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.
This funding supported the creation of a ecohydrological observatory, which was installed at the Mpala Research Center in central Kenya. Over the past 5 years, this observatory has been used to study how dryland ecosystems respond to climate variability, and to develop new methodologies for using stable isotopes as a tracer of water fluxes through landscapes. Understanding the coupled interactions between hydrology and ecology requires new measurements of environmental process at the landscape level. This project employed the use of stable isotopes of hydrogen and oxygen as a tool for partitioning land surface water vapor fluxes into evaporation and transpiration components. Our research has advanced understanding related to both theoretical (e.g., kinetic fractionations in soil evaporation) and practical questions (e.g., the effect of vegetation structure on evaporation/transpiration partition) related to stable isotope hydrology. These efforts have led to improved understanding of how drylands will to changing rainfall distributions, and the ways in which vegetation structure and function are coupled in dryland ecosystems. The funding supported a PhD student, two postdoctoral research associates, more than 15 undergraduate interns, and four undergraduate thesis projects. A new course was developed - Water, Energy, and Ecosystems, and was taught based on the science and data conducted in this project. Overall, this research led to the publication of at least 20 peer-reivewed research manuscripts, which have been cited over 400 times.
