CAREER: An ecohydrological framework for understanding land degradation in dryland ecosystems

这一建议将提供一个新的视角，说明影响旱地植被结构的因素如何支配植物用水的时空格局以及从个体到景观尺度的水分利用效率。为了实现这一目标，将在位于肯尼亚中部莱基皮亚区的普林斯顿附属姆帕拉研究中心和保护协会(MRCC)建立一个生态水文观测站。该天文台将能够使用集成的离轴空腔光谱仪(ICOS)来测量陆地表面和大气之间水蒸气通量的同位素组成，从而分别解决景观尺度的蒸发和蒸腾问题。之前的校准和验证活动已经证明了该传感器用于E/T分配的实用性，以及在0.5赫兹的采样频率下使用ICOS激光吸收方法直接表征通量的可能性。三个特定的假设构成了研究和教育计划的核心：(1)可以根据约束优化来描述限水植被的结构/功能组织，该约束优化寻求在一系列尺度和环境中最大化水分利用(保持高蒸腾)和最小化水分胁迫(防止低水分利用效率)；(2)生态系统尺度上植物水分利用的功能多样性和水分利用效率(如时间和空间上不同的树木/草水分利用)对于维持旱地生态系统的生态和水文功能至关重要；(3)各种水分平衡成分(如蒸腾/蒸发)的相对大小的变化可作为旱地生态系统土地退化的诊断标志。为了解决这些假设，拟议的研究计划将结合核心区的连续生态水文测量和同时在管理和实验操纵的地块进行密集的短期观测。连续数据和有针对性的测量相结合，将允许直接测试生态系统对受草食和土地利用强度不同影响的地貌的水供应的反应。这项研究的结果为制定土地利用战略提供了一个独特的量化框架，这些战略力求防止土地退化并维持依赖旱地生态系统的牧民社会。

This proposal will provide a novel perspective regarding how factors that impact dryland vegetation structure govern spatial and temporal patterns of plant water use and water use efficiency from individual to landscape scales. To accomplish this goal, an ecohydrological observatory will be created at the Princeton-affiliated Mpala Research Centre & Conservancy (MRCC), located in the Laikipia District of central Kenya. The observatory will be capable of resolving landscape-scale evaporation and transpiration separately using an integrated off-axis cavity spectrometer (ICOS) to measure the isotopic composition of water vapor fluxes between the land surface and the atmosphere. Prior calibration and validation activities have demonstrated the utility of this sensor for E/T partitioning, and the possibility of direct flux characterization using ICOS laser absorption methods at sampling frequencies of 0.5 Hz.Three specific hypotheses form the heart of the research and educational plans: (1) The structural/functional organization of water-limited vegetation may be described according to a constrained optimization that seeks to both maximize water use (maintain high transpiration) and also minimize water stress (prevent low water use efficiency) across a range of scales and settings; (2) Ecosystem-scale functional diversity in plant water use and water use efficiency (e.g. differing tree/grass water use in time and space) are critical to the maintenance of ecological and hydrological function in dryland ecosystems; and (3) Shifts in the relative magnitude of various water balance components (e.g. transpiration/ evaporation) may be used as diagnostic signatures of land degradation in dryland ecosystems. To address these hypotheses, the proposed research program will combine continuous ecohydrological measurements at a core site with simultaneous intensive shorter-term observations in both managed and experimentally manipulated plots. The combination of continuous data as well as targeted measurements will allow for direct tests of ecosystem response to water availability in landscapes differentially impacted by herbivory and land use intensity. The results of this research provide a unique quantitative framework for the development of land use strategies that seek to prevent land degradation and sustain pastoralist societies that depend on dryland ecosystems.