CZO: Critical Zone Observatory--Snowline Processes in the Southern Sierra Nevada

南塞拉利昂临界区观测站位于海拔1700-2100米，跨越了雨雪过渡，这一地区特别容易受到气候和土地覆盖大变化的影响。快速的季节变化和降水的陡峭梯度使该地区成为研究临界区过程如何对扰动做出反应以及水循环如何驱动临界区过程的极好的天然实验室。特别是，气候中当地明显的高度和与方位有关的梯度的差异为在一系列面向过程的丰富的跨学科研究中用空间取代时间提供了机会。CZO与USFS国王河实验流域并置，利用多个、仪器齐全和具有特色的流域和长期数据集为研究提供社区平台。来自CZO的四个关键发现帮助塑造了当前的研究议程。首先是利用一套丰富的水量平衡、通量塔和树液流蒸散(ET)测量方法，关闭水源集水区和更大尺度的水量平衡。其次，多条证据与中海拔地区全年生长季节一致，中山森林在冬季表现出耐寒能力，并通过深深扎根来避免夏末干旱胁迫。第三，与附近低海拔限水和高海拔冷限水生态系统相比，中等海拔地区的ET值相对较高。第四，深层扎根和土壤发育对维持高净初级生产力(NPP)很重要，超过三分之一的ET来自1米以下的深度。这些结果为当前的研究提供了一个统一的主题，涉及：i)土壤水分和地形变异性如何相互作用影响土壤形成和风化；ii)土壤水分、蒸腾和径流对融雪和降雨的响应如何受景观变异性控制；iii)植被和生态系统的分布和功能如何以及为什么随气候变化而变化；iv)景观异质性和植被如何相互作用影响水、能量、养分和净初级生产力的循环。CZO研究的基础是丰富的野外测量计划、地球化学分析和良好集成的建模工作，这些工作将提供对气候、水文、(生物)地球化学、生态系统和景观过程之间联系的基本了解。南塞拉利昂CZO是一个数据丰富的社区平台，用于过程级别的研究，增强多个个人和研究小组的科学。它位于一个具有典型的快速季节变化的海拔范围内，在1-2个月的时间里，从积雪到湿润的土壤再到干燥的土壤。如果在当前的雨雪转换中，雪在很大程度上被雨水取代，区域气候变暖将使这种变化发生得更早或完全消除。这将导致季节性到年际临界区过程中与水有关的重大变化，涉及风化、风化层形成、养分循环以及植被/生态系统的分布和功能。主要是针叶混交林的森林密度很高，灾难性火灾的威胁很大，增加了较长期临界区进程发生重大变化的可能性。CZO的研究正在提供前所未有的预测能力，以预测相关的水文、(生物)地球化学、生态系统和景观过程将如何应对扰动，即温度、降水模式和人类对森林的管理的变化。它为内华达山脉和其他季节性积雪覆盖的山区森林的基于科学的植被和水管理提供了坚实的基础。此外，CZO为设计和评估更广泛的研究和应用观测网络提供了基础。最后，研究团队使用CZO的数据和知识来增强数千名初中生和参与大学的许多本科生的科学体验。

The Southern Sierra Critical Zone Observatory (CZO), located at 1700-2100 m elevation, crosses the rain-snow transition, an area that is particularly vulnerable to large changes in climate and landcover. Rapid seasonal changes and steep gradients in precipitation make this zone an excellent natural laboratory for studying how critical-zone processes respond to perturbations and how the water cycle drives critical-zone processes. In particular, differences along locally pronounced altitudinal and aspect-related gradients in climate offer the opportunity to substitute space for time in a rich cross-disciplinary array of process-oriented studies. Collocated with the USFS Kings River Experimental Watersheds, the CZO takes advantage of multiple, well-instrumented and characterized catchments and long-term data sets to provide a community platform for research. Four key findings from the CZO have helped shape the current research agenda. First is closing the water budget at headwater catchment and larger scales using a rich set of water-balance, flux-tower and sap-flow measurements of evapotranspiration (ET). Second, multiple lines of evidence are consistent with a year-round growing season at mid elevation, with mid-montane forests exhibiting cold tolerance in winter and avoiding late-summer drought stress through deep rooting. Third, rates of ET at mid-elevation are high relative to nearby lower-elevation water-limited and higher-elevation cold-limited ecosystems. Fourth, deep rooting and soil development are important for sustaining high rates of net primary productivity (NPP), with over one-third of ET coming from depths below 1m. Together these results provide a unifying theme for the current research, which addresses: i) how soil moisture and topographic variability interact to influence soil-formation and weathering, ii) how the response of soil moisture, ET and streamflow, to snowmelt and rainfall is controlled by landscape variability, iii) how and why vegetation and ecosystem distribution and function vary with climate, iv) and how landscape heterogeneity and vegetation interact to influence cycling of water, energy, nutrients, and NPP. The foundation of CZO research is a rich field measurement program, with geochemical analyses and a well-integrated modeling effort that will provide a fundamental understanding of the links among climatic, hydrologic, (bio)geochemical, ecosystem and landscape processes. The Southern Sierra CZO is a data-rich community platform for process-level research that enhances the science of multiple individuals and research groups. It is located in an elevation range that has characteristically rapid seasonal changes, going from snowcover to wet soil to dry soil over a 1-2 month period. Regional climate warming will make such changes occur earlier or eliminate them entirely if snow is largely replaced by rain at the current rain-snow transition. This will lead to major water- related changes in seasonal-to-interannual critical-zone processes involving weathering, regolith formation, nutrient cycling, and vegetation/ecosystem distribution and function. Forest density of the largely mixed-conifer forest and the threat of catastrophic fire are very high, raising the chances of major changes in longer-term critical-zone processes. Research at the CZO is providing an unprecedented predictive ability for how the linked hydrologic, (bio)geochemical, ecosystem and landscape processes will respond to perturbations, i.e. changes in temperature, precipitation patterns, and human management of forests. It is providing a solid foundation for science-based vegetation and water management in the Sierra Nevada and other seasonally snow-covered mountain forests. Further, the CZO provides a basis for the design and evaluation of broader observation networks for both research and applications. Finally, the research team uses CZO data and knowledge to enhance the science experience of thousands of middle and high school students and also many undergraduate students at the participating universities.