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通常认为在高海拔地区，尤其是林线地区，由于土壤含水量以及降水明显增加，水分不是最重要的限制因子，相反较低的CO2浓度及极端的寒冷限制了植物的生长和繁殖，限制了植物在特定区域的分布。最近几年林线地区水分胁迫对植物生长以及分布的影响已经受到了广泛的注意，并得到相关野外实验数据的支持。通过研究林线不同植物种是否存在水分胁迫，有可能为林线植物生长及物种分布的限制因素提供新的解释。植物的稳定碳同位素组成（δ13C值）包含了大量的环境信息，反映植物长期的水分利用效率，为研究植物与环境之间的相互作用关系以及植物对极端环境的生物学适应提供了一个行之有效的手段，已经广泛地应用于全球变化、植物生态学及相关领域。被称为世界“第三极”的青藏高原因其海拔高而形成独特的气候环境，形成极其脆弱而且对全球变化非常敏感的生态系统，是研究生态系统结构和功能及其对全球变化响应的理想区域。在藏东南地区，由方枝柏和急尖长苞冷杉构成的高山林线是全球海拔最高的林线之一，相对较多的降水以及高海拔的低温环境形成了该地区典型的冷湿气候特征，为揭示植物与环境之间的关系以及植物对全球变化的响应机理提供了理想的天然实验室。本论文拟在色齐拉山口高山林线展开研究，通过测定不同生活型植物的δ13C 值、叶性状及淀粉、非结构性碳含量在海拔、冠层高度以及叶龄间的分布特征，试图从水分利用的角度阐明冷湿气候下高山林线植物的相关生理生态学特性。比较研究不同生活型植物水分利用状况，揭示低温可能导致的水分胁迫在多大程度上解释林线植物叶δ13C值的变化，探讨沿树高变化的水力限制假说（Hydraulic limitation hypothesis）能否部分解释林线交错带乔木物种被低矮的灌木及草本植物所替代的生物地理现象。研究结果总结如下： 1、通过对色齐拉山阴坡冷杉林线植物叶δ13C 值的测定，从物种、生活型（常绿乔木、常绿灌木、落叶灌木及草本）两个水平研究该地区植物的水分利用策略是否存在明显的分异。所测定的隶属于18科、28属的31种植物叶片的δ13C值介于-30.24‰和-25.39‰之间，平均值为-27.68‰。常绿灌木的黄杯杜鹃与海绵杜鹃叶δ13C值无显著性差异，落叶灌木的山生柳、西南华楸以及冰川茶藨子等也无显著性差异。相反，不同生活型植物之间叶δ13C值普遍差异显著（P < 0.001），其排列顺序为：常绿乔木（冷杉）（-27.27‰） > 常绿灌木（-27.56‰） > 落叶灌木 （-27.93‰）= 草本（-27.91‰）。本研究结果表明在色齐拉山高山林线地带，尽管水分相对充足，但不同生活之间存在明显不同的碳同位素分馏。对于同一坡向的灌木类型，同一生活型的叶δ13C值无显著差异，而不同生活型植物δ13C值差异显著，说明植物水分利用策略的变化主要是由于生活型的变化引起的，即不同生活型植物的叶δ13C值可综合反映不同功能类群植物的水分利用策略变化。 2、测定了色齐拉山阴坡冷杉林线不同海拔高度植物当年叶和一年叶δ13C 值、相关叶性状及淀粉、非结构性碳含量，在物种及生活型二个层次上研究植物叶δ13C 值随海拔的分布模式以及叶δ13C 值与相关叶性状及淀粉、非结构性碳含量的关系。冷杉（乔木）和常绿灌木的δ13C 值随海拔上升表现出显著增加趋势，其当年叶和一年叶的变化趋势相同；落叶灌木和草本没有显著的海拔变化。冷杉δ13C 值的变化幅度最大，每上升100m增加0.61‰，常绿灌木、落叶灌木及草本植物分别为0.30‰/100m、0.03‰/100m和0.12‰/100m，说明不同生活型植物对相同的环境变化表现出不同的响应程度，冷杉对环境的变化更为敏感。另一方面，常绿灌木比叶面积随海拔升高而显著降低；冷杉比叶面积表现出与常绿灌木相似的海拔变化趋势，但没有达到显著性水平；落叶灌木及草本叶性状特征没有明显的海拔变化；所有植物叶片淀粉及非结构性碳含量均无显著性海拔差异（即光合产物的转移和分配没有海拔变化）。一系列回归分析表明，冷杉和灌木叶δ13C值均与比叶面积呈负相关，但与单位重量或面积氮、磷含量相关关系没有一致性，表明比叶面积的海拔变化对叶片δ13C值的影响主要是通过增加叶片厚度，从而使大气CO2在叶片内部的传输距离增加，进入叶片内部的CO2含量下降，导致对13C的分馏减少；所有植物叶δ13C 值的变化与淀粉及非结构性碳含量无相关性（即光合产物的转移和分配对叶δ13C 值的影响很小）。上述研究结果表明，植物叶稳定碳同位素比值的海拔变化主要受环境因子的控制，尤其是较低的大气温度及土壤低温：一方面低气温限制了植物叶片内外的CO2浓度差（即低水汽压差），另一方面土壤低温限制了水分在植物体内的输送而导致水分胁迫，植物通过增加叶厚（即低比叶面积）来提高其对水分资源的利用效率（即高δ13C 值）。乔木物种由于树高及生长季低温限制了水分的吸收及传输，使其对水分胁迫的敏感性高于灌木及草本，导致δ13C 值随海拔的变幅明显高于其它生活型。 3、通过分析乔木（急尖长苞冷杉）及常绿灌木（海绵杜鹃和黄杯杜鹃）不同年龄叶片δ13C 值、比叶面积、单位重量及单位面积氮、磷含量（Nmass, Narea, Pmass, Parea）、淀粉及非结构性碳含量随取样高度的分布，研究冠层高度及叶龄对稳定碳同位素组成及相关叶性状和淀粉、非结构性碳含量的相对影响，检验是否能用水力限制假说（Hydraulic limitation hypothesis）来解释叶片δ13C 值随冠层高度的变化特征。随着取样高度的增加，三个物种的叶δ13C 值均显著增加，不同叶龄δ13C 值的变化趋势相同，而且在不同海拔地带三个物种的变化趋势也相同，单位高度的叶δ13C 变化率相似（冷杉0.18-0.20‰/m vs 杜鹃灌木0.16-0.17‰/m），表明存在水力限制的物理过程。同时，单位重量叶氮、磷含量及其淀粉及非结构性碳含量沿树高均没有显著变化，表明光合作用的分馏过程不能解释叶δ13C沿树高的垂直变化。受叶龄的影响，δ13C 值与比叶面积呈负相关，而与叶氮、磷含量呈正相关，与淀粉及非结构性碳含量无明显相关关系。因此，叶气孔导度和光合能力随冠层高度和叶龄的耦合变化决定了δ13C的变化，其中沿冠层高度的水力变化所形成的水势梯度是δ13C 值变化的主要控制因子，说明在冷湿气候下的林线地区植物叶片很可能普遍存在水分胁迫，尤其是中上部的叶片。另外，在较高海拔处即林线上限，三个物种的δ13C 变化率最大，表明高海拔处低温对水分胁迫的影响尤为明显。 4、上述研究结果指出，在一定土壤低温阈值下，高大乔木物种被低矮的灌木及草本植物所替代，不但有利于改善土壤温度及提高植物根系对水分的吸收能力，而且减少植物体内的水分传输路径，进而增加冠层叶片的光合能力。

It is generally considered that water availability is not the most limiting resource at high elevations, especially at timberline area because at the upper timberline locations soil water content is high and precipitation is often abundant where the low partial pressure of CO2 and extreme cold may limit the growth, development and distribution of plant species. Recently, the effect of water stress on plant growth and distribution at the timberline has received varying attention and has been supported by some field experiments. By comparing the extent of water stress of different plants at timberline area, it may help us to further explain limiting factors for plant growth and distribution. Stable carbon isotope composition, emerged as one of the more powerful tools for studying the interactions between plants and their environments, as well as for understanding adaptations of plants to natural environments, has been widely used in plant ecology, global change and related research fields. The stable isotope compositions of plant organic matters contain an integrated record of environmental changes and could be used as an index for assessing long term water-use efficiency (WUE) of plants. The Tibetan Plateau, the third pole of the world, is the ideal and important place for the research of structure and function of nature ecosystems and its response to climate change due to the frangibility and sensitivity of ecosystems under the extreme environmental conditions driven by high-altitude climates. On the south-east edge of the Tibetan Plateau where is charactirized by a cold and humid climate because of high evelation and relatively abundant precipitation, the alpine timberline dominated by Sabina saltuaria or Abies george is one of the highest altitude timberline in the world and will play an important role in studying plant responses to global climate change. In this thesis, we measured the variations in leaf δ13C and related leaf traits (specific leaf area, SLA; mass- and area-based nitrogen concentration, Nmass and Narea) and biochemical compositions (starch; none-structural carbohydrates, NSC) associated with altitude, canopy height, and leaf age across different plant species and life forms growing at the north-facing Abies timberline of the Sergyemla Mts, southeastern Tibet Plateau. We focused on the understanding of differences in the eco-physiological characteristics related to water use strategy across plant species and life forms. By comparing the water use efficiency of different plant species and life forms, we tried to determine if water stress induced by low air or soil temperature at the timberline could explain altitudinal variations in leaf δ13C. We further examined whether the canopy height-induced hydraulic limitation previously suggested in the literature might exist in the timberline plants and then could be used to partly explain the vegetation changes from tree species to shrub or grass species. The main results indicated as follows: 1、Variations in leaf δ13C across evergreen trees, evergreen shrubs, deciduous shrubs and forbs at the timberline were examined at both species and life-form levels to understand the differences in water use strategy of plants. The δ13C values of 31 species belonging to 18 families and 28 genera ranged from -30.24‰ to -25.39‰, with an average of -27.68‰. At the species level, no significant differences in δ13C values were found in evergreen shrubs (R. wardii, R. pingianum) or in deciduous shrubs (S. oritrepha, R. glaciale and S. rehderiana). However, significant differences in δ13C were found among life forms (P < 0.001): evergreen trees (-27.27‰) > evergreen shrubs (-27.56‰) > deciduous shrubs (-27.93‰) and forbs (-27.91‰). The results indicated that although soil water availability was high at the timberline, the significant difference in isotope discrimination occurred among life forms, suggesting different water use efficiency of plants. There was no difference in δ13C of plants within the same life form plants but a significant difference between different life forms, indicating that the variation of water use efficiency was dependent on life form identities. The δ13C values of different life-form plants would be an integrated indicator of changes in water use strategy among functional groups at the timberline. 2、By measuring the δ13C values and related leaf traits of current year and 1-yr leaves across species and life forms along an altitudinal transect, we examined the altitudinal patterns of δ13C values at the species and life form levels and the relationships of δ13C values to environmental factors and leaf traits. With increasing altitude, the δ13C values of current and 1-yr old leaves significantly increased in Abies trees and evergreen shrubs but varied little in deciduous shrubs and grasses. The increased rates of δ13C with altitude were: 0.61‰/100m for Abies trees, 0.30‰/100m for evergreen shrubs, 0.12‰/100m, 0.03‰/100m for grasses and deciduous shrubs. The results suggested that plants with different life forms showed different responses to the similar increase in altitude, and Abies trees were more sensitive to altitudinal changes. The significant increase in SLA was found in evergreen shrubs, and Abies trees showed the similar trend though no significant difference existed in current or 1-yr old leaves. In contrast, deciduous shrubs and grasses did not show significantly altitudinal variations in leaf treats. The starch and non-structural carbon concentrations of leaves across species and life forms varied little with altitude. Regression analyses indicated that the δ13C in Abies or evergreen shrubs was negatively correlated with SLA but showed inconsistently relationships with area or mass-based N and P concentrations, suggesting that the effect of SLA on δ13C values was a result of the increased pathway of CO2 from air to leaf and then decreased intercellular CO2 because of increased leaf thickness (lower SLA). For any life form species, no significant relationship was found between δ13C values and starch or non-structural carbon contents, suggesting no post-photosynthetic discrimination at the timberline. Our data indicated that the δ13C values of plant leaves were mainly controlled by environmental factors, especially low air and soil temperatures that determined leaf δ13C. The low soil-temperature enhanced hydraulic limitation might contribute to the increase of δ13C values with altitude. In this case, plants would improve their use efficiency of water resource (high δ13C values) by increasing leaf thickness (low SLA). For tree species, the high canopy stature and accompanied low air- and soil-temperature at the timberline limited the water uptake by roots and its transportation rate in the soil and plant, leading to a higher sensitivity to water stress in trees than in shrubs and grasses as indicated by the highest altitudinal variations in δ13C of tree leaves. 3、δ13C values and related leaf traits of different aged leaves collected from different canopy positions for Abies trees and two evergreen shrub species were used to determine the relative effects of sampled canopy height and leaf age on δ13C and related leaf traits, and to examined if hydraulic limitation hypothesis could be used to explain canopy-vertical patterns in δ13C. With increased canopy heights, δ13C values increased significantly both for Abies and two evergreen shrubs. The increasing trends of δ13C were similar for different aged leaves. The rate of δ13C increase per unit height was similar between Abies trees and two shrub species (0.18-0.20‰/m vs 0.16-0.17‰/m) across different altitudes, suggesting that the water stress would be a physical process, not a biological phenomenon. No significant variations in Nmass, Pmass, starch and NSC contents with canopy heights were found across Abies trees and evergreen shrubs, indicating little discrimination during photosynthetic process. Because of leaf age effects, leaf δ13C was negatively correlated with SLA but positively correlated with mass or area-based N or P concentrations. Therefore, the combination of variations in stomatal conductance and photosynthetic capacity determined the differences of δ13C values. The height-induced hydraulic limitation would be the limited factor influencing the variations of δ13C values along canopy gradients, suggesting that water stress of plant leaves, especially for leaves lived in middle or high canopy positions might occur even under the cold and humid environment at the timberline. In addition, the variation extent of δ13C values along canopy gradients was higher at the high altitude (i.e. upper timberline), suggesting that the water stress induced by low temperature was more prounced at high altitude. 4、Given a threshold of low soil temperature at the timberline, tree species with high canopy stature would be replaced by low shrubs and/or grasses, which not only improve soil temperature and enhance the uptake of water or other nutrients by roots, but also decrease the pathway of water transportation in the plant. In this case, whole-canopy carbon gain would be improved accordingly.

中文摘要........................................................................................................................ I
ABSTRACT..................................................................................................................V
目录..............................................................................................................................IX
第一章 前言..................................................................................................................1
第二章 研究综述..........................................................................................................5
第一节 高山林线研究进展...................................................................................5
1.1 高山林线定义及其类型...........................................................................5
1.2 高山林线研究的几个阶段.......................................................................6
1.3 有关林线形成的假说...............................................................................7
1.4 高山林线植物的生理生态学特性............................................................9
1.4 我国林线研究概况.................................................................................12
第二节 稳定碳同位素技术在植物生理生态学中的应用.................................12
2.1 光合作用稳定碳同位素分馏及其影响因素.........................................13
2.2 稳定碳同位素技术在植物生理生态学中的应用.................................16
第三节 我国稳定碳同位素生理生态学研究状况.............................................20
第三章 研究地区的自然概况....................................................................................23
第一节 色齐拉山自然概况.................................................................................23
1.1 地理位置.................................................................................................23
1.2 地质地貌.................................................................................................23
1.3 水文特征..................................................................................................23
1.4 气候特征..................................................................................................24
1.5 土壤.........................................................................................................24
1.6 植被类型..................................................................................................25
第二节 研究区域概况.........................................................................................26
2.1 研究区域植被组成.................................................................................27
2.2 研究区域土壤理化性质.........................................................................29
2.3 研究区域气候特征.................................................................................29
第四章 叶稳定碳同位素组成随物种及生活型的变化特征....................................31
1 引言...................................................................................................................31
2 材科与方法.......................................................................................................33
2.1 样品的采集.............................................................................................33
2.2 样品的稳定碳同位素分析分析..............................................................33
2.3 统计分析.................................................................................................34
3 结果...................................................................................................................34
3.1 叶稳定碳同位素值指示研究区植物属于C3 光合途径........................34
3.2 同一生活型不同物种间的δ13C 值差异...............................................37
3.3 不同生活型之间的δ13C 值差异...........................................................39
4 讨论...................................................................................................................39
4.1 不同生活型植物叶片δ13C 值与水分利用策略的变化.......................39
4.2 高山林线地区植物功能群的划分.........................................................41
第五章 不同生活型植物叶δ13C 值及其相关叶性状海拔分异..............................43
1 引言...................................................................................................................43
2.1 样品的采集.............................................................................................46
2.2 样品的分析.............................................................................................46
2.3 统计分析.................................................................................................47
3 结果与分析.......................................................................................................47
3.1 在物种及生活型水平上δ13C 值的海拔分异.......................................47
3.2 相关叶性状、淀粉及非结构性碳含量的海拔分异..............................50
3.3 叶δ13C 值与相关叶性状、淀粉及非结构性碳含量的相关关系......53
4 讨论....................................................................................................................56
4.1 不同生活型植物δ13C 值的海拔分异及其生态学意义......................56
4.2 叶性状的海拔分异.................................................................................59
4.3 植物叶δ13C 值随海拔变化的因素分析................................................60
第六章 冠层高度和叶龄对植物δ13C 值及其相关叶性状的影响..........................65
1 引言...................................................................................................................65
2 材科与方法.......................................................................................................67
2.1 样品的采集.............................................................................................67
2.2 样品的分析.............................................................................................67
2.3 统计分析.................................................................................................67
3 结果...................................................................................................................67
3.1 δ13C 值、叶性状、淀粉及非结构性碳含量随取样高度的变化规律.67
3.2 δ13C 值、叶性状、淀粉及非结构性碳含量随叶片年龄的变化.........68
3.3 δ13C 值与相关叶性状、淀粉及非结构性碳含量的关系.....................73
4 讨论...................................................................................................................75
4.1 δ13C 值随取样高度的变化及影响因素分析.........................................75
4.2 叶龄对δ13C 值、叶性状、淀粉及非结构性碳含量的影响...............77
4.3 δ13C 值与淀粉及非结构性碳含量的关系............................................79
第七章 结论与展望....................................................................................................81
参考文献......................................................................................................................85
附录............................................................................................................................105
致谢............................................................................................................................109