Reviews and syntheses: On the roles trees play in building and plumbing the Critical Zone

Abstract

<p><strong>Abstract.</strong> 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, <i>H</i>, is large, trees primarily shape the CZ through biogeochemical reactions within the rooting zone. (3) In forested uplands, the thickness of mobile soil, <i>h</i>, can evolve toward a steady state because of feedbacks related to root disruption and tree throw. (4) In settings where <i>h</i> << <i>H</i> 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.</p>

The Demographics of Water: A Review of Water Ages in the Critical Zone

“…Moreover, recent development of in situ membrane‐based probes for direct measurements of soil water isotopes (e.g., Rothfuss et al, ; Volkmann & Weiler, ) and plant xylem water isotopes (Volkmann, Kuhnhammer, et al, ) allow for continuous observations along soil profiles or within trees. Such new in situ continuous measurement methods allow for new insights into processes at the soil‐plant interface and at the same time highlight the need for research about stable isotopic variation in subsurface waters used by plants (Brantley et al, ).…”

Patterns in Soil Chemical Weathering Related to Topographic Gradients and Vegetation Structure in a High Andean Tropical Ecosystem

“…The plant canopy intercepts rainfall by its leaves, stems, and branches; plants control soil water losses through transpiration; and their roots and vacant macropores generally increase the soil's infiltration capacity and vertical preferential flow. Vegetation‐driven change in water flow paths and rates in the soil system can have an effect on soil water residence time and soil solute fluxes (Drever , ; see also Brantley et al, ; Kelly et al, ).…”

Water mining from the deep critical zone by apple trees growing on loess

“…Although significant new findings have come from coupled exploration of plant water use and groundwater recharge tradeoffs (Brooks, Barnard, Coulombe, & McDonnell, ), much of the work to date has been exclusive to shallow soil profiles. We know that trees plumb and exploit the critical zone (Brantley et al, ) but our inability to measure these processes limits our understanding of the connections, couplings, and controls of deep roots and soil water dynamics. Recent work that has gone deeper into the critical zone has revealed new behaviours as linked to tree water sourcing of rock moisture (Rempe & Dietrich, ) and down‐valley subsidy of groundwater to their parent watersheds (Ameli, Gabrielli, Mortgenster, & Mcdonnell, ).…”