Towards a plant-based proxy for the isotope ratio of atmospheric water vapor

对自然系统中稳定氧同位素的分析可以阐明植物对环境变化的生理反应，从酶的尺度上到整个植物和生态系统，再到地球。植物改变环境水中不同的地区性和季节性同位素信号，在大气二氧化碳、O2和植物有机物质中产生独特的示踪剂。这些信号允许在不同的时间和空间尺度上分配总的二氧化碳通量，重建从季节到世纪的生长环境，并估计数千年的全球生产力。叶片-水的同位素比是所有这些应用的核心，并受三个物理因素控制：(I)相对湿度，(Ii)植物根水的同位素比和(Iii)大气水汽的同位素比。我们对大气水汽在任何空间或时间尺度上的同位素比缺乏了解，导致在解释二氧化碳、O2和植物有机物的稳定同位素比时产生了很大的误差。通过获得大气水蒸气在时间和空间上的同位素比率的综合估计，将最大限度地减少这一误差。这项研究的目的是开发和测试模型来解释观察到的附生CAM植物Tillandsia usneides的叶水和纤维素的氧同位素比率。假设叶水和叶有机物质的同位素比受大气水汽的同位素比控制，并反映大气水汽的同位素比。这些预测将通过实验室实验和实地活动相结合的方式进行验证。将在弗吉尼亚州和佛罗里达州两个同位素不同的地点测定年度和每月时间尺度的大气水汽同位素比值的当代值；这些地区的历史值将使用植物标本重建。这项研究的更广泛影响代表了大气水汽氧同位素比值平均值在前所未有的空间和时间尺度上的表征。这项叶水平生态生理研究的最终实现成果将是一张从美国弗吉尼亚州向南穿过热带地区到阿根廷的时间整合大气水汽地图。这一产品将惠及包括生态系统到全球范围的生物生产力估计、法医应用、重建过去气候和水文模型在内的广泛科学领域。此外，这项提议还将培训一名技术人员、一名研究生和几名本科生，学习稳定同位素的理论和应用。这些专门的分析技能在公共和私营部门都很有市场。

The analysis of stable oxygen isotopes in natural systems allows for the elucidation of plant physiological responses to environmental change from the scales of enzymes up through whole plants and ecosystems to the planet. Plants modify the distinct regional and seasonal isotopic signals in environmental water yielding unique tracers in atmospheric CO2, O2 and plant organic material. These signals allow for the partitioning of gross CO2 fluxes on a variety of temporal and spatial scales, the reconstruction of growth environment from seasons to centuries and the estimation of global productivity over millennia. The isotope ratio of leaf-water is central to all of these applications, and is controlled by three physical factors: (i) relative humidity, (ii) the isotope ratio of plant root water and (iii) the isotope ratio of atmospheric water vapor. Our lack of knowledge of the isotope ratio of atmospheric water vapor at any spatial or temporal scale leads to large errors in the interpretation of stable isotope ratios of CO2, O2, and plant organics. This error would be minimized by obtaining integrated estimates of the isotope ratio of atmospheric water vapor through time and space. The objectives of the proposed research are to develop and test models to explain the observed oxygen isotope ratios of leaf water and cellulose of the epiphytic CAM plant Tillandsia usneoides. It is hypothesized that the isotope ratio of leaf water and consequently leaf organic material is controlled by and reflects the isotope ratio of atmospheric water vapor. These predictions will be tested through a combination of laboratory experiments and field campaigns. Contemporary values of the isotope ratio of atmospheric water vapor for annual and monthly timescales will be determined at two isotopically distinct locations in Virginia and Florida; historical values in these regions will be reconstructed using herbarium specimens.The broader impacts of this research represent a characterization of mean values for the oxygen isotope ratio of atmospheric water vapor on an unprecedented spatial and temporal scale. The realized final product of this leaf-level ecophysiological research would be a map of time-integrated atmospheric water vapor from Virginia, USA southwards through the tropics to Argentina. Such a product would benefit broad disciplines of scientific pursuits including ecosystem to global scale estimates of biological productivity, forensic applications, reconstruction of past climates and hydrological models. Additionally, this proposal will train a technician, a graduate student and several undergraduate students in the theory and applications of stable isotopes. These specialized analytical skills are highly marketable in both the public and private sectors.