Hydrologic regulation of plant rooting depth

Abstract

Plant rooting depth affects ecosystem resilience to environmental stress such as drought. Deep roots connect deep soil/groundwater to the atmosphere, thus influencing the hydrologic cycle and climate. Deep roots enhance bedrock weathering, thus regulating the long-term carbon cycle. However, we know little about how deep roots go and why. Here, we present a global synthesis of 2,200 root observations of >1,000 species along biotic (life form, genus) and abiotic (precipitation, soil, drainage) gradients. Results reveal strong sensitivities of rooting depth to local soil water profiles determined by precipitation infiltration depth from the top (reflecting climate and soil), and groundwater table depth from below (reflecting topography-driven land drainage). In well-drained uplands, rooting depth follows infiltration depth; in waterlogged lowlands, roots stay shallow, avoiding oxygen stress below the water table; in between, high productivity and drought can send roots many meters down to the groundwater capillary fringe. This framework explains the contrasting rooting depths observed under the same climate for the same species but at distinct topographic positions. We assess the global significance of these hydrologic mechanisms by estimating root water-uptake depths using an inverse model, based on observed productivity and atmosphere, at 30″ (∼1-km) global grids to capture the topography critical to soil hydrology. The resulting patterns of plant rooting depth bear a strong topographic and hydrologic signature at landscape to global scales. They underscore a fundamental plant-water feedback pathway that may be critical to understanding plant-mediated global change.

Keywords: global change biology; infiltration depth; plant rooting depth; soil hydrology; water table depth.

Fig. 1.

Schematic of soil water profiles along a drainage gradient, wetted from above by rain infiltration and from below by groundwater capillary rise, with a dry gap that diminishes downslope. Along this gradient, plant rooting depths vary systematically (see text). SI Appendix, Fig. S3 gives examples of published root images at different drainage positions.

Fig. 2.

Maximum observed rooting depth (in meters). Points overlap due to multiple samples in a small area. The deepest root is shown at sites with multiple samples. Inset gives frequency distribution (0.2-m bin width).

Fig. 3.

Rooting depth (log-scale) vs. (A) mean annual rainfall, (B) soil texture (SI Appendix, Table S1A), (C) depth of soil barriers, (D) growth form (SI Appendix, Table S1B), (E) genera (SI Appendix, Table S1C), and (F) WTD, giving Pearson correlation coefficient r (on original data) and sample size N out of 2,200 observations.

Fig. 4.

A hydrologic framework for interpreting plant rooting depth along the climate gradient (vertical axis) defining regional patterns in infiltration depth and frequency, and land drainage gradient (horizontal axis) defining local patterns in groundwater accessibility and oxygen stress. See text for discussions of the cases.

Fig. 5.

Schematic of how soil texture regulates the two soil water fluxes and profiles: from the top, precipitation infiltration flux (green arrows) and the resulting soil water profile (green dashed line), and from below, the groundwater capillary rise (blue arrows) and resulting soil water profile (blue dashed line), in (A) fine-textured, for example, clay, (B) medium-textured, for example, silt, and (C) coarse-textured, for example, sandy, soils. The width of the arrow indicates flux rate, and the length indicates flux reach, with equal precipitation and WTD.

Fig. 6.

Inverse-model results of 10-y mean maximum depth (in meters) of root water uptake (Upper). Insets reveal strong local topographic influence. The frequency distribution (Lower, 0.2-m bin width), over vegetated surface only, suggests large model–observation discrepancy, which may imply observation bias (undersampling of very shallow and very deep roots). The oscillations in the model distribution are due to soil water uptake crossing discrete soil layers.

Fig. 7.

Modeled uptake profiles at six grid cells with rooting depth observations (, , –48), corresponding to six cases in the conceptual model of Fig. 4. The colored panels plot monthly root water uptake (in millimeters per day per meter soil depth) over the 10 y of simulation, at different soil depth (in meters), with monthly mean infiltration depth (blue line) WTD (gray line). The 10-y mean uptake profile is shown to the Right. The map at Center is a reduced version of Fig. 2. The model exhibits the same behavior as observations under different climate/drainage combinations. In the desert, observations are of single plants not detectable by satellites, so model results of a nearby grid are shown, which have higher leaf area index and thus deeper uptake than observed.