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

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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.