植被冠层叶水同位素富集及数值模拟研究

Understanding the controls on temporal variation in plant leaf δD and δ18O values is critical for contributing to improve understanding of the coupled carbon–water dynamics of the biosphere and to interpreting a wide range of proxies for past environments. There have been already several studies performed to evaluate the models in field conditions, nevertheless, there are only a few studies that have performed model-measurement comparisons at the canopy or ecosystem scale rather than at the leaf scale, few studies have evaluated these models in multispecies natural community and access them at different time resolution over full growing seasons. How to upscale those modeling work in leaf scale to ecosystem scale considering canopy structure, spatial heterogeneity, and isotopic non-steady–state or Péclet effects for simulating canopy foliage water isotopic enrichment remain challenge. The project intends to using models of varying complexity, previously conducted at the leaf scale for the isotope composition of canopy leaf water with the aim of obtaining as close as possible an observation at the ecosystem scale. Comparisons of canopy leaf water enrichment process and dynamics will be made among different climate characteristics (cold, arid, semi-arid, and humid) and alpine vegetation types (alpine meadow, splendid achnatherum, Dasiphorafruticosa shrub) in order to improve the understanding of the canopy water isotopic enrichment process and mechanism and make progress on hydrological cycle with isotopic tracer in cold and semi-arid region of our country.

深入理解叶水稳定同位素富集过程对研究陆-气交换，蒸散发分离，古气候反演等生物、水文及气象过程都具有重要的科学意义。然而，绝大多数研究仍局限于叶片尺度或控制实验，较少研究在自然群落冠层尺度上，尤其在我国高寒及干旱地区开展。如何耦合同位素过程及参数化生态系统的稳态或非稳态，运移及扩散现象（Péclet效应）来探究冠层叶水富集规律仍存在诸多挑战及不确定性。本项目拟参考国际叶水同位素富集主流模型, 采取现场观测及数值模拟方法，尺度上推至生态系统，研究不同气候特征下（高寒、干旱、半干旱及湿润）及高寒区不同植被类型（高寒草甸、芨芨草、金露梅灌丛）冠层叶水富集过程及动态，比对分析观测及模拟结果，提高对陆地植物冠层水同位素富集过程及机制的认知，旨在为我国高寒，半干旱区的同位素示踪生态水文过程方面做出贡献。

深入理解叶片水同位素（H2O18 或HDO16）富集过程对研究陆-气交换，蒸散发拆分，古气候反演等生物、水文及气象过程都具有重要的科学意义。然而，对叶片尺度研究，虽然有成熟的模型，但在生态系统尺度研究较为缺乏，尤其缺乏异质性较大多物种生态系统（如草地生态系统性）。如何耦合及参数化生态系统水同位素的稳态(SSA)或非稳态（NSS），运移及扩散现象（Péclet效应）来探究冠层叶水富集规律存在诸多挑战。本项目参考了国际叶水同位素富集主流模型, 采取现场观测及数值模拟方法，将尺度上推至生态系统，构建了冠层叶水富集模型，研究了冠层叶水富集过程及动态。本研究基于对草地生态系统，以及玉米农田生态系统的野外观测与取样，实验室同位素分析。并基于野外实地观测结果水热通量（蒸散发，净辐射，土壤热通量）以及同位素信息（叶片水，冠层水同位素）对模型进行了全面的率定。研究结果表明: 草地及农田群落冠层叶水同位素存在显著的异质性（单个叶片顶端，中端及低端，物种间差异，冠层上下层差异）；在2个站点模拟率定结果表明： SSA是草地生态系统和农田生态系统模拟效果最好的子模型，即叶水同位素稳态假设作为冠层叶水同位素富集是可以接受的。研究强调尽管叶水同位素存在较大的时空异质性，多物种群落冠层叶水同位素是一种系统行为，其日变化及季节变动可以被预估。诸多环境因子中，相对湿度由于其高敏感性和季节变动是控制冠层叶片同位素富集的关键因子。随着植被生长，其冠层的叶水富集逐渐减小，该趋势是由于生态系统蒸腾比（T/ET）的逐渐增加，进而引起冠层温度降低，其相对湿度减小引起的。以上生态系统尺度同位素观测及模拟结果，对提高陆地植物冠层水同位素富集过程及机制的认知，深化对半干旱区的同位素示踪生态水文过程研究。