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Biogeosciences An interactive open-access journal of the European Geosciences Union
doi:10.5194/bg-2017-61
© Author(s) 2017. This work is distributed
under the Creative Commons Attribution 3.0 License.
Reviews and syntheses
03 Mar 2017
Review status
A revision of this discussion paper is under review for the journal Biogeosciences (BG).
Reviews and syntheses: On the roles trees play in building and plumbing the Critical Zone
Susan L. Brantley1, David M. Eissenstat2, Jill A. Marshall3, Sarah E. Godsey4, Zsuzsanna Balogh-Brunstad5, Diana L. Karwan6, Shirley A. Papuga7, Joshua Roering3, Todd E. Dawson8, Jaivime Evaristo9, Oliver Chadwick10, Jeffery J. McDonnell9, and Kathleen C. Weathers11 1Earth and Environmental Systems Institute and Department of Geosciences, Pennsylvania State University, PA, USA
2Department of Ecosystem Science and Management, Pennsylvania State University, PA, USA
3Department of Geological Sciences, University of Oregon, Eugene, OR, USA
4Department of Geosciences, Idaho State University, Pocatello, ID, USA
5Department of Geology and Environmental Sciences, Hartwick College, Oneonta, NY, USA
6Department of Forest Resources, University of Minnesota, Saint Paul, MN, USA
7School of Natural Resources and Environment, University of Arizona, Tucson, AZ, USA
8Department of Integrative Biology, Univerisity of California, Berkeley, CA, USA
9School of Environment and Sustainability, University of Saskatchewan, Saskatoon, Canada
10Department of Geography, University of California-Santa Barbara, Santa Barbara, CA, USA
11Cary Institute of Ecosystems Studies, Millbrook, NY, USA
Abstract. Trees, the most successful biological power plants on earth, build and plumb the critical zone (CZ) in ways that we do not yet understand. To encourage exploration of the character and implications of interactions between trees and soil in the CZ, we propose nine hypotheses that can be tested at diverse settings. Depending upon one's disciplinary background, many of the hypotheses may appear obviously true or obviously false. We infer from this lack of agreement that the following nine hypotheses are important and must be tested to advance critical zone science. (1) Tree roots can only physically penetrate and biogeochemically comminute the immobile substrate underlying mobile soil when that underlying substrate is fractured or pre-weathered. (2) In settings where the depth of weathered material, H, is large, trees primarily shape the CZ through biogeochemical reactions within the rooting zone. (3) In forested uplands, the thickness of mobile soil, h, can evolve toward a steady state because of feedbacks related to root disruption and tree throw. (4) In settings where h << H and the rate of uplift and erosion are low, the uptake of phosphorus into trees is buffered by the fine-grained fraction of the soil, and the ultimate source of this phosphorus is dust. (5) In settings of limited water availability, trees maintain the highest density of functional roots at depths where water can be extracted over most of the growing season with the least amount of energy expenditure. (6) Trees grow the majority of their roots in the zone where the most growth-limiting resource is abundant, but they also grow roots at other depths to forage for other resources and to hydraulically redistribute those resources to depths where they can be taken up more efficiently. (7) Trees rely on matrix water in the unsaturated zone that at times may have an isotopic composition distinct from the gravity-drained water that transits from the hillslope to groundwater and streamflow. (8) Mycorrhizal fungi can use matrix water directly but trees can only use this water by accessing it indirectly through the fungi. (9) Even trees growing well above the valley floor of a catchment can directly affect stream chemistry where changes in permeability near the rooting zone promote intermittent zones of water saturation and downslope flow of water to the stream.

Citation: Brantley, S. L., Eissenstat, D. M., Marshall, J. A., Godsey, S. E., Balogh-Brunstad, Z., Karwan, D. L., Papuga, S. A., Roering, J., Dawson, T. E., Evaristo, J., Chadwick, O., McDonnell, J. J., and Weathers, K. C.: Reviews and syntheses: On the roles trees play in building and plumbing the Critical Zone, Biogeosciences Discuss., doi:10.5194/bg-2017-61, in review, 2017.
Susan L. Brantley et al.
Susan L. Brantley et al.
Susan L. Brantley et al.

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Short summary
This review represents the outcome from an invigorating workshop discussion that involved tree physiologists, geomorphologists, ecologists, geochemists, and hydrologists and developed nine hypotheses that could be tested. We argue these hypotheses point to the essence of issues we must explore if we are to understand how the natural system of the earth surface evolves, and how humans will affect its evolution. This paper will create discussion and interest both before and after publication.
This review represents the outcome from an invigorating workshop discussion that involved tree...
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