Reviews and syntheses: on the roles trees play in building and plumbing the critical zone

Abstract

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. The hypotheses are roughly divided into those about the architecture (building) and those about the water (plumbing) in the CZ, but the two functions are intertwined. Depending upon one's disciplinary background, many of the nine hypotheses listed below may appear obviously true or obviously false. (1) Tree roots can only physically penetrate and biogeochemically comminute the immobile substrate underlying mobile soil where that underlying substrate is fractured or pre-weathered. (2) In settings where the thickness 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 rates 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 length 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 anPublished by Copernicus Publications on behalf of the European Geosciences Union. 5116 S. L. Brantley et al.: Reviews and syntheses: on the roles trees play in building and plumbing the critical zone 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. By testing these nine hypotheses, we will generate important new cross-disciplinary insights that advance CZ science.

Using StorAge Selection functions to quantify ecohydrological controls on the time-variant age of evapotranspiration, soil water, and recharge

“…The approach is an innovative means to address the ecohydrological controls of atmospheric fluxes on soil water, including the sources of fluxes and the potential for ecohydrologic separation of 5 "energetically available" water for uptake (McDonnell, 2014;Good et al, 2015;Brantley et al, 2017). The simple assumption here that all soil water (both fast and slow flow domain) was available for was shown, with calibration of source with depth, to reproduce the xylem isotopic measurements reasonably well.…”

“…The characterization of the water ages of evaporation and root-uptake as well as their fluxes from discrete depths have been subject to limited investigation. The identification of root-water uptake ages and fluxes is often difficult; this is due to the often unknown root densities and soil moisture distributions that influence the spatial location of preferential root-uptake volumes (Brantley et al, 2017). Isotopes have been shown to be a useful tool to identify root-uptake source through mass-balance of 10 soil isotopic compositions and xylem water samples (Ogle et al, 2014;Geris et al, 2017;Sprenger et al, 2017b).…”

Designing a network of critical zone observatories to explore the living skin of the terrestrial Earth

“…A full elucidation of hypotheses is beyond the scope of this paper and only a subset is shown in Table 4. Many have been published in collaborative papers (Rempe and Dietrich, 2014;Riebe et al, 2016;Li et al, 2017;Pelletier et al, 2017;Yan et al, 2017;Brantley et al, 2017a). Here we summarize three multidisciplinary discoveries that have large implications for the prediction of flow paths relevant to the largest supply of accessible and drinkable water available to humans -water contained in rock and regolith (Fetter, 2001;Banks et al, 2009).…”

Ideas and perspectives: Strengthening the biogeosciences in environmental research networks

“…The biogeosciences can potentially enrich CZOs by bringing novel and advanced biological and ecological research to the otherwise predominant Earth science focus of CZOs (e.g., Chen et al, 2016;Brantley et al, 2017a). The critical zone is called "critical" because all of Earth's diverse life-forms depend entirely on the structure and function of the critical zone.…”

“…Growing numbers of biologists and geologists are working together on the biogeoscience of societally important issues (Hedin et al, 2002;Hinckley et al, 2016;Field et al, 2016;O'Neill and Richter, 2016;Wymore et al, 2017;Brantley et al, 2017a). Top-tier, multidisciplinary journals now publish biogeoscience papers, and professional ecological and geological societies have new biogeoscience journals and subdivisions.…”