The phylogeny of Critical Zone Observatories, or how to better structure existing observation networks to match the whole system approach
Authors: Braud Isabelle , Gaillardet Jérôme , Mercier François , Galle Sylvie , Entringer Virginie 
Published: March 2021

&lt;p&gt;Implementing the Whole System Approach for long-term ecosystem, critical zone and socio-ecological system research requires going beyond existing structuration of scientific communities and observation networks. Indeed, existing observation networks were often built independently from each other, on a very disciplinary basis, with their own scientific objectives, funding mechanisms and institutional constraints. To tackle the observation challenges of the &amp;#8220;new climatic regime&amp;#8221; in the Anthropocene, a new type of observational platforms, more compatible with a scientific systemic approach needs to be built taking into account the history and institutional contexts of long-term observatories.&lt;/p&gt;&lt;p&gt;&lt;br&gt;We have attempted to represent the diversity of critical zone observatories, sites and network of observatories that exist and that have been founded by different research institutions in France over the last 40 years and that are now gathered in the OZCAR Critical Zone network. Our representation encapsulates three main characteristics: the spatial scales of investigation (from the plot scale to the continental-scale watershed), the diversity of monitored compartments (catchments, glaciers, peatlands, aquifers&amp;#8230;), and the institutional dimension (labeling and founding at the national level).&amp;#160; We found that a representation in the form of a tree, mimicking the phylogenetic tree of life, named the OZCAR-tree, was offering a visualization tool able to capture the philosophy and rationale of the network and was useful to improve the communication with the neighboring infrastructures, users and stakeholders. The branches of the tree represent the nested monitored scales, with the small branches of the tree representing monitored parcels or small catchments. The trunks represent networks of sites investigating the same compartment. For monitored catchments, the representation directly shows the various sampled scales and their nested organization from upstream to downstream. At each site, colored pie charts allow us to visualize rapidly the types of data that are collected, each part of the pie being a component of the critical zone (atmosphere, soil water, aquifers, vegetation, snow, ice&amp;#8230;). This visualization directly shows the focus of the various sites, the completeness of measurements conducted by the different scientists, but also the missing compartments. It also shows that, if the network, as a whole is able to sample the various compartments and variables required for implementing the whole system approach, it is rarely the case when considering individual sites.&lt;/p&gt;&lt;p&gt;Beyond being a visualization tool, the OZCAR-tree helps representing the requirements of a &amp;#8220;whole critical zone approach&amp;#8221;. Because all compartments of the critical zone are connected vertically and horizontally by processes and fluxes of energy and matter, the tree is meant to represent all the components to be monitored and what should be the spatial architecture of a monitoring network fulfilling the disciplinary questions and approaches. The tree is therefore an illustration of a conceptual and idealized network (devoid of cost issues) of terrestrial surfaces monitoring infrastructure respectful of disciplinary approaches.&lt;/p&gt;&lt;p&gt;Finally, this representation is open to ecological and socio-ecological communities and may serve as a template for fostering collaboration with ecological and socio-ecological communities and networks and implementing observation platforms at the scale of changing territories.&lt;/p&gt;