This content will become publicly available on November 4, 2025
Woody encroachment—the expansion of woody shrubs into grasslands—is a widely documented phenomenon with global significance for the water cycle. However, its effects on watershed hydrology, including streamflow and groundwater recharge, remain poorly understood. A key challenge is the limited understanding of how changes to root abundance, size and distribution across soil depths influence infiltration and preferential flow. We hypothesised that woody shrubs would increase and deepen coarse‐root abundance and effective soil porosity, thus promoting deeper soil water infiltration and increasing soil water flow velocities. To test this hypothesis, we conducted a study at the Konza Prairie Biological Station in Kansas, where roughleaf dogwood (
- Award ID(s):
- 2034232
- PAR ID:
- 10553636
- Publisher / Repository:
- Wiley Blackwell (John Wiley & Sons)
- Date Published:
- Journal Name:
- Ecohydrology
- ISSN:
- 1936-0584
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
More Like this
-
Woody encroachment is a widespread phenomenon in grassland ecosystems, driven by overgrazing, fire suppression, nitrogen deposition and climate change, among other environmental changes. The influence of woody encroachment on processes such as chemical weathering however is poorly understood. In particular, for fast reactions such as carbonate weathering, root traits associated with woody encroachment (e.g., coarser, deeper, and longer residence times) can potentially change fluxes of inorganic carbon into streams and back to the atmosphere, providing CO2-climate feedbacks. Here we examine the influence of deepening roots arising from woody encroachment on catchment water balance and carbonate weathering rates at Konza a tallgrass prairie within a carbonate terrain where woody encroachment is suspected to drive the groundwater alkalinity upwards. We use a watershed reactive transport model BioRT-Flux-PIHM to understand the ramifications of deepening roots. Stream discharge and evapotranspiration (ET) measurements were used to calibrate the hydrology model. The subsurface CO2 concentration, water quality data for groundwater, stream, soil water and precipitation were used to constrain the soil respiration and carbonate dissolution reaction rates. The hydrology model has a Nash-Sutcliffe Efficiency value of 0.88. Modelling results from numerical experiments indicate that woody encroachment results in overall lower stream flow due to higher ET, yet the groundwater recharge is higher due to deep macropore development from deepening roots. The deeper macropores enhance carbonate weathering rate as more acidic, CO2-rich water recharges the deeper calcite bedrock. Accounting for the change in inorganic carbon fluxes caused by such land use changes gives a better estimate of carbon fluxes in the biosphere. Such knowledge is essential for effective planning of climate change mitigation strategies.more » « less
-
Abstract In savannas, partitioning of below‐ground resources by depth could facilitate tree–grass coexistence and shape vegetation responses to changing rainfall patterns. However, most studies assessing tree versus grass root‐niche partitioning have focused on one or two sites, limiting generalization about how rainfall and soil conditions influence the degree of rooting overlap across environmental gradients.
We used two complementary stable isotope techniques to quantify variation (a) in water uptake depths and (b) in fine‐root biomass distributions among dominant trees and grasses at eight semi‐arid savanna sites in Kruger National Park, South Africa. Sites were located on contrasting soil textures (clayey basaltic soils vs. sandy granitic soils) and paired along a gradient of mean annual rainfall.
Soil texture predicted variation in mean water uptake depths and fine‐root allocation. While grasses maintained roots close to the surface and consistently used shallow water, trees on sandy soils distributed roots more evenly across soil depths and used deeper soil water, resulting in greater divergence between tree and grass rooting on sandy soils. Mean annual rainfall predicted some variation among sites in tree water uptake depth, but had a weaker influence on fine‐root allocation.
Synthesis . Savanna trees overlapped more with shallow‐rooted grasses on clayey soils and were more distinct in their use of deeper soil layers on sandy soils, consistent with expected differences in infiltration and percolation. These differences, which could allow trees to escape grass competition more effectively on sandy soils, may explain observed differences in tree densities and rates of woody encroachment with soil texture. Differences in the degree of root‐niche separation could also drive heterogeneous responses of savanna vegetation to predicted shifts in the frequency and intensity of rainfall. -
Abstract Here we use Richards Equation models of variably saturated soil and bedrock groundwater flow to investigate first‐order patterns of the coupling between soil and bedrock flow systems. We utilize a Monte Carlo sensitivity analysis to identify important hillslope parameters controlling bedrock recharge and then model the transient response of bedrock and soil flow to seasonal precipitation. Our results suggest that hillslopes can be divided into three conceptual zones of groundwater interaction, (a) the zone of lateral unsaturated soil moisture accumulation (upper portion of hillslope), (b) the zone of soil saturation and bedrock recharge (middle of hillslope) and (c) the zone of saturated‐soil lateral flow and bedrock groundwater exfiltration (bottom of hillslope). Zones of groundwater interaction expand upslope during periods of precipitation and drain downslope during dry periods. The amount of water partitioned to the bedrock groundwater system a can be predicted by the ratio of bedrock to soil saturated hydraulic conductivity across a variety of hillslope configurations. Our modelled processes are qualitatively consistent with observations of shallow subsurface saturation and groundwater fluctuation on hillslopes studied in our two experimental watersheds and support a conceptual model of tightly coupled shallow and deep subsurface circulation where groundwater recharge and discharge continuously stores and releases water from longer residence time storage.
-
Soil sampling pits across three hillslope positions - toeslope, backslope, and summit - were dug in 2020 in watershed N4D (burned every 4 years) and N1D (burned annually) to characterize the impacts of woody encroachment on subsurface soil physical, chemical, and biological properties. Pits were hand-dug to 120 cm in the toeslope position and to 60 cm deep at the backslope and summit positions. Soil pits in N4D were dug directly under dogwood shrubs (Cornus drumondii) while pits in N1B were dug under grasses and forbs. Soil pit faces were photographed to determine root fractions with depth, soil monoliths were take to charaterize soil macroporosity with depth while soil cores were taken in each horizon for water retention analysis. Soil sensors were also installed at four soil depths at the toeslope position and 3 soil depths at the backslope and summit positions to record half hourly soil moisture, soil temperature, soil water potential, soil electrical conductivity, and soil carbon dioxide, and soil oxygen. In addition, geophysical measurements were taken in N4D using time-lapse electrical resistivity.more » « less
-
Abstract Hydraulic redistribution is the transport of water from wet to dry soil layers, upward or downward, through plant roots. Often in savanna and woodland ecosystems, deep‐rooted trees, and shallow‐rooted grasses coexist. The degree to which these different species compete for or share soil‐water derived from precipitation or groundwater, as well as how these interactions are altered by hydraulic redistribution, is unknown. We use a multilayer canopy model and field observations to examine how the presence of deep, but tree‐root accessible, groundwater impacts seasonal patterns of hydraulic redistribution, and interaction between coexisting vegetation species in a semiarid riparian woodland (US‐CMW). Based on the simulation, trees absorb moisture at the water table (∼10 m depth) and release it in the shallow soil depth (0–3 m) during the dry pre‐monsoon season. We observed the occurrence of a new convergent hydraulic redistribution pattern during the monsoon season, where moisture is transported from both the near‐surface (0–0.5 m) and the water table to intermediate soil layers (1–5 m) through tree roots. We found that hydraulic redistribution demonstrates a growth facilitation effect at this site, supporting 49% of growing season tree transpiration and 14% of the grass transpiration. Compared to a similarly structured upland savanna without accessible groundwater, the riparian site shows an increased amount of hydraulically redistributed water and more facilitative water use between coexisting grasses and trees. These results shed light on the linkage between accessible groundwater and the role of hydraulic redistribution on the interaction between deep‐rooted and shallow‐rooted vegetation.