Different responses of ecosystem carbon exchange to warming in three types of alpine grassland on the central Qinghai–Tibetan Plateau

Climate is a driver of terrestrial ecosystem carbon exchange, which is an important product of ecosystem function. The Qinghai–Tibetan Plateau has recently been subjected to a marked increase in temperature as a consequence of global warming. To explore the effects of warming on carbon exchange in grassland ecosystems, we conducted a whole‐year warming experiment between 2012 and 2014 using open‐top chambers placed in an alpine meadow, an alpine steppe, and a cultivated grassland on the central Qinghai–Tibetan Plateau. We measured the gross primary productivity, net ecosystem CO₂ exchange (NEE), ecosystem respiration, and soil respiration using a chamber‐based method during the growing season. The results show that after 3 years of warming, there was significant stimulation of carbon assimilation and emission in the alpine meadow, but both these processes declined in the alpine steppe and the cultivated grassland. Under warming conditions, the soil water content was more important in stimulating ecosystem carbon exchange in the meadow and cultivated grassland than was soil temperature. In the steppe, the soil temperature was negatively correlated with ecosystem carbon exchange. We found that the ambient soil water content was significantly correlated with the magnitude of warming‐induced change in NEE. Under high soil moisture condition, warming has a significant positive effect on NEE, while it has a negative effect under low soil moisture condition. Our results highlight that the NEE in steppe and cultivated grassland have negative responses to warming; after reclamation, the natural meadow would subject to loose more C in warmer condition. Therefore, under future warmer condition, the overextension of cultivated grassland should be avoided and scientific planning of cultivated grassland should be achieved.     

增温对青藏高原高寒草原生态系统碳交换的影响

碳交换是影响草地生态系统碳汇功能的关键过程,对气候变暖极为敏感。青藏高原分布着大面积的高寒草原,其碳汇功能对气候变暖的响应对区域碳循环过程具有重要的影响。为探究高寒草原生态系统碳交换过程对增温的响应,2012—2014年,在青藏高原班戈县进行了模拟增温对高寒草原生态系统碳交换过程影响的研究。结果表明,增温对高寒草原碳交换各组分的影响存在年际差异,但总体上对碳交换存在负面影响。3年平均结果显示,增温显著降低了高寒草原地上生物量、总生态系统生产力(GEP)、生态系统呼吸(ER)和净生态系统碳交换量(NEE)(P0.05),平均降幅分别为15.1%、36.8%、19.2%和51.5%。增温条件下3年平均土壤呼吸(SR)较对照无显著变化(P0.05),但2013年增温显著降低了SR(P0.05),降幅达18.1%。增温对SR与ER的比值具有一定的促进作用,最高增幅达到40.0%。GEP、ER、SR和NEE与土壤温度和土壤水分无显著相关(P0.05),而GEP、ER和NEE与空气温度呈显著的负相关关系(P0.05)。增温引起的干旱胁迫以及地上生物量降低是导致高寒草原NEE降低的主要原因。研究表明,全球变暖会一定程度降低青藏高原高寒草原的碳汇功能。     

青藏高原高寒草甸和荒漠碳交换特征及其气象影响机制

以青藏高原玛沁地区高寒草甸和沱沱河地区高寒荒漠草原为观测研究站,利用涡动协方差技术获取高寒生态系统水平上的CO₂通量以及水和能量通量,通过REddyProc、随机森林(Random Forest, RF)进行了数据后处理,探究了不同下垫面典型环境因子对净生态系统CO₂交换量(Net Ecosystem Exchange, NEE)的影响机制。结果表明：1)玛沁高寒草甸在6—7月以吸收为主,表现为碳汇,吸收峰值出现在11:00—12:00(北京时,下同)之间,而在3、4、5、8月以排放为主,表现为碳源,排放峰值出现在21:00—23:00之间;沱沱河高寒荒漠在3—8月以吸收为主,表现为净碳汇,吸收峰值出现在13:00—14:00之间;整个生长季前后(3—8月),玛沁和沱沱河的累计NEE分别为79.50 g C/m²和79.24 g C/m²,都表现为碳汇。2)不同尺度不同下垫面,气象因子对NEE的重要程度不同,小时尺度上,高寒草甸辐射对NEE的重要性最大,高寒荒漠草原蒸散发对NEE的重要性最大;日尺度...     

A growing season climatic index to simulate gross primary productivity and carbon budget in a Tibetan alpine meadow

Accurate estimation of gross primary production (GPP) of ecosystem is needed to evaluate terrestrial carbon cycle at various spatial and temporal scales. Eddy covariance (EC) technique provides continuous measurements of net ecosystem CO₂ exchange (NEE) and can be used to separate GPP from NEE in real time series. However, seasonal and inter-annual variation and consequently ecosystem carbon budget is still very difficult to simulate from climatic and environment. To address this limitation, we develop a growing season indicator (GSI) based on low temperature and soil water stress to model and predict intra and inter-annual dynamic of gross primary productivity (GPP). Validation of this new index was conducted using continuous six-year consective EC measurement from 2004 to 2009 at a Tibetan alpine meadow. Simulated GPP agreed well with the observed GPP in terms of seasonal and inter-annual variation. The six-year correlation coefficients on seasonal scale between GSI and scalar GPP derived from EC reached more than 0.85 no matter in dry years or wet years. In addition, the temporal GPP estimation derived from GSI model was quite similar to those from observed values by EC measurement. Moreover, accumulated GSI values can predict annual variability of net ecosystem production (NEP). Higher yearly accumulated GSI corresponded to more annual NEP. When cumulative GSI arrived up to 92, the target ecosystem was a carbon sink. This is probably a threshold which Tibetan alpine meadow changes from carbon source to carbon sink. It is indicated that the GSI model is a simple, alternative approach to estimating GPP and has the potential to simulate spatial GPP in a larger scale. However, the performance of GSI model in other vegetation types or regions still needs a further verification.     

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.     

Estimation of aboveground biomass using in situ hyperspectral measurements in five major grassland ecosystems on the Tibetan Plateau

Aims There are numerous grassland ecosystem types on the Tibetan Plateau. These include the alpine meadow and steppe and degraded alpine meadow and steppe. This study aimed at developing a method to estimate aboveground biomass (AGB) for these grasslands from hyperspectral data and to explore the feasibility of applying air/satellite-borne remote sensing techniques to AGB estimation at larger scales.Methods We carried out a field survey to collect hyperspectral reflectance and AGB for five major grassland ecosystems on the Tibetan Plateau and calculated seven narrow-band vegetation indices and the vegetation index based on universal pattern decomposition (VIUPD) from the spectra to estimate AGB. First, we investigated correlations between AGB and each of these vegetation indices to identify the best estimator of AGB for each ecosystem type. Next, we estimated AGB for the five pooled ecosystem types by developing models containing dummy variables. At last, we compared the predictions of simple regression models and the models containing dummy variables to seek an ecosystem type-independent model to improve prediction of AGB for these various grassland ecosystems from hyperspectral measurements.Important findings When we considered each ecosystem type separately, all eight vegetation indices provided good estimates of AGB, with the best predictor of AGB varying among different ecosystems. When AGB of all the five ecosystems was estimated together using a simple linear model, VIUPD showed the lowest prediction error among the eight vegetation indices. The regression models containing dummy variables predicted AGB with higher accuracy than the simple models, which could be attributed to the dummy variables accounting for the effects of ecosystem type on the relationship between AGB and vegetation index (VI). These results suggest that VIUPD is the best predictor of AGB among simple regression models. Moreover, both VIUPD and the soil-adjusted VI could provide accurate estimates of AGB with dummy variables integrated in regression models. Therefore, ground-based hyperspectral measurements are useful for estimating AGB, which indicates the potential of applying satellite/airborne remote sensing techniques to AGB estimation of these grasslands on the Tibetan Plateau.     

珠三角城市绿地CO2通量的季节特征

城市绿地是城市碳循环的重要组成部分,利用长期定位观测资料估算珠三角典型城市绿地的CO2通量,可以为应对气候变化、评价区域碳源汇提供参考。应用2009、2010年,东莞市植物园内的涡度相关法CO2通量定位观测资料,分析了净生态系统交换量(NEE)的年变化及其与气象要素的关系,结果表明:(1)年平均NEE总量为-104.2 gC.m-.2a-1,表明城市绿地生态系统具有固碳能力。(2)NEE随光温条件变化呈现明显的季节动态,12至3月表现为碳源,其他月份表现为碳汇。(3)根据白天NEE与光合有效辐射(PAR)逐月拟合Michaelis-Menten方程,得到年平均表观初始光能利用率(α)为(0.00134±0.00035)mgCO.2μmol-1光子,年平均光饱和生态系统生产力(Pmax)为(1.006±0.283)mgCO.2m-.2s-1。(4)利用夜间呼吸(Reco)与5 cm土壤温度(Ts)拟合指数方程,得到年平均Reco总量为1378.1 gC.m-.2a-1。(5)NEE与PAR、气温(Ta)和饱和水压差(VPD)的相关性分析显示,NEE与PAR偏相关系数的绝对值大于Ta和VPD,表明PAR对NEE的影响最大。     

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。     

青藏高原高寒灌丛2003-2016年生长季的不同月份CO2通量对气温日较差的响应

全球气候变化引起的气温日较差（ADT）减小，将会对高寒生态系统的碳平衡造成深刻影响。基于涡度相关系统，利用2003-2016年的涡度相关系统观测资料，做了青藏高原高寒灌丛在生长季（6-9月）不同月份的ADT对CO₂通量影响的研究。结果表明：2003-2016年的生长季中，最高气温（MaxTa）和最低气温（MinTa）呈先升高后降低的单峰变化趋势，ADT没有呈现明显的变化趋势。逐日总初级生产力（GPP）和生态系统呼吸（Re）呈先增加后降低的单峰趋势，逐日净生态系统CO₂交换（NEE）呈先下降后上升的"V"型变化趋势。高寒灌丛在生长季为碳汇，整个生长季总NEE、GPP和Re平均值分别为（-161.2±30.1）、（501.9±60.2）、（340.7±54.4） gCm^-2。在高寒灌丛生长季（6-9月）的每个月份，MaxTa、MinTa和ADT分别是GPP（P<0.001）、Re（P<0.001）和NEE（P<0.01）变化的主要控制因子。高寒灌丛的ADT的增大有利于生态系统的碳固持，暗示在未来气候变化背景下ADT的减小将会削弱高寒灌丛生态系统的碳汇能力。     

黄河三角洲芦苇湿地生长季净生态系统CO2交换及其环境调控机制

采用涡度相关法,对2011年生长季的黄河三角洲芦苇湿地净生态系统CO₂交换(NEE)进行了观测,研究湿地NEE的变化规律及其影响因子.结果表明: 不同月份芦苇湿地的NEE日变化均呈“U”形曲线,CO₂最大净吸收率和释放率的日均值分别为(0.44±0.03)和(0.16±0.01) mg CO₂·m^-2·s^-1;芦苇湿地NEE、生态系统呼吸(R_eco)、总初级生产力(GPP)的季节变化均呈现生长旺季(7-9月)较高、生长初期(5-6月)和生长末期(10-11月)较低的趋势;R_eco和NEE在8月达到峰值,GPP在7月达到峰值.芦苇湿地生态系统的CO₂交换受到光合有效辐射(PAR)、土壤温度(T_s)和土壤体积含水量(SWC)的共同影响.白天NEE与PAR呈直角双曲线关系;5 cm深处T_s与夜间生态系统呼吸(R_eco,n)呈指数关系,生态系统呼吸的温度敏感性(Q₁₀)为2.30,SWC和T_s是影响芦苇湿地R_eco,n的主要因子.在整个生长季,黄河三角洲芦苇湿地生态系统是一个明显的CO₂的汇,总净固碳量为780.95 g CO₂·m^-2.     

西藏高原高寒草原生态系统植被C／N值的分布特征及其影响因素

以西藏高原高寒草原生态系统的4个自然地带（高山草原、高山灌丛草甸、山地半荒漠与荒漠以及山地灌丛草原）的19个草地型植被为研究对象,采用野外调查与室内分析相结合的方法,对高寒草原生态系统植被C／N值的分布特征及其影响因素进行了研究。结果表明：西藏高原高寒草原植被C／N值总体上呈现出东西部低而中间高的态势以及斑块状交错分布的格局。不同自然地带间和不同草地型间植被地上部分和根系的C／N值有明显差异,且地上部分的C／N值均大于根系。19个草地型植被地上部分的平均C／N值为34．17,变异系数为35．87％;根系的平均C／N值为29．58,变异系数为40．02％。4个自然地带植被地上部分的平均C／N值为31．98,变异系数为13．82％;根系的平均C／N值为31．86,变异系数为16．92％。回归分析结果显示：植被地上部分C／N值与地上部生物量以及土壤全N和全K含量呈显著正相关、与植被高度呈显著负相关;根系C／N值与海拔和20～30em土壤容重呈显著正相关、与年均降水量和年均蒸发量呈显著负相关,这些因子均为影响西藏高原高寒草原植被C／N值的关键环境因子。总体上看,地理因子、气候因子和土壤物理因子对西藏高原高寒草原生态系统植被C／N值的影响不显著,而植被因子和土壤化学因子则对其有显著影响。     

氮磷共限制青藏高原高寒草甸生态系统碳吸收

随着人类活动加剧,青藏高原高寒草地面临外来资源输入的威胁,而外源资源输入如氮、磷、钾(N、P、K)及其交互作用如何影响高寒草地生态系统碳循环尚不明确.本研究在藏北高寒草甸进行了连续3年N、P、K元素交互的添加试验,测定群落盖度和生态系统碳交换等数据,旨在阐明资源添加对高寒草甸生态系统碳交换过程的影响.结果 表明:在植物...     

Effects of vegetation control on ecosystem water use efficiency within and among four grassland ecosystems in China

Through 2–3-year (2003–2005) continuous eddy covariance measurements of carbon dioxide and water vapor fluxes, we examined the seasonal, inter-annual, and inter-ecosystem variations in the ecosystem-level water use efficiency (WUE, defined as the ratio of gross primary production, GPP, to evapotranspiration, ET) at four Chinese grassland ecosystems in the Qinghai-Tibet Plateau and North China. Representing the most prevalent grassland types in China, the four ecosystems are an alpine swamp meadow ecosystem, an alpine shrub-meadow ecosystem, an alpine meadow-steppe ecosystem, and a temperate steppe ecosystem, which illustrate a water availability gradient and thus provide us an opportunity to quantify environmental and biological controls on ecosystem WUE at different spatiotemporal scales. Seasonally, WUE tracked closely with GPP at the four ecosystems, being low at the beginning and the end of the growing seasons and high during the active periods of plant growth. Such consistent correspondence between WUE and GPP suggested that photosynthetic processes were the dominant regulator of the seasonal variations in WUE. Further investigation indicated that the regulations were mainly due to the effect of leaf area index (LAI) on carbon assimilation and on the ratio of transpiration to ET ( T /ET). Besides, except for the swamp meadow, LAI also controlled the year-to-year and site-to-site variations in WUE in the same way, resulting in the years or sites with high productivity being accompanied by high WUE. The general good correlation between LAI and ecosystem WUE indicates that it may be possible to predict grassland ecosystem WUE simply with LAI. Our results also imply that climate change-induced shifts in vegetation structure, and consequently LAI may have a significant impact on the relationship between ecosystem carbon and water cycles in grasslands.     

Ecosystem Carbon Fluxes in Response to Warming and Clipping in a Tallgrass Prairie

Global warming and land-use change could have profound impacts on ecosystem carbon (C) fluxes, with consequent changes in C sequestration and its feedback to climate change. However, it is not well understood how net ecosystem C exchange (NEE) and its components respond to warming and mowing in tallgrass prairie. We conducted two warming experiments, one long term with a 1.7°C increase in a C₄-dominated grassland (Experiment 1), and one short term with a 2.8°C increase in a C₃-dominated grassland (Experiment 2), to investigate main and interactive effects of warming and clipping on ecosystem C fluxes in the Great Plains of North America during 2009–2011. An infrared radiator was used to simulate climate warming and clipping once a year mimicked mowing in both experiments. The results showed that warming significantly increased ecosystem respiration (ER), slightly increased GPP, with the net outcome (NEE) being little changed in Experiment 1. In contrast, warming significantly suppressed GPP and ER in both years, with the net outcome being enhanced in NEE (more C sequestration) in 2009–2010 in Experiment 2. The C₄-dominated grassland showed a much higher optimum temperature for C fluxes than the C₃-dominated grassland, which may partly contribute to the different warming effects in the two experiments. Clipping significantly enhanced GPP, ER, and NEE in both experiments but did not significantly interact with warming in impacting C fluxes in either experiment. The warming-induced changes in ecosystem C fluxes correlated significantly with C₄ biomass proportion but not with warming-induced changes in either soil temperature or soil moisture across the plots in the experiments. Our results demonstrate that carbon fluxes in the tallgrass prairie are highly sensitive to climate warming and clipping, and C₃/C₄ plant functional types may be important factor in determining ecosystem response to climate change.     

The interactive effects of nitrogen addition and increased precipitation on gross ecosystem productivity in an alpine meadow

氮水添加对高寒草甸生态系统生产力的影响 降水变化和大气氮沉降增加对草原生态系统碳交换具有重要的影响,进而影响草地生产力、群落组成和生态系统功能。然而,氮水添加对高寒草甸生态系统碳交换的影响目前尚不清楚。因此,本研究在青藏高原高寒草甸布设氮水添加试验,设置4种不同处理：对照、 加氮、加水和同时添加氮水,对生态系统碳交换过程进行了连续4年的原位观测。研究结果发现,氮添加可以增加总生态系统生产力(GEP)、植物地上生物量、群落盖度和群落加权平均高度(CWMh),而水分添加没有显著影响。生态系统碳交换对氮水添加的响应在干湿年存在显著差异。水分添加仅在干旱年对净生态系统碳交换(NEE)具有显著影响,原因是GEP的增加量大于生态系统呼吸(ER)。相反,氮添加仅在湿润年显著提高了生态系统碳交换,其中GEP的增加归因于NEE的增加量大于ER。结构方程结果表明,氮添加主要通过增加优势种的盖度从而提高NEE。本研究强调了降水和优势物种在调节高寒草甸生态系统响应环境变化中的重要作用。     

Carbon dynamics of terrestrial ecosystems on the Tibetan Plateau during the 20th century: an analysis with a process‐based biogeochemical model

Aim The Tibetan Plateau accounts for about a quarter of the total land area of China and has a variety of ecosystems ranging from alpine tundra to evergreen tropics. Its soils are dominated by permafrost and are rich in organic carbon. Its climate is unique due to the influence of the Asian monsoon and its complex topography. To date, the carbon dynamics of the Tibetan Plateau have not been well quantified under changes of climate and permafrost conditions. Here we use a process‐based biogeochemistry model, the Terrestrial Ecosystem Model (TEM), which was incorporated with a soil thermal model, to examine the permafrost dynamics and their effects on carbon dynamics on the plateau during the past century. Location The Tibetan Plateau. Methods We parameterize and verify the TEM using the existing data for soil temperature, permafrost distribution and carbon and nitrogen from the region. We then extrapolate the model and parameters to the whole plateau. Results During the 20th century, the Tibetan Plateau changed from a small carbon source or neutral in the early part of the century to a sink later, with a large inter‐annual and spatial variability due to changes of climate and permafrost conditions. Net primary production and soil respiration increased by 0.52 and 0.22 Tg C year^?1, respectively, resulting in a regional carbon sink increase of 0.3 Tg C year^?1. By the end of the century, the regional carbon sink reached 36 Tg C year^?1 and carbon storage in vegetation and soils is 32 and 16 Pg C, respectively. On the plateau, from west to east, the net primary production, soil respiration and net ecosystem production increased, due primarily to the increase of air temperature and precipitation and lowering elevation. In contrast, the decrease of carbon fluxes from south to north was primarily controlled by precipitation gradient. Dynamics of air temperature and associated soil temperature and active layer depth resulted in a higher plant carbon uptake than soil carbon release, strengthening the regional carbon sink during the century. Main conclusions We found that increasing soil temperature and deepening active layer depth enhanced soil respiration, increasing the net nitrogen mineralization rate. Together with the effects of warming air temperature and rising CO₂ concentrations on photosynthesis, the stronger plant nitrogen uptake due to the enhanced available nitrogen stimulates plant carbon uptake, thereby strengthening the regional carbon sink as the rate of increase net primary production was faster than that of soil respiration. Further, the warming and associated soil thermal dynamics shifted the regional carbon sink from the middle of July in the early 20th century to early July by the end of the century. Our study suggests that soil thermal dynamics should be considered for future quantification of carbon dynamics in this climate‐sensitive region.     

增温对青藏高原高寒草甸生态系统固碳通量影响的模拟研究

低温被广泛认为是高寒草甸生态系统首要限制性因子,因此增温可能会在某种程度上促进初级生产力,但是也可能由于土壤水分、N素营养状况的改变形成新胁迫而抑制生产力提高。此外,生态系统呼吸由于增温而提高的幅度也可能高于初级生产力提高的幅度,造成总碳库平衡的改变。利用青藏高原海北高寒草甸实测数据对生态系统过程模型Biome-BGC(V.4.2)进行了参数化,并利用研究区实测土壤水分(0-40 cm)和其它观测数据对模型进行了检验,证明模型模拟结果较为可靠。模型使用2005-2008年的海北气象站实测气象数据包括气温、降水等作为驱动数据,模拟了增温1.2-1.7℃下青藏高原海北定位站高寒草甸生态系统碳通量的变化,并整合分析增温试验平台上已发表的试验,与模拟结果进行对比,探讨增温对海北高寒草甸生态系统碳收支的可能影响。结果表明:2005-2008年青藏高原高寒草甸生态系统为弱的碳汇,短期增温导致系统净碳固定增加。增温直接影响系统碳通量,也通过土壤水分和土壤矿化氮变化间接影响碳通量,相比土壤水分和氮素,增温对影响碳通量变化过程中的效应更大;研究也揭示,在增温条件下,植物对土壤矿化氮的吸收量小于有机质分解产生的土壤矿化氮量,土壤矿化氮含量增加。     

青海湖北岸草甸草原CO2通量年际动态及其驱动机制

青藏高原草甸草原是生态系统中重要的植被类型,准确评估高寒草甸草原生态系统碳源汇状况及碳储量变化尤为重要。基于涡度相关系统观测,分析了2009年至2016年8年期间青海湖北岸草甸草原环境因子以及碳通量的变化特征,运用结构方程模型(SEM)分析环境因子对总初级生产力(GPP)、净生态系统CO₂交换量(NEE)、生态系统呼吸(Re)的调控机制。结果表明：2009—2016年8年NEE日均值在-2.02—0.88 gC m^-2 d^-1之间,5—9月NEE为负值,表现为碳吸收,雨热同期的6、7、8月是CO₂净吸收最强的时期,平均每月吸收CO₂ 39.85 gC m^-2 month^-1,NEE负值日数约占全年的48%,10月—翌年4月为正值,表现为碳释放,初春3月和秋末11月是CO₂净释放最强的时期;Re日均值为1.69 gC m^-2 d^-1,受季节温度的影响,呈夏季强,冬季弱的态...     

Vegetation and Soil 15N Natural Abundance in Alpine Grasslands on the Tibetan Plateau: Patterns and Implications

The natural abundance of nitrogen (N) stable isotopes (δ¹⁵N) has the potential to enhance our understanding of the ecosystem N cycle at large spatial scales. However, vegetation and soil δ¹⁵N patterns along climatic and edaphic gradients have not yet been fully understood, particularly for high-altitude ecosystems. Here we determined vegetation and soil δ¹⁵N in alpine grasslands on the Tibetan Plateau by conducting four consecutive regional surveys during 2001–2004, and then examined their relationships with both climatic and edaphic variables. Our results showed that both vegetation and soil N in Tibetan alpine grasslands were more ¹⁵N-enriched than global averages. Vegetation δ¹⁵N did not exhibit any significant trend along the temperature gradient, but decreased significantly with an increase in precipitation amount. In contrast, soil δ¹⁵N did not vary with either mean annual temperature or precipitation. Our results also indicated that soil δ¹⁵N exhibited a slight increase with clay content, but decreased with soil carbon:nitrogen ratio. A general linear model analysis revealed that variations in vegetation δ¹⁵N were dominantly determined by climatic variables, whereas soil δ¹⁵N was related to edaphic variables. These results provide clues for potential climatic and edaphic regulations on ecosystem N cycle in these high-altitude regions.     

Current challenges in distinguishing climatic and anthropogenic contributions to alpine grassland variation on the Tibetan Plateau

Quantifying the impact of climate change and human activities on grassland dynamics is an essential step for developing sustainable grassland ecosystem management strategies. However, the direction and magnitude of climate change and human activities in driving alpine grassland dynamic over the Tibetan Plateau remain under debates. Here, we systematically reviewed the relevant studies on the methods, main conclusions, and causes for the inconsistency in distinguishing the respective contribution of climatic and anthropogenic forces to alpine grassland dynamic. Both manipulative experiments and traditional statistical analysis show that climate warming increase biomass in alpine meadows and decrease in alpine steppes, while both alpine steppes and meadows benefit from an increase in precipitation or soil moisture. Overgrazing is a major factor for the degradation of alpine grassland in local areas with high level of human activity intensity. However, across the entire Tibetan Plateau and its subregions, four views characterize the remaining controversies: alpine grassland changes are primarily due to (1) climatic force, (2) nonclimatic force, (3) combination of anthropogenic and climatic force, or (4) alternation of anthropogenic and climatic force. Furthermore, these views also show spatial inconsistencies. Differences on the source and quality of remote sensing products, the structure and parameter of models, and overlooking the spatiotemporal heterogeneity of human activity intensity contribute to current disagreements. In this review, we highlight the necessity for taking the spatiotemporal heterogeneity of human activity intensity into account in the models of attribution assessment, and the importance for accurate validation of climatic and anthropogenic contribution to alpine grassland variation at multiple scales for future studies.