The Critical Zone (CZ), a term first coined by the US National Research Council, encompasses the thin outer veneer of Earth's surface extending from the top of the vegetation canopy down to the subsurface depths of fresh groundwater. Research in the Critical Zone Observatory (CZO) network is accomplished by tightly integrating geological and ecological sciences with computational simulation. CZ science hypothesizes that to fully understand ecosystem metabolism, nutrient cycling and water flow, one must study the terrestrial ecosystem from the top of the canopy to unweathered bedrock. Including the propogation of biological and ecological effects over 10s of meters belowground depth, whether measured by CO2 and O2 content, mineral weathering fronts, groundwater and stream water chemistry, plant rooting, or deep microbial communities is likely to provide new insights into ecosystem function. Thus, the CZO strategy represents a unique opportunity to transform our understanding of coupled Earth surface processes, and to address quantitatively the impacts of climate and land use change and the value of CZ functions and services. CZOs address the challenge of understanding terrestrial life's support system from the deep geologic past to that of human life spans.

The science of the nine U.S. CZOs can be summarized as focused on the following questions: 1) what controls CZ properties and processes? 2) what will be the response of CZ structure, and its stores and fluxes, to climate and land use change? and, 3) how can improved understanding of the CZ be used to enhance ecosystem resilience and sustainability, and restore ecosystem functions? These questions clearly align, entirely or in part, with core research themes of the LTER Network. Thus a primary motivation for this workshop is to discuss, identify and refine scientific questions that could best be answered through LTER-CZO collaboration.

The development of CZOs and the evolving CZO network has been loosely framed on the concept of environmental gradients in order to explore and understand the full range of Earth surface conditions and processes. The breadth of environmental gradient space occupied by the nine U.S. CZOs is somewhat limited, however. In addition, while more geographically expansive, the LTER network could benefit from additional landscape coverage. Thus, a secondary consideration in discussions of LTER-CZO collaborations is to increase coverage of environmental space and the development of more complete coverage for environmental gradients in the U.S. There is also great potential for disciplinary sharing between the LTER and CZO networks. The LTER network could benefit from the deep earth and geological approaches in the CZO network and the CZO network could benefit from the ecological and biological richness present in the LTER network.

In this working group we will: 1) discuss, identify and refine scientific questions that can be answered by establishing environmental gradients using LTER and CZO sites in North America; 2) write a vision and mission statement for the working group; and, 3) outline and organize writing assignments for completion of a white paper to be shared with the CZO and LTER leadership and greater communities during Fall 2015, to be formalized by submission to Eos, ESA Bulletin and the CZO and LTER newsletters prior to the Fall 2015 AGU meeting.