Using digital repeat photography and eddy covariance data to model grassland phenology and photosynthetic CO2 uptake

The continuous and automated monitoring of canopy phenology is of increasing scientific interest for the multiple implications of vegetation dynamics on ecosystem carbon and energy fluxes. For this purpose we evaluated the applicability of digital camera imagery for monitoring and modeling phenology and physiology of a subalpine grassland over the 2009 and 2010 growing seasons.We tested the relationships between color indices (i.e. the algebraic combinations of RGB brightness levels) tracking canopy greenness extracted from repeated digital images against field measurements of green and total biomass, leaf area index (LAI), greenness visual estimation, vegetation indices computed from continuous spectroradiometric measurements and CO₂ fluxes observed with the eddy covariance technique. A strong relationship was found between canopy greenness and (i) structural parameters (i.e., LAI) and (ii) canopy photosynthesis (i.e. Gross Primary Production; GPP). Color indices were also well correlated with vegetation indices typically used for monitoring landscape phenology from satellite, suggesting that digital repeat photography provides high-quality ground data for evaluation of satellite phenology products.We demonstrate that by using canopy greenness we can refine phenological models (Growing Season Index, GSI) by describing canopy development and considering the role of ecological factors (e.g., snow, temperature and photoperiod) controlling grassland phenology. Moreover, we show that canopy greenness combined with radiation use efficiency (RUE) obtained from spectral indices related to photochemistry (i.e., scaled Photochemical Reflectance Index) or meteorology (i.e., MOD17 RUE) can be used to predict daily GPP.Building on previous work that has demonstrated that seasonal variation in the structure and function of plant canopies can be quantified using digital camera imagery, we have highlighted the potential use of these data for the development and parameterization of phenological and RUE models, and thus point toward an extension of the proposed methodologies to the dataset collected within PhenoCam Network.     

Response of the carbon cycle in sub-arctic black spruce forests to climate change: Reduction of a carbon sink related to the sensitivity of heterotrophic respiration

Assessment of the response of the carbon cycle to climate change is important for predicting future climate, especially in northern high latitudes, where the warming may be highly pronounced. In this paper, three years of data measured by the eddy covariance method were used to verify the carbon dynamics of a black spruce forest in interior Alaska with the BIOME-BGC model. The model was modified and calibrated to the cold regions. The calibrated model simulation satisfactorily performed reproduction of the observed net ecosystem exchange (NEE), gross primary productivity (GPP), and ecosystem respiration (RE). The daily output of the model also correlated with field-based estimates of NEE (42%), GPP (89%), and RE (89%). The model simulation showed that the heterotrophic respiration occupied a substantial portion of total carbon exchange, and that inter-annual variation of the heterotrophic respiration determined the sink/source strength of carbon. According to our simulation, the black spruce forest in interior Alaska was a net carbon sink between 1949 and 2005, and the trend of the projected sink strength was not obvious in the recent climate warming. The model analysis suggested that the change in the seasonal weather pattern strongly affected the carbon balance. A potentially increased carbon sink due to future warming climate and elevated CO₂ will be at least partly offset by a concurrent increase of VPD, since the response of the stomatal conductance to increased VPD could increase the soil moisture, and thus stimulate heterotrophic respiration, rather than net primary productivity. The results of this study should be useful to forecast the response of the carbon cycle to warming in black spruce forests of interior Alaska.     

耦合植被物候的新疆绿洲土壤盐分信息挖掘

随着土壤环境问题涉及的尺度日趋增大，小区域斑块化盐渍化信息的提取难以了解土壤环境总体的变化趋势。本文以野外监测的南北疆典型绿洲区域——渭库绿洲和艾比湖流域为分析靶区，通过实测数据建立土壤–环境关系，并通过MODIS EVI数据反演得到植被物候特征，耦合植被物候、植被指数、盐度指数、地表温度和地形参数作为随机森林(random forest, RF)模型的输入因子，预测新疆绿洲区域土壤盐分含量信息并绘制土壤盐分空间分布图。结果表明：通过深入挖掘植被物候信息，物候参数在预测土壤盐分方面具有较高的相对重要性，代表生物积累量的LSI和SSI参数表征土壤盐渍化的能力较强，优于其他几个物候参数。耦合物候参数后土壤盐分信息预测精度明显提高，决定系数R~2从0.53提升到0.61。经模型反复迭代进一步筛选出适合研究区的23个环境参数，大幅提升了预测精度(R~2=0.73, RMSE=5.19, MAE=3.59)。从得到的盐渍化空间分布特征来看，新疆绿洲大部分区域分布的是非盐渍化土和轻盐渍化土，且普遍分布在绿洲内部，中度及以上盐渍化土多分布在绿洲外围，总体盐渍化水平依次为：伊犁平原<北疆绿洲&l...     

Climatic controls and ecosystem responses drive the inter-annual variability of the net ecosystem exchange of an alpine meadow

Seven years of continuous eddy covariance measurements at an alpine meadow were used to investigate the impacts of climate drivers and ecosystem responses on the inter-annual variability (IAV) of the net ecosystem exchange (NEE). The annual cumulative value of NEE was positive (source) in 2003, 2005 and 2009 (50, 15 and 112 g m⁻² respectively) and negative (sink) in 2004, 2006, 2007 and 2008 (29, 75, 110 and 28 g m⁻² respectively). The IAV of carbon dioxide fluxes builds up in two phenological phases: the onset of the growing season (triggered by snow melting) and the canopy re-growth after mowing. Respiratory fluxes during the non-growing season were observed to increase IAV, while growing season uptake dampened it. A novel approach was applied to factor out the two main sources of IAV: climate drivers’ variability and changes in the ecosystem responses to climate. Annual values of carbon dioxide fluxes were calculated assuming (a) variable climate and variable ecosystem response among years, (b) variable climate and constant ecosystem response and (c) constant climate and variable ecosystem response. The analysis of flux variances calculated under these three assumptions indicates the occurrence of an important negative feedback between climate and ecosystem responses. Due to this feedback, the observed IAV of NEE is lower than one would expect for a given climate variability, because of the counteracting changes in ecosystem responses. This alpine meadow therefore demonstrates the ability to acclimatise and to limit the IAV of carbon fluxes induced by climate variability.     

2001-2020年南盘江流域植被物候时空变化及其对气候的响应

植被物候对气候敏感易观测,是观测生态变化与全球气候变化的重要指标,研究植被物候变化及其对气候的响应对于了解全球气候变化与植被之间的复杂关系具有重要意义。为揭示近20年来南盘江流域植被物候时空变化及其对气候的响应特征,基于2001—2020年增强型植被指数(MOD13Q1-EVI),运用S-G滤波、动态阈值法、Sen斜率分析法、相关性分析法等方法,获取了南盘江流域植被物候参数,分析其时空分布特征,利用偏相关系数分析了气候变化对SOS,EOS的影响。结果表明:(1)2001—2020年,南盘江流域物候特征变化较大,SOS(Start of the growing season)呈提前趋势,EOS(End of the growing season)和LOS(Length of the growing season)呈推迟延长趋势,物候变化具有空间异质性,不同子流域植被物候存在较大差异。(2)植被物候变化受到地形的影响,EOS随海拔升高,结束时间提前; SOS随海拔变化的规律与LOS的规律相反,都存在1 000 m,2 000 m和2 600 m分界线。(3)南盘江流域SOS变化与气温、降水呈正相关关系,EOS与气温呈正相关关系、与降水呈负相关关系。该研究可为南盘江流域的生态环境保护与植被资源可持续发展提供科学依据。     

陕西黄土台塬区植被-土壤系统碳收支估算

[目的]为土地利用的低碳战略提供决策依据。[方法]基于GIS和RS分析与处理NDVI数据和Landsat遥感影像数据源,采用CASA光能利用模型对陕西黄土台塬区植被—土壤系统的碳收支进行估算。[结果]1990—2010年陕西黄土台塬区植被—土壤系统碳源效应明显。其中,咸阳台塬区、宝鸡台塬区和西安台塬区有所减弱,但渭南台塬区和铜川台塬区有所增强。研究时段内,陕西黄土台塬区的高碳源区的面积由38.22%缩减为21.13%,中碳源区由57.40%扩大为67.71%,低碳塬区由3.43%扩大为7.76%;碳汇区由0.96%扩大为3.40%。[结论]1990—2010年该区建设用地占用耕地、林地、草地,土地利用结构变化不利于碳固定,提高复种指数有利于降低植被—土壤系统的碳排放。     

Assessing net ecosystem carbon exchange of U.S. terrestrial ecosystems by integrating eddy covariance flux measurements and satellite observations

More accurate projections of future carbon dioxide concentrations in the atmosphere and associated climate change depend on improved scientific understanding of the terrestrial carbon cycle. Despite the consensus that U.S. terrestrial ecosystems provide a carbon sink, the size, distribution, and interannual variability of this sink remain uncertain. Here we report a terrestrial carbon sink in the conterminous U.S. at 0.63 pg C yr⁻¹ with the majority of the sink in regions dominated by evergreen and deciduous forests and savannas. This estimate is based on our continuous estimates of net ecosystem carbon exchange (NEE) with high spatial (1 km) and temporal (8-day) resolutions derived from NEE measurements from eddy covariance flux towers and wall-to-wall satellite observations from Moderate Resolution Imaging Spectroradiometer (MODIS). We find that the U.S. terrestrial ecosystems could offset a maximum of 40% of the fossil-fuel carbon emissions. Our results show that the U.S. terrestrial carbon sink varied between 0.51 and 0.70  pg C yr⁻¹ over the period 2001-2006. The dominant sources of interannual variation of the carbon sink included extreme climate events and disturbances. Droughts in 2002 and 2006 reduced the U.S. carbon sink by ∼20% relative to a normal year. Disturbances including wildfires and hurricanes reduced carbon uptake or resulted in carbon release at regional scales. Our results provide an alternative, independent, and novel constraint to the U.S. terrestrial carbon sink.     

灌水和放牧对荒漠草原生态系统碳交换的影响

依托内蒙古乌兰察布市四子王旗境内短花针茅荒漠草原的10a放牧强度（不放牧、轻度放牧、重度放牧）试验平台,于2012年和2013年在荒漠草原生长季（5-10月）,设计放牧嵌套灌水的控制试验,以不灌水和不放牧为对照,研究净生态系统碳交换速率（net ecosystem exchange,NEE）、净生态系统呼吸速率（ecosystem respiration,RE）和总生态系统碳交换速率（gross ecosystem exchange,GEE）对不同水分条件（灌水、非灌水）和不同放牧强度的响应。结果表明：（1）荒漠草原生长季表现为净碳吸收（NEE和GEE<0）。不放牧条件下灌水通过增加土壤体积含水量（soli volumetric water content,Vwc）间接增加RE和生态系统碳吸收（P<0.01）,NEE、RE和GEE对灌水带来的土壤水分变化较为敏感。（2）不灌水条件下,NEE、RE和GEE在不同放牧强度处理中的差异并不显著,但8月后放牧带来的ΔNEE和ΔGEE正偏差显著大于8月前的负偏差（P<0.001）。（3）灌水和放牧的交互作用对NEE、RE和GEE均无显著影响,但放牧条件下增加Vwc可以增加生态系统碳吸收和呼吸作用。区分放牧强度时,RE和GEE对Vwc的线性回归模型斜率的绝对值在不放牧时最大（分别为0.26和0.61）,说明放牧导致荒漠草原生态系统碳交换对土壤水分变化的敏感性减小。     

北京植被净初级生产力对物候变化的响应

植被净初级生产力（Net Primary Productivity,NPP）对物候的响应是全球气候变化背景下的重要研究内容,气候变化对植物物候与NPP的影响仍需明了,物候的时空变异规律更需深入探讨。该研究基于2001-2020年MODIS归一化植被指数（Normalized Difference Vegetation Index,NDVI）数据提取北京植被物候信息,利用CASA（Carnegie-Ames-Stanford-Approach）模型模拟NPP,通过线性回归、趋势分析和结构方程模型等方法,阐明NPP与物候时空变化特征,探究气象因素和物候变化对NPP的影响。结果显示：1）2001-2020年超过70 %的区域出现生长季开始（Start of Growing Season,SOS）逐渐提前,平均每年提前0.53 d。超过90%的区域生长结束期（End of Growing Season,EOS）逐渐推迟,平均每年推迟0.51 d。2）SOS提前和生长季（Length of Growing Season,LOS）延长均对NPP增长产生显著影响（P<0.05）。SOS每提前1 d,NPP增长3.74 g/m2;LOS每延长1 d,NPP增长2.56 g/m2 。秋季EOS推迟对NPP变化影响不显著。3）春季和秋季,气候通过改变物候（SOS、EOS）对NPP的间接影响大于直接影响,而夏季温度和降雨对NPP的直接影响更大。该研究表明春季物候变化是NPP年际变异的重要驱动因子,春季物候提前导致NPP年总量增加。研究结果是都市区植被生产力如何响应气候变化认识的重要补充。     

Eight years of continuous carbon fluxes measurements in a Portuguese eucalypt stand under two main events: Drought and felling

This paper reports on results from eddy covariance measurements of carbon uptake and evapotranspiration in the eucalypt site of Espirra in Southern Portugal (38°38′N, 8°36′W). This site was included in the “Carboeurope” European network and is part of a 300 ha eucalypt forest, with about 1100 trees ha⁻¹, intensively managed as a coppice for pulp production and characterized by a 12-month annual growing period. The climate is of Mediterranean type with a long term (1961-1990) annual average precipitation of 709 mm and an annual average air temperature of 15.90 °C. During the measurement period (2002-2009) two main events took place, which changed the annual sink pattern of the forest: a drought period of two years (2004-2005) and a tree felling (October and November 2006). We analyzed the daily, seasonal and inter-annual variation of carbon uptake and evapotranspiration, and their relationships with the events and the variability of the main meteorological variables. Before the felling, annual net ecosystem exchange (NEE) increased from −865.56 g C m⁻² in 2002 to −356.64 g C m⁻² in 2005 together with a deep decrease in rainfall from 748 mm in 2002 to 378.58 mm and 396.64 mm in 2004 and 2005, respectively. For the same period, seasonal patterns of carbon uptake showed maximum values in April and decreased in July-August. The eucalypt stand recovered its carbon sink ability since June 2007 and had a NEE of −209.01 g C m⁻² in 2009. After the felling, the carbon uptake occurred from mid-February to mid-October, following an almost opposite pattern than that of the trees in the term of their productive cycle. A quantitative approach using generalized estimating equations (GEEs) was made for the period before the felling to relate monthly NEE and GPP with accumulated photosynthetic active radiation, water vapour pressure and precipitation. In conclusion, our study showed the relevant effects of water stress and anthropogenic interventions in the daily, seasonal and annual patterns of carbon uptake, under a context of good environmental conditions for carbon sequestration.     

在纤毛狼尾草为主的草原上植物叶片以及生态系统的气体交换对夏季降水的响应特征

Sporadic rain events that occur during summer play an important role in the initiation of biological activity of semi-arid grasslands.To understand how ecosystem processes of a buffel grass(Cenchrus ciliaris L.)-dominated grassland respond to summer rain events,an LI 6 400 gas exchange system was used to measure the leaf gas exchange and plant canopy chambers were used to measure net ecosystem CO2exchange(NEE) and ecosystem respiration(Reco), which were made sequentially during periods before rain(dry) and after rain(wet). Gross ecosystem photosynthesis(GEP) was estimated from NEE and Reco fluxes, and light use efficiency parameters were estimated using a rectangular hyperbola model. Prior to the monsoon rain, grassland biomass was non-green and dry exhibiting positive NEE(carbon source) and low GEP values during which the soil water became increasingly scarce. An initial rain pulse(60 mm) increased the NEE from pre-monsoon levels to negative NEE(carbon gain) with markedly higher GEP and increased green biomass. The leaf photosynthesis and leaf stomatal conductance were also improved substantially. The maximum net CO2uptake(i.e.,negative NEE) was sustained in the subsequent period due to multiple rain events. As a result, the grassland acted as a net carbon sink for 20 d after first rain. With cessation of rain(drying cycle), net CO2 uptake was reduced to lower values. High sensitivity of this grassland to rain suggests that any decrease in precipitation in summer may likely affect the carbon sequestration of the semiarid ecosystem.     

Carbon stocks and carbon fluxes from a 10‐year prescribed burning chronosequence on a UK blanket peat

Prescribed burning is a common land management technique in many areas of the UK uplands. However, concern has been expressed at the impact of this management practice on carbon stocks and fluxes found in the carbon‐rich peat soils that underlie many of these areas. This study measured both carbon stocks and carbon fluxes from a chronosequence of prescribed burn sites in northern England. A range of carbon parameters were measured including above ground biomass and carbon stocks; net ecosystem exchange (NEE), net ecosystem respiration (R_eco) and photosynthesis (P_g) from closed chamber methods; and particulate organic carbon (POC). Analysis of the CO₂ data showed that burning was a significant factor in measured CO₂ readings but that other factors such as month of sampling explained a greater proportion of the variation in the data. Carbon budget results showed that whereas all the plots were net sources of carbon, the most recent burn scars were smaller sources of carbon compared with the older burn scars, suggesting that burning of Calluna‐dominated landscapes leads to an ‘avoided loss’ of carbon. However, this management intervention did not lead to a transition to a carbon sink and that for carbon purposes, active peat‐forming conditions are desirable.     

江苏省农田植被净碳汇时空格局分析

科学地开展区域农田植被净碳汇及其时空格局研究,对采取合理的农业管理措施,达到农田生态系统减源增汇目的具有重要意义。以江苏省13个地级市为研究单元,采用2000—2015年农作物产量、农田投入等相关统计数据估算江苏省农田植被净碳汇,综合运用重心移动、空间自相关分析等方法分析其时空格局。结果表明:(1)2000—2015年,苏北及苏中地区农田植被净碳汇总量呈波动上升趋势,苏南地区总体呈下降趋势;江苏省各市农田植被净碳汇强度总体呈上升趋势;(2)江苏省农田植被净碳汇重心基本分布在江苏省宝应县境内,呈多次振荡式向西北方向迁移;(3)江苏省农田植被净碳汇表现为"北热南冷"相对稳定的空间格局,沿海和内陆地区的农田植被净碳汇水平明显高于沿江地区。总体而言,江苏省农田植被表现为较强的碳汇功能,苏北和苏中地区是未来江苏省农田植被净碳汇的主要潜力区,应根据地区差异,通过制定差别化的农业管理措施发挥农田生态系统整体的生态屏障作用。     

The REFLEX project: Comparing different algorithms and implementations for the inversion of a terrestrial ecosystem model against eddy covariance data

We describe a model-data fusion (MDF) inter-comparison project (REFLEX), which compared various algorithms for estimating carbon (C) model parameters consistent with both measured carbon fluxes and states and a simple C model. Participants were provided with the model and with both synthetic net ecosystem exchange (NEE) of CO₂ and leaf area index (LAI) data, generated from the model with added noise, and observed NEE and LAI data from two eddy covariance sites. Participants endeavoured to estimate model parameters and states consistent with the model for all cases over the two years for which data were provided, and generate predictions for one additional year without observations. Nine participants contributed results using Metropolis algorithms, Kalman filters and a genetic algorithm. For the synthetic data case, parameter estimates compared well with the true values. The results of the analyses indicated that parameters linked directly to gross primary production (GPP) and ecosystem respiration, such as those related to foliage allocation and turnover, or temperature sensitivity of heterotrophic respiration, were best constrained and characterised. Poorly estimated parameters were those related to the allocation to and turnover of fine root/wood pools. Estimates of confidence intervals varied among algorithms, but several algorithms successfully located the true values of annual fluxes from synthetic experiments within relatively narrow 90% confidence intervals, achieving >80% success rate and mean NEE confidence intervals <110 gC m⁻² year⁻¹ for the synthetic case. Annual C flux estimates generated by participants generally agreed with gap-filling approaches using half-hourly data. The estimation of ecosystem respiration and GPP through MDF agreed well with outputs from partitioning studies using half-hourly data. Confidence limits on annual NEE increased by an average of 88% in the prediction year compared to the previous year, when data were available. Confidence intervals on annual NEE increased by 30% when observed data were used instead of synthetic data, reflecting and quantifying the addition of model error. Finally, our analyses indicated that incorporating additional constraints, using data on C pools (wood, soil and fine roots) would help to reduce uncertainties for model parameters poorly served by eddy covariance data.     

模拟降水变化下黄土丘陵区草地群落特征及其物候期响应

在全球气候变化背景下,黄土高原降水格局呈现出季节波动增强和极端降水事件增加趋势,降水格局变化势必会引起草地群落特征和物候期发生重要改变。为研究黄土丘陵区草地生态系统对降水改变的响应,选取黄土丘陵区退耕草地为研究对象,连续定位观测自然恢复小区不同降雨梯度下(0,±20%,±40%,±60%)物候动态变化,探究了物候期和群落结构对降水变化的响应状况。结果表明:(1)不同降水条件下群落物种组成及丰度存在明显差异,与对照组相比,增雨与减雨均提高了物种的种类。增雨60%群落种类最多,为18种。(2)降水梯度递增处理下,群落盖度呈先增加后降低趋势,地上生物量随呈单峰趋势。增雨20%处理和减雨20%处理均能够显著提高群落植被盖度和地上生物量,地上生物量在增雨处理下明显高于减雨处理。(3)丰富度和Shannon-Wiener指数总体上都呈增加趋势,减雨20%,40%,60%均使Pielou均匀度指数降低。(4)不同降水条件对物种物候期的影响显著(p<0.05),群落返青期、繁殖期和和枯黄期在减雨处理下显著提前,而增雨处理则使其繁殖期和枯黄期发生推迟。综上,群落物种组成、植被盖度和地上生物量在不同降水处理下表现出显著差异,而物种多样性对降水变化响应不敏感。     

Partitioning carbon fluxes in a Mediterranean oak forest to disentangle changes in ecosystem sink strength during drought

Net carbon flux partitioning was used to disentangle abiotic and biotic drivers of all important component fluxes influencing the overall sink strength of a Mediterranean ecosystem during a rapid spring to summer transition. Between May and June 2006 we analyzed how seasonal drought affected ecosystem assimilation and respiration fluxes in an evergreen oak woodland and attributed variations in the component fluxes (trees, understory, soil microorganisms and roots) to observations at the ecosystem scale. We observed a two thirds decrease in both ecosystem carbon assimilation and respiration (R_eco) within only 15 days time. The impact of decreasing R_eco on the ecosystem carbon balance was smaller than the impact of decreasing primary productivity. Flux partitioning of GPP and R_eco into their component fluxes from trees, understory, soil microorganisms and roots showed that declining ecosystem sink strength was due to a large drought and temperature-induced decrease in understory carbon uptake (from 56% to 21%). Hence, the shallow-rooted annuals mainly composing the understory have a surprisingly large impact on the source/sink behavior of this open evergreen oak woodland during spring to summer transition and the timing of the onset of drought might have a large effect on the annual carbon budget. In response to seasonal drought R_eco was increasingly dominated by respiration of heterotrophic soil microorganisms, while the root flux was found to be of minor importance. Soil respiration flux decreased with drought but its contribution to total daily CO₂-exchange increased by 11.5%. This partitioning approach disentangled changes in respiratory and photosynthetic ecosystem fluxes that were not apparent from the eddy-covariance or the soil respiration data alone. By the novel combination of understory vs. overstory carbon flux partitioning with soil respiration data from trenched and control plots, we gained a detailed understanding of factors controlling net carbon exchange of Mediterranean ecosystems.     

基于物候参数和面向对象法的濒海生态脆弱区植被遥感提取

用遥感数据快速准确地提取植被信息对生态环境监测和发展气候模型具有基础性和关键性的意义。由于中国黄河三角洲地区植被类型插花分布,传统遥感提取方法精度较低。该文选取近代黄河三角洲典型生态脆弱区为研究区,基于MODIS和LANDSAT8数据,通过提取研究区的物候参数和不同分辨率遥感影像的融合处理,根据植被类型斑块大小确定分割尺度,根据典型植被类型的物候特征、光谱和空间等特征值构建分类规则,利用分区策略自上而下进行面向对象分类。结果表明,该方法总体精度为80.75%,Kappa系数0.79,高于传统物候和面向对象分类方法。广生态幅的棉田与其他植被的光谱混淆是传统面向对象分类方法植被分类精度低的主要原因,利用物候参数进行植被分区能规避棉田和自然植被的光谱混淆,有利于对植被类型的区分。分类结果与当地植被分布情况相符,可以用于研究区植被类型的精细提取。     

基于拟合物候参数的植被遥感决策树分类

针对目前遥感分类较少利用植被物候特征参数的现状,该研究以内蒙古自治区额尔古纳、根河两市作为试验区,基于2012年根河土地覆盖数据,采用双Logistic函数拟合的方法对跨度范围为2011年7月下旬至2013年7月下旬的时间序列MODIS EVI数据进行植被物候参数的分析与提取,并依据物候参数特征构建决策树对试验区土地覆盖进行分类。研究表明,不同植被的物候有较明显的特征,森林、草原与作物的生长季开始时间依次滞后,作物的生长季最短,森林与草原生长季基本持平;利用植被物候特征参数进行决策树分类,作物和森林2种植被类型取得较好分类效果,分类的总体精度达到73.67%,优于该区域MODIS土地覆盖产品的总体精度(66.08%)。该研究可为呼伦贝尔地区生态环境评价和农、牧、林业生产活动提供一定的参考。     

Influences of recovery from clear-cut, climate variability, and thinning on the carbon balance of a young ponderosa pine plantation

From 1999 to 2002, the variations in carbon flux due to management practices (shrub removal, thinning) and climate variability were observed in a young ponderosa pine forest originated from clear-cutting and plantation in 1990. These measurements were done at the Blodgett Forest Ameriflux site located in the Sierra Nevada Mountains of California. Thinning in spring 2000 decreased the leaf area index (LAI) by 34% and added 496 g C m⁻² of wood and leaf debris at the soil surface. Total ecosystem respiration was not significantly affected by thinning (1261 g C m⁻² in 1999 and 1273 g C m⁻² in 2000), while canopy photosynthesis decreased by 202 g C m⁻². As a result the ecosystem shifted from a net sink of CO₂ in 1999 (−201 g C m⁻²) to a small net source in 2000 (13 g C m⁻²). Woody and leaf debris resulting from thinning only accounted for maximum 1% and 7% of the total respiration flux, respectively. Thinning did not affect the relative proportion of the different components of respiration to an observable degree. Low soil water availability in summer 2001 and 2002 decreased the proportion of soil respiration to the total respiration. It also imposed limitations on canopy photosynthesis: as a result the ecosystem shifted from a sink to a source of carbon 1 month earlier than in a wetter year (1999). The leaf area index and biomass of the stand increased rapidly after the thinning. The ecosystem was again a sink of carbon in 2001 (−97 g C m⁻²) and 2002 (−172 g C m⁻²). The net carbon uptake outside the traditionally-defined growing season can be important in this ecosystem (NEE = −50 g C m⁻² in 2000), but interannual variations are significant due to differences in winter temperatures.