Knowledge Resource Center for Ecological Environment in Arid Area
| 基于遥感的旱区生态系统多稳态刻画及其退化早期信号研究 | |
| 其他题名 | Studies on Characterization of Alternative Stable States and Early Signals of Degradation in Dryland Ecosystems Based on Remote Sensing |
| 赵颜创 | |
| 出版年 | 2018 |
| 学位类型 | 博士 |
| 导师 | 王心源 |
| 学位授予单位 | 中国科学院大学 |
| 中文摘要 | 旱区生态系统是陆地生态系统的重要组成部分，由于降雨稀少、蒸发强烈、植被覆盖度低而成为生态脆弱区，极易受到气候变化和人类活动的影响，面临着各种生态环境问题，土地退化是其中最严重的问题之一。旱区生态系统的迟滞现象使得退化恢复非常困难，许多学者集中于研究早期信号来应对旱区的土地退化。当前大多数早期信号的研究是在生态系统多稳态的理论框架下进行的，尽管计算机模拟的理论模型研究表明旱区生态系统具有多稳态的特征，稳态之间会发生非线性的突变转换，然而基于真实旱区生态系统的多稳态研究案例却相对缺乏。在过去的几十年里，利用遥感指标揭示旱区土地退化过程机制的研究受到了越来越多的关注。遥感的优势在于其能快速地以较低成本获取长时间、大范围表征生态系统状态的参数，这为研究真实旱区生态系统的多稳态提供了可能。Berdugo et al.（2017）基于地面样点数据，对比分析了植被幂律分布指数、植被覆盖度和生态系统多功能指数（一种综合反映土壤碳氮磷循环的指数）在干旱生态系统多稳态和早期预警方面的适用性，发现只有生态系统多功能指数能反映干旱生态系统的双稳态特征。地面测量的生态系统多功能指标往往需要大量资金、劳力和时间，很难在大尺度范围内应用和推广。因此，本学位论文结合卫星遥感在大尺度研究中的技术优势，首先以地面生态系统多功能数据为基准，筛选适用于旱区生态系统多稳态研究的遥感指标。并进一步以非洲Sahel地区、亚洲蒙古高原为热带和温带旱区的案例地，进行生态系统多稳态刻画及其退化早期信号辨识的对比研究。得出以下主要结论：（1）植被指数和降雨利用率这两个常用遥感指标均与生态系统多功能指数存在着显著正相关关系，但遥感植被指数与生态系统多功能指数的关系受降雨的影响很大。与植被指数相比，降雨利用率受降雨影响较弱，因而与生态系统多功能的关系更密切。但需要注意的是，在使用遥感降雨利用率作为生态系统多功能的指标时，须满足降雨利用率的前提假设，即（1）降雨和植被净初级生产力（或其代理指标）之间存在着一个正的显著线性关系；（2）降雨利用率和降雨的线性关系接近于0。除此之外，降雨利用率与土壤碳、氮循环指数具有显著相关性，而与磷循环指数相关性不显著。（2）遥感地表反照率与生态系统多功能性之间存在着负相关关系。大体上，生态系统多功能指数与黑空地表反照率之间的相关性强于其与白空地表反照率的相关性。碳、氮循环指数与可见光黑空反照率的相关性强于其与近红外黑空反照率的相关性；而磷循环指数与近红外黑空反照率的相关性强于其与可见光黑空反照率的相关性。两种植被指数（归一化植被指数和增强型植被指数）均与生态系统多功能指数和碳氮循环指数呈显著正相关关系，并且相关强度要高于地表反照率与生态系统功能的相关性。但需要注意的是，植被指数与磷循环指数间的相关性却不显著。在MODIS的7个窄带黑空地表反照率中，红波段黑空反照率（0.620-0.670 μm, 中心波长0.645 μm）与生态系统多功能指数和碳、氮循环指数的相关性最强。而第七波段（2.105-2.155 μm, 中心波长2.13 μm）与磷循环指数相关性最强。（3）非洲Sahel（热带旱区）的案例研究结果表明：三种（短波、近红外和可见光）遥感黑空地表反照率均显示 Sahel地区存在两种稳态，即低反照率稳态和高反照率稳态。高、低反照率大致分别对应着低、高植被覆盖度，但植被覆盖度随着干旱梯度呈现连续变化，不能刻画研究区的多稳态特征现象。黑空地表反照率的两种稳态在干旱度0.67和0.81之间共同出现，高、低反照率稳态分别对应的植被类型是稀树草地和无树草地。当干旱度达到0.81时，系统的稳态表现出一个急剧的、不连续的突变上升，伴随着植被覆盖度由~67%迅速降为~6%，暗示当干旱度超过0.81时，树态会发生退化，转为草态。在反照率稳态共现区内，当干旱度增加至0.74时，两种反照率稳态的势能量发生了显著变化，即低反照率稳态势能量开始大于高反照率稳态势能量，该显著变化具有作为稳态间临界转换（即退化）早期信号的潜力，据此提取了研究范围内的脆弱区，即易发生土地退化的区域，提取结果与其他学者利用不同方法得到的脆弱区相一致。（4）蒙古高原（温带旱区）的案例研究结果表明：蒙古高原短波白空地表反照率的概率密度分布具有双峰特征，表明该地区可能存在高、低两种白空反照率模式。势分析模型结果显示在干旱度为0.35至0.45之间，有高、低两种白空地表反照率稳态持续性同时出现，高、低白空反照率稳态所对应的植被类型分别是草地和针叶林。反照率双稳态共现区内，当干旱度超过0.38后，低反照率稳态的势能量开始大于高反照率稳态的势能量，系统倾向于向高反照率稳态转换，暗示尽管针叶林可以在大于0.38的干旱度与草地共存，但在这样的干旱水平下，草地比针叶林更适合生存。当干旱度超过0.45时，地表反照率有一个不连续的突变增加，暗示针叶林有可能发生退化，转为草地。（5）遥感地表反照率在Sahel和蒙古高原均可以揭示研究区的多稳态。但在两个地区，稳态共现区和稳态临界转换的阈值却明显不同。这很有可能与温带旱区在干旱度较低的情况下，无树草原态就大片存在有关。此外，在蒙古高原和Sahel的稳态共现区内，均出现了低反照率稳态的势能量大于高反照率稳态的势能量的显著变化，暗示系统倾向于由低稳态反照率（对应的植被类型为树态）向高反照率稳态（对应着植被类型为草态）转换。因此无论是蒙古高原还是Sahel地区，低反照率稳态的势能量开始大于高反照率稳态的势能量均有潜力作为系统临界转换（退化）的早期信号。总体而言，本学位论文为真实旱区生态系统双稳态的存在提供了直接证据。通过对比不同气候背景下旱区的多稳态共现区和稳态间临界转换阈值的异同，为揭示多稳态产生机制和设计具有普遍适用性的生态系统退化早期信号提供理论基础，从而为政府和管理部门在应对旱区土地退化提供决策依据与科学支持。 |
| 英文摘要 | Drylands are critical terrestrial environments, considered to be Earth's most abundant biome and known for their extensive global. They are regarded as vulnerable areas to climatic variations and human activities at global scale because of low precipitation, high evapotranspiration and low vegetation cover. Land degradation has become one of the most serious social-economic-environmental issues in more than one hundred countries of the world. Considering the difficulty of reversion and the low restoration success of many dry degraded areas, many research efforts have focused on developing useful indicators to detect it at the early stages. The early signals of land degradation has often been studied in the framework of critical transitions. According to this theory, drylands exhibit two stable states (non-degraded vs. desertified) and the transition between them, triggered by disturbances such as increased aridity and grazing pressure, is abrupt and sudden, rather than gradual and continuous. Multiple theoretical studies have suggested the existence of alternative stable states and of abrupt, non-linear, shifts between them in drylands. However, empirical verification of alternative stable states in the field has been scarce so far. In the past decades, remote sensing has become a valuable tool to obtain parameters characterizing dryland ecosystems and gain more and more attentions because it can provide detailed information quickly about ecosystem state across wide regions and over increasingly long series of years at a low cost. Thus, it is a potential valuable approach to supply a solution for observation of ecosystem state, which is the key access to validate the existence of alternative stable states in real drylands ecosystems.Berdugo et al. (2017) compared the performance of three indicators, i.e., power law range, vegetation cover and ecological multifunctionality index (MUL), to reveal the presence of alternative stable states in dryland based on field data, and he found only ecological multifunctionality index can show two stable states. Therefore, our study firstly employed the ecological multifunctionality index as the reference, to select the remotely sensed indicators which has potential to reveal the alternative stable states. Then, we carried out the case studies on alternative stable states and early signs of degradation in the Sahel (tropical drylands) and Mongolian Plateau (temperate drylands) by indicator derive from remote sensing. The main conclusions are as follows.(1) The linear relationships of EVI (Enhanced Vegetation Index) and RUE (Rain-Use Efficiency, defined as aboveground net primary production (ANPP) divided by rainfall) with MUL are both significant positive. However, RUE explained more variation (about 44%) in EMI than summed EVI (about 32%) did. The results obtained by partial correlation analysis by controlling the rainfall showed that correlation coefficient between summed EVI and MUL, increased about 20%, while correlation coefficient between RUE and MUL increased very slightly (about 3%). Similar results were also found by using the Normalized Difference Vegetation Index (NDVI). These facts indicated that both remotely sensed vegetation indices and RUE could be indicators of ecological multifunctioning. However, RUE was better due to its robustness to rainfall. Also, we must take care that the core assumptions related to the RUE should be fulfilled before using it as an indicator, i.e., (1) a significant positive linear correlation between precipitation and the ANPP proxy applied, (2) a near-zero correlation between RUE and precipitation. The relationships between RUE and nutrient cycles showed that in comparison to phosphorus cycle (PCY), the carbon and nitrogen cycles (CCY and NCY) had an apparent higher weight in determining the relationship between RUE and MUL.(2) There was a negative relationship between land surface albedo and multifunctionality. Black-sky albedo had a stronger correlation with multifunctionality than white-sky albedo. Visible black-sky albedo showed the strongest correlation with multifunctionality (MUL, -0.314), as well as with functions related to carbon (-0.216) and nitrogen cycling (-0.410), while near-infrared (-0.339) and shortwave black-sky albedo (-0.325) showed stronger correlations with functions related to phosphorus cyclingthan visible black-sky albedo (-0.233) did. VIs showed significant positive correlations with MUL, CCY, and NCY, and the magnitudes were higher than those observed between albedo metrics and the multifunctionality indices. However, VIs were not correlated with PCY, which had significant correlations with both shortwave and near-infrared albedo. Among the 7 narrowband albedos from MODIS, the red band albedo has the best ability to indicate MUL, CCY and NCY, while the narrowband albedo near 2.13 μm has the best correlation with PCY. Though the magnitudes of the correlations observed were not high, which may result from the wide variability in soil and vegetation types in our dataset, our findings indicate that remotely sensed albedo correlates to multifunctionality, which has been linked to alternative states in global drylands.(3) In the case study of the Sahel (tropical drylands), three remotely sensed albedo (shortwave, visible and near-infrared black-sky) metrics the albedo metrics evaluated showed two stable states (low and high) in this area, and exhibited an abrupt and discontinuous increase with increases in aridity. The two albedo states co-occurred along aridity values ranging from 0.67 to 0.82. Low and high albedo stable states were associated with high and low vegetation cover, respectively, which showed a rapid but continuous decrease along these aridity values. By identifying regions on the edge between high and low albedo states, we also identified vulnerable areas within the Sahel where critical transitions may occur in the future as a consequence of forecasted increases in aridity due to climate change. Our findings indicate that remotely sensed albedo indices have potential to assess the vulnerability of sites to experience discontinuous transitions, which might be linked to the onset of dryland desertification.(4) In the case study of Mongolian Plateau (temperate drylands), probability density distribution of shortwave white-sky albedo showed double peaks, which indicated there were possible two albedo modes in the study area. Potential analysis model showed there were two white-sky albedo states (low and high) co-occurred persistently in aridity range from 0.35 to 0.45. The low and high albedo stated were represented by coniferous forest and grass, respectively. When aridity researched 0.38, potential energy of the high albedo state became less than that of low albedo state, and it implied the low albedo state tended to tip into the high albedo states. The low albedo states exhibited an abrupt and discontinuous increase with when aridity exceeded 0.45. This implied at this aridity, coniferous forest may started to degrade to grass.(5) Land surface albedo derived from remote sensing can reveal two stable states in both Sahel and Mongolian Plateau. But the aridity range during which two albedo states co-occurred and the threshold at which critical transition occurred were different in the two areas. In additions, in both Sahel and Mongolian Plateau, potential energy of the high albedo state became less than that of low albedo state in the aridity range during which two albedo states co-occurred. It implied the low albedo state tended to tip into the high albedo states, accompanied by trees degraded to grass. Therefore, the turnover of the potential energy have the potential to be an early-warning signal of critical transitions (i.e., degradation) in drylands.Totally, this study provids direct proofs for the existence of alternative stable states in real dryland ecosystems. The results are helpful for uncovering the mechanism of appearance of alternative stable states and developing general early-warning signals of critical transitions by comparative studies in drylands under contrasting climate. This study can provide decision-making basis and scientific support for the government and management departments when combating dryland degradation. |
| 中文关键词 | 多稳态 ; 早期信号 ; 旱区 ; 生态系统退化 ; 遥感 |
| 英文关键词 | alternative stable states，early signals，drylands，ecosystem degradation，remote sensing |
| 语种 | 中文 |
| 国家 | 中国 |
| 来源学科分类 | 地图学与地理信息系统 |
| 来源机构 | 中国科学院遥感与数字地球研究所 |
| 资源类型 | 学位论文 |
| 条目标识符 | http://119.78.100.177/qdio/handle/2XILL650/288201 |
| 推荐引用方式 GB/T 7714 | 赵颜创. 基于遥感的旱区生态系统多稳态刻画及其退化早期信号研究[D]. 中国科学院大学,2018. |
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