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区域和季节同位素信号，在大气CO2、O2和植物有机物中产生独特的示踪剂。这些信号允许在各种时间和空间尺度上划分总二氧化碳通量，重建从季节到世纪的生长环境，以及估计千年的全球生产力。叶片水分的同位素比率是所有这些应用的核心，并由三个物理因素控制：(i)相对湿度，（ii）植物根系水分的同位素比率和（iii）大气水蒸气的同位素比率。由于缺乏对大气水汽在任何时空尺度上的同位素比值的了解，导致对CO2、O2和植物有机物稳定同位素比值的解释存在很大误差。通过对大气水汽的同位素比值进行时空综合估计，可以使这一误差降到最低。本研究的目的是建立和测试模型来解释附生CAM植物Tillandsia usneoides叶片水分和纤维素的氧同位素比值。假设叶片水分和叶片有机质的同位素比值受大气水汽的同位素比值控制并反映大气水汽的同位素比值。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检验。将在弗吉尼亚和佛罗里达两个同位素不同的地点测定年和月时间尺度上大气水蒸气同位素比率的当代值；这些地区的历史价值将利用植物标本馆标本重建。本研究的更广泛影响代表了在前所未有的时空尺度上大气水蒸气氧同位素比值平均值的特征。这一叶片级生态生理学研究的最终成果将是一张从美国弗吉尼亚州向南通过热带地区到阿根廷的时间积分大气水汽图。这样的产品将有利于广泛学科的科学追求，包括生态系统到全球尺度的生物生产力估计、法医应用、重建过去的气候和水文模型。此外，该计划还将培训一名技术员、一名研究生和几名本科生，学习稳定同位素的理论和应用。这些专业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.