasonal changes in net ecosystem CO₂ exchange (NEE) and its components – gross ecosystem photosynthesis (GEP) and ecosystem respiration (ER) – in response to controlling factors during the second (2011) and third (2012) years of stand establishment in the southern Great Plains of the United States (Chickasha, OK). Larger vapor pressure deficit (VPD > 3 kPa) limited photosynthesis and caused asymmetrical diurnal NEE cycles (substantially higher NEE in the morning hours than in the afternoon at equal light levels). Consequently, rectangular hyperbolic light–response curve (NEE partitioning algorithm) consistently failed to provide good fits at high VPD. Modified rectangular hyperbolic light–VPD response model accounted for the limitation of VPD on photosynthesis and improved the model performance significantly. The maximum monthly average NEE reached up to ?33.02 ± 1.96 μmol CO₂ m^?2 s^?1 and the highest daily integrated NEE was ?35.89 g CO₂ m^?2 during peak growth. Although large differences in cumulative seasonal GEP and ER were observed between two seasons, total seasonal ER accounted for about 75% of GEP regardless of the growing season lengths and differences in aboveground biomass production. It suggests that net ecosystem carbon uptake increases with increasing GEP. The ecosystem was a net sink of CO₂ during 5–6 months and total seasonal uptakes were ?1128 ± 130 and ?1796 ± 217 g CO₂ m^?2 in 2011 and 2012, respectively. In conclusion, our findings suggest that the annual carbon status of a switchgrass ecosystem can be a small sink to small source in this region if carbon loss from biomass harvesting is considered. However, year‐round measurements over several years are required to assess a long‐term source‐sink status of the ecosystem.     

Net ecosystem carbon dioxide exchange over grazed steppe in central Mongolia

This paper presents results of 1 year (from March 25, 2003 to March 24, 2004, 366 days) of continuous measurements of net ecosystem CO₂ exchange (NEE) above a steppe in Mongolia using the eddy covariance technique. The steppe, typical of central Mongolia, is dominated by C₃ plants adapted to the continental climate. The following two questions are addressed: (1) how do NEE and its components: gross ecosystem production (GEP) and total ecosystem respiration (R_eco) vary seasonally? (2) how do NEE, GEP, and R_eco respond to biotic and abiotic factors? The hourly minimal NEE and the hourly maximal R_eco were −3.6 and 1.2 μmol m⁻² s⁻¹, respectively (negative values denoting net carbon uptake by the canopy from the atmosphere). Peak daily sums of NEE, GEP, and R_eco were −2.3, 3.5, and 1.5 g C m⁻² day⁻¹, respectively. The annual sums of GEP, R_eco, and NEE were 179, 138, and −41 g C m⁻², respectively. The carbon removal by sheep was estimated to range between 10 and 82 g C m⁻² yr⁻¹ using four different approaches. Including these estimates in the overall carbon budget yielded net ecosystem productivity of −23 to +20 g C m⁻² yr⁻¹. Thus, within the remaining experimental uncertainty the carbon budget at this steppe site can be considered to be balanced. For the growing period (from April 23 to October 21, 2003), 26% and 53% of the variation in daily NEE and GEP, respectively, could be explained by the changes in leaf area index. Seasonality of GEP, R_eco, and NEE was closely associated with precipitation, especially in the peak growing season when GEP and R_eco were largest. Water stress was observed in late July to early August, which switched the steppe from a carbon sink to a carbon source. For the entire growing period, the light response curves of daytime NEE showed a rather low apparent quantum yield (α=−0.0047 μmol CO₂ μmol⁻¹ photons of photosynthetically active radiation). However, the α values varied with air temperature (T_a), vapor pressure deficit, and soil water content.     

腾格里荒漠红砂-珍珠群落CO2收支变化及其不同观测方法间的比较

由于荒漠生态系统植被覆盖度低、生产力低下,其在全球碳循环中的作用被长期忽视。为探讨荒漠生态系统碳收支各组分的变化规律,以腾格里荒漠红砂(Reaumuria soongorica Maxim.)-珍珠(Salsola passerina Beg.)群落为研究对象,采用静态箱式法研究了该群落的净生态系统CO2交换量(NEE)、生态系统呼吸、土壤呼吸的日变化规律,同时将该方法所获得的NEE结果与涡动相关法观测的结果进行了比较。结果表明:(1)红砂-珍珠群落NEE的日变化表现为,在6:00—9:00左右出现一个CO2吸收的高峰值,随后在12:00—15:00左右出现一个CO2释放高峰值。红砂种群、珍珠种群和整个群落NEE的平均值分别为0.018、0.020和0.028 mg CO2m-2s-1;(2)红砂种群、珍珠种群、土壤及整个群落生态系统呼吸速率的日变化规律一致,均表现为明显的单峰变化趋势,在12:00—15:00左右出现一个CO2释放的高峰值。红砂种群、珍珠种群、土壤和整个群落的生态系统呼吸的平均值分别为:0.121、0.062、0.029和0.040 mg CO2m-2s-1。以盖度为加权因子计算得到红砂种群、珍珠种群和土壤呼吸占生态系统呼吸的比例分别为:9%、21%和70%,由此可见,生态系统呼吸主要来源于土壤呼吸。(3)将箱式法和涡动相关法观测的NEE进行比较,结果表明两种方法观测的NEE变化规律基本一致,相关系数达到0.7。采用箱式法观测的NEE高于涡动相关法观测的结果,平均值分别0.028 mg CO2m-2s-1(箱式法)和0.015 mg CO2m-2s-1(涡动相关法),涡动相关法的观测结果与箱式法观测结果的比值为0.54。综上可得,荒漠生态系统土壤呼吸的变化速率决定了生态系统呼吸的变化规律,采用箱式法可能高估了荒漠生态系统CO2的释放量。     

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.     

CO2 exchange in three Canadian High Arctic ecosystems: response to long-term experimental warming

Carbon dioxide exchange, soil C and N, leaf mineral nutrition and leaf carbon isotope discrimination (LCID‐Δ) were measured in three High Arctic tundra ecosystems over 2 years under ambient and long‐term (9 years) warmed (～2°C) conditions. These ecosystems are located at Alexandra Fiord (79°N) on Ellesmere Island, Nunavut, and span a soil water gradient; dry, mesic, and wet tundra. Growing season CO₂ fluxes (i.e., net ecosystem exchange (NEE), gross ecosystem photosynthesis (GEP), and ecosystem respiration (R_e)) were measured using an infrared gas analyzer and winter C losses were estimated by chemical absorption. All three tundra ecosystems lost CO₂ to the atmosphere during the winter, ranging from 7 to 12 g CO₂‐C m^?2 season^?1 being highest in the wet tundra. The period during the growing season when mesic tundra switch from being a CO₂ source to a CO₂ sink was increased by 2 weeks because of warming and increases in GEP. Warming during the summer stimulated dry tundra GEP more than R_e and thus, NEE was consistently greater under warmed as opposed to ambient temperatures. In mesic tundra, warming stimulated GEP with no effect on R_e increasing NEE by ～10%, especially in the first half of the summer. During the ～70 days growing season (mid‐June–mid‐August), the dry and wet tundra ecosystems were net CO₂‐C sinks (30 and 67 g C m^?2 season^?1, respectively) and the mesic ecosystem was a net C source (58 g C m^?2 season^?1) to the atmosphere under ambient temperature conditions, due in part to unusual glacier melt water flooding that occurred in the mesic tundra. Experimental warming during the growing season increased net C uptake by ～12% in dry tundra, but reduced net C uptake by ～20% in wet tundra primarily because of greater rates of R_e as opposed to lower rates of GEP. Mesic tundra responded to long‐term warming with ～30% increase in GEP with almost no change in R_e reducing this tundra type to a slight C source (17 g C m^?2 season^?1). Warming caused LCID of Dryas integrafolia plants to be higher in dry tundra and lower in Salix arctic plants in mesic and wet tundra. Our findings indicate that: (1) High Arctic ecosystems, which occur in similar mesoclimates, have different net CO₂ exchange rates with the atmosphere; (2) long‐term warming can increase the net CO₂ exchange of High Arctic tundra by stimulating GEP, but it can also reduce net CO₂ exchange in some tundra types during the summer by stimulating R_e to a greater degree than stimulating GEP; (3) after 9 years of experimental warming, increases in soil carbon and nitrogen are detectable, in part, because of increases in deciduous shrub cover, biomass, and leaf litter inputs; (4) dry tundra increases in GEP, in response to long‐term warming, is reflected in D. integrifolia LCID; and (5) the differential carbon exchange responses of dry, mesic, and wet tundra to similar warming magnitudes appear to depend, in part, on the hydrologic (soil water) conditions. Annual net ecosystem CO₂‐C exchange rates ranged from losses of 64 g C m^?2 yr^?1 to gains of 55 g C m^?2 yr^?1. These magnitudes of positive NEE are close to the estimates of NPP for these tundra types in Alexandra Fiord and in other High Arctic locations based on destructive harvests.     

基于模型数据融合的中国温带和亚热带典型森林生态系统碳通量模拟

生态系统碳循环过程对水分响应的研究已成为全球变化关注的焦点问题之一。基于长白山温带针阔混交林与千烟洲亚热带人工针叶林观测站2003—2009年生长季的碳通量(NEE)和气象观测数据,综合考虑水分对光合、呼吸作用的影响,构建不同的NEE模型,并应用模型数据融合方法优化模型参数、遴选最适模型,系统分析了水分因子对不同森林生态系统碳循环的影响。结果表明:(1)优化后的模型参数均能被NEE实测数据较好约束。长白山生长季的光合、呼吸参数值均高于千烟洲,未考虑空气饱和水汽压差(VPD)的模型高估了千烟洲温度敏感性参数(Q10)值、低估了千烟洲基础呼吸速率参数(BR)值;(2)仅考虑VPD对光合作用影响的模型是长白山生长季碳通量模拟的最优模型,但模拟精度提高不显著。不同模型间碳通量组分模拟结果差异较小;(3)考虑VPD和土壤含水量对光合、呼吸作用共同影响的模型是千烟洲生长季碳通量模拟的最优模型,并且显著提高了模拟精度。未考虑水分的模型在生长季高估了总生态系统生产力(GEP)总量2.0%(21.85 g C/m~2),同时更大幅度地高估了生态系统呼吸(RE)总量4.4%(38.02 g C/m~2),从而导致NEE总量低估于实测值7.8%(18.55 g C/m~2)。     

High Arctic Dry Heath CO2 Exchange During the Early Cold Season

Climate change may alter the terrestrial ecosystem carbon balance in the Arctic, and previous studies have emphasized the importance of cold season gas exchange when considering the annual carbon balance. Here, we examined gross ecosystem production (GEP), ecosystem respiration (R _eco) and net ecosystem exchange (NEE) during autumn at a high arctic dry open heath, over a period where air temperatures decreased from +9.8 to ?16.5°C. GEP declined by 95–100% during autumn but GEP significantly different from 0 was measured on October 8 despite sub-zero temperatures. R _eco declined by 90% and dominated NEE throughout the study as the ecosystem on all measurement days was a source of atmospheric CO₂. We estimated net September carbon losses (NEE) to be 17?g?CO₂?m^?2, emphasizing the importance of autumn in relation to annual carbon budgets. The study site has been subjected to 14 summers of water addition, and occasional pulses of nitrogen (N) addition in a fully factorial design. N addition enhanced GEP up to 17-fold during September, although there was no effect in October when GEP was very low. Summer water addition decreased autumn R _eco by up to 25%. Both N amendment and water addition decreased carbon loss, that is, increased NEE; N amendment increased NEE on all dates by 13–64% whereas water addition increased NEE by 20–54% late in September and onward, demonstrating the importance of nutrient and water availability on carbon balance in high arctic tundra, also during the autumn freeze-in.     

Role of vegetation in determining carbon sequestration along ecological succession in the southeastern United States

Vegetation plays a central role in controlling terrestrial carbon (C) exchange, but quantifying its impacts on C cycling on time scales of ecological succession is hindered by a lack of long‐term observations. The net ecosystem exchange of carbon (NEE) was measured for several years in adjacent ecosystems that represent distinct phases of ecological succession in the southeastern USA. The experiment was designed to isolate the role of vegetation – apart from climate and soils – in controlling biosphere–atmosphere fluxes of CO₂ and water vapor. NEE was near zero over 5 years at an early successional old‐field ecosystem (OF). However, mean annual NEE was nearly equal, approximately ?450 g C m^?2 yr^?1, at an early successional planted pine forest (PP) and a late successional hardwood forest (HW) due to the sensitivity of the former to drought and ice storm damage. We hypothesize that these observations can be explained by the relationships between gross ecosystem productivity (GEP), ecosystem respiration (RE) and canopy conductance, and long‐term shifts in ecosystem physiology in response to climate to maintain near‐constant ecosystem‐level water‐use efficiency (EWUE). Data support our hypotheses, but future research should examine if GEP and RE are causally related or merely controlled by similar drivers. At successional time scales, GEP and RE observations generally followed predictions from E. P. Odum's ‘Strategy of Ecosystem Development’, with the surprising exception that the relationship between GEP and RE resulted in large NEE at the late successional HW. A practical consequence of this research suggests that plantation forestry may confer no net benefit over the conservation of mature forests for C sequestration.     

Six-year Stable Annual Uptake of Carbon Dioxide in Intensively Managed Humid Temperate Grassland

Variability and future alterations in regional and global climate patterns may exert a strong control on the carbon dioxide (CO₂) exchange of grassland ecosystems. We used 6 years of eddy-covariance measurements to evaluate the impacts of seasonal and inter-annual variations in environmental conditions on the net ecosystem CO₂ 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 CO₂ occurred in this grassland with a narrow range in the annual NEE from −245 to −284 g C m⁻² y⁻¹, with the exception of 2008 in which the NEE reached −352 g C m⁻² y⁻¹. 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 CO₂, in this intensively managed grassland, may be expected to continue even under predicted future climatic changes in the humid temperate climate region.     

Response of carbon fluxes to drought in a coastal plain loblolly pine forest

Full accounting of ecosystem carbon (C) pools and fluxes in coastal plain ecosystems remains less studied compared with upland systems, even though the C stocks in these systems may be up to an order of magnitude higher, making them a potentially important component in regional C cycle. Here, we report C pools and CO₂ exchange rates during three hydrologically contrasting years (i.e. 2005–2007) in a coastal plain loblolly pine plantation in North Carolina, USA. The daily temperatures were similar among the study years and to the long‐term (1971–2000) average, whereas the amount and timing of precipitation differed significantly. Precipitation was the largest in 2005 (147 mm above normal), intermediate in 2006 (48 mm below) and lowest in 2007 (486 mm below normal). The forest was a strong C sink during all years, sequestering 361 ± 67 (2005), 835 ± 55 (2006) and 724 ± 55 (2007) g C m^?2 yr^?1 according to eddy covariance measurements of net ecosystem CO₂ exchange (NEE). The interannual differences in NEE were traced to drought‐induced declines in canopy and whole tree hydraulic conductances, which declined with growing precipitation deficit and decreasing soil volumetric water content (VWC). In contrast, the interannual differences were small in gross ecosystem productivity (GEP) and ecosystem respiration (ER), both seemingly insensitive to drought. However, the drought sensitivity of GEP was masked by higher leaf area index and higher photosynthetically active radiation during the dry year. Normalizing GEP by these factors enhanced interannual differences, but there were no signs of suppressed GEP at low VWC during any given year. Although ER was very consistent across the 3 years, and not suppressed by low VWC, the total respiratory cost as a fraction of net primary production increased with annual precipitation and the contribution of heterotrophic respiration (R_h) was significantly higher during the wettest year, exceeding new litter inputs by 58%. Although the difference was smaller during the other 2 years (R_h : litterfall ratio was 1.05 in 2006 and 1.10 in 2007), the soils lost about 109 g C m^?2 yr^?1, outlining their potential vulnerability to decomposition, and pointing to potential management considerations to protect existing soil C stocks.