KMS XINJIANG INSTITUTE OF ECOLOGY AND GEOGRAPHY，CAS
| 梭梭幼苗干旱死亡过程中的水力结构变化与碳分配动态 | |
| 张尧 | |
| Subtype | 博士 |
| Thesis Advisor | 李彦 |
| 2016 | |
| Degree Grantor | 中国科学院大学 |
| Place of Conferral | 北京 |
| Degree Discipline | 生态学 |
| Keyword | 干旱死亡机制 水力失效 碳饥饿 生物量分配 异速生长 再萌芽 |
| Abstract | 根据全球变化模型预测，未来地球表面温度将持续升高，降水模式发生改变，干旱事件的频次和强度增加。干旱事件的频发将导致植被变化。当前全球干旱导致的植物死亡报道越来越多，尤其在干旱和半干旱生态系统更为严重。对植物死亡的生理机制认识不足，会限制我们对未来气候变化情景下植被动态的预测。“水力失效”和“碳饥饿”是植物干旱死亡的两个主流机制，分别基于植物水、碳平衡对干旱的响应而提出。水碳生理变化的先后顺序和相互作用决定着植物的干旱死亡机制。 古尔班通古特沙漠为我国第二大沙漠，建群种梭梭（Haloxylon ammodendron）耐瘠薄、抗旱性极强，具有优良的固沙特性，对于荒漠生态系统的稳定与维持意义重大。然而近年来，在气候变化和人类活动的影响下，古尔班通古特沙漠南缘天然梭梭林分布面积减小，盖度下降。当夏季干旱来临，土壤水分有效性降低，梭梭幼苗死亡率升高，不利于幼苗的建成、种群的更新和分布。了解建成阶段梭梭幼苗水碳动态对干旱的响应有助于理解其干旱死亡机制。另外，幼苗根系结构和功能发展尚不完善，比成年个体更容易遭受水分胁迫，因此其生物量分配模式和根系对生存的作用值得深入探讨。 本研究通过温室控制实验，对一年生梭梭幼苗在干旱死亡过程中的水碳生理、形态指标进行跟踪测定，确定各指标变化的先后顺序，量化地上、地下组织在干旱下的存活时间，探讨根系生理和形态对梭梭幼苗干旱存活的重要作用，分析生物量分配模式对干旱的响应以及对复水后恢复的作用，并进行梭梭幼苗胚根生长动力学特征初探。 结果表明：（1）干旱下梭梭幼苗根系存活时间（死于干旱第70天）是地上部分存活时间（死于干旱第35天）的2倍。碳水化合物优先向根系分配，维持根系呼吸，延长根系的存活时间。同时，确定了其干旱死亡过程中水、碳相关生理指标发生变化的时间顺序，探讨了此物种在干旱下对维护水、碳安全的权衡及策略。（2）水力功能的失调限制了非结构性碳水化合物在地上部分的利用，导致梭梭幼苗地上部分的枯萎，表明水、碳共同调控梭梭幼苗在干旱下的生存。地上部分死亡后，存活的根系有利于幼苗的再萌芽，其再萌芽能力依赖于优先分配于根系的非结构性碳水化合物。（3）提出了此物种的死亡判定标准：地上组织和根系气体交换接近于零，并且复水后再萌芽能力为零，即为真正的死亡。此标准建立了“胁迫”和“死亡”的界限，为死亡机制研究设定了前提。（4）梭梭幼苗优先向地下分配生物量，且这种分配模式不受干旱的影响，具有表观可塑性。在长期环境压力下，固定的生物量分配模式有利于降低可塑性代价，增加适应性。梭梭幼苗固定的生物量分配有利于其复水后的恢复。（5）梭梭种子萌发初期胚根伸长率最大的区域通常是离根尖最远的部分，距根尖2 mm范围内始终未见任何伸长迹象。而曲率变化最大的区域通常在根尖附近，形成两个中心点，位置依根长的增加而改变。伸长生长和弯曲生长在时间上交替进行，在空间上基本不重合。 本研究表明，荒漠灌木梭梭具有“与生俱来”的根保护策略：在干旱胁迫下优先投资根系，牺牲地上部分，在降低地上碳需求的前提下满足根系的碳消耗，保证根系的存活。根系积累的碳水化合物是驱动再萌芽的能量源。而优先分配于地下的生物量日积月累形成了庞大的根系，在沙漠长期干旱的环境中进行有效的形态调整以维持供需水平衡。生理响应和形态调整、水平衡和碳平衡共同作用，调控梭梭幼苗在荒漠环境下的生存。 |
| Other Abstract | Climate change models generally agree that sustained temperature increase will happen on the global surface, and shifts in precipitation will result in higher drought intensity and frequency in the near future. Under this climate change background, changes in vegetation will be driven by drought globally. Evidence of drought-induced tree mortality has been reported in recent years in many regions of the world，especially in arid and semi-arid ecosystems. Lacking understanding of the plant mortality mechanisms by drought will hamper the predictions of the vegetation dynamic under climate change background. There are two mainstream hypotheses explaining the plant mortality mechanisms: hydraulic failure and carbon starvation, proposed base on the water and carbon balance response of plants to drought, respectively. Time sequence of water and carbon associated variations and interaction of the two during drought ultimately determines the cause of death. The Gurbantonggut Desert is the second largest desert of China. Haloxylon ammodendron, as the constructive species in Gurbantonggut Desert, has fortissimo barren tolerance, drought resistance and fine sand-fixation features that is of great significance to maintain desert ecosystem stability. In recent years, however, with the influence of climate change and human activities, both the distribution areas and coverage of H. ammodendron in the south rim of Gurbantonggut Desert are decreasing. When summer drought comes, H. ammodendron seedlings mortality increased with the reduced soil moisture availability, which will hamper the establishment, recruitment and distribution of this species. Knowing water and carbon dynamics in response to drought of H. ammodendron seedlings in the establishment period is helpful to understand the mortality mechanism. In addition, young seedlings are more likely subjected to water deficiency compared to adult individuals due to the lack of fully developed root system. Consequently, the biomass allocation patterns and the role of the root in regulating survival for H. ammodendron seedlings need to be further investigated. In the current study, a greenhouse experiment was performed with first-year seedlings of H. ammodendron. Physiological and morphological traits were monitored throughout the drought to confirm their time sequence of occurrence. Also, the survival time for above- and belowground parts were quantified respectively, and the importance of root physiology and morphology in individual survival was discussed. Additionally, the response of biomass allocation pattern to drought was examined and its role in recovery after re-watering was analyzed. At last, a preliminary study was conducted to understand the kinematic growth characteristics of H. ammodendron seedlings radicle. The results showed that: (1) the quantified survival duration showed that the survival time of the seedling root system (died at 70 d of drought) was double the survival time for the shoot (died at 35 d). Difference in survival time between compartments resulted from sustained root respiration supported by increased non-structural carbohydrates ratio in the root under drought. Meawhile, the time sequence of variations in water and carbon associated physiological indicators was determined in the process of drought to death, and the trade-offs and strategy between water and carbon safty were discussed. (2) Hydraulic disfunction killed the aboveground parts by limiting the availability of stored non-structural carbohydrates, which emphasized the interaction and interdependence of the two mechanisms of hydraulic failure and carbon starvation; after the death of the aboveground parts, the live root contributed to resprouting following drought, and the resprout capacity relies on the carbohydrates that preferentially invested to root. (3) Proposed the mortality criterion for this species: the individual ‘real’ die when both occurrence in aboveground tissues and root gas exchang arrive almost zero as well as the resprout capacity get to zero. This criterion builds the boundary between ‘stress’ and ‘mortality’, and sets the premise for mortality mechanism study. (4) Biomass of H. ammodendron seedlings was preferentially allocated to roots along a fixed allometric trajectory, irrespective of water availability. In prolonged stressful environments, fixed biomass allocation patterns may reduce plasticity costs while increase fitness. The fixed biomass allocation of H. ammodendron seedlings contributes to recovery after re-watering. (5) At the beginning of H. ammodendron germination, the root region that has the highest growth rate locates farthest from the root tip. The elongation was not observed in the region that 2 mm away from the root tip all the time. And maximum curvature usually occurred in the regions that near the root tip, forming two centers and had changed positions with the growth of radicle. Elongation growth and bending growth alternates in time, little or no overlap in spatial. The study suggests that the desert shrub H. ammodendron has an "intrinsic" root-protect strategy: sacrificing aboveground parts during drought contributes to reduction of carbon demand, while preferentially invest to belowground parts to ensure the survival of root. Meanwhile, the stored root non-structural carbohydrates are the energy sources for resprouting. Accumulation of biomass formed large roots, which can maintain water supply - demand balance through effective morphological adjustment in prolonged drought of desert. Hence, the survival of H. ammodendron seedlings in such environment is both regulated by physiological and morphological adjustments, as well as water and carbon interactions. |
| Subject Area | 生态学 |
| Language | 中文 |
| Document Type | 学位论文 |
| Identifier | http://ir.xjlas.org/handle/365004/14738 |
| Collection | 研究系统_荒漠环境研究室 |
| Affiliation | 中科院新疆生态与地理研究所 |
| Recommended Citation GB/T 7714 | 张尧. 梭梭幼苗干旱死亡过程中的水力结构变化与碳分配动态[D]. 北京. 中国科学院大学,2016. |
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