Woody plant encroachment modifies carbonate bedrock: field evidence for enhanced weathering and permeability

Abstract

Little is known about the effects of woody plant encroachment—a recent but pervasive phenomenon—on the hydraulic properties of bedrock substrates. Recent work using stream solute concentrations paired with weathering models suggests that woody plant encroachment accelerates limestone weathering. In this field study, we evaluate this hypothesis by examining bedrock in the Edwards Plateau, an extensive karst landscape in Central Texas. We compared a site that has been heavily encroached by woody plants (mainly Quercus fusiformis and Juniperus ashei), with an adjacent site that has been maintained free of encroachment for the past eight decades. Both sites share the same bedrock, as confirmed by trenching, and originally had very few trees, which enabled us to evaluate how encroachment impacted the evolution of hydraulic properties over a period of no more than 80 years. Using in situ permeability tests in boreholes drilled into the weathered bedrock, we found that the mean saturated hydraulic conductivity of the bedrock was higher—by an order of magnitude—beneath woody plants than in the areas where woody plants have been continuously suppressed. Additionally, woody plant encroachment was associated with greater regolith thickness, greater plant rooting depths, significantly lower rock hardness, and a 24–44% increase in limestone matrix porosity. These findings are strong indicators that woody plant encroachment enhances bedrock weathering, thereby amplifying its permeability—a cycle of mutual reinforcement with the potential for substantial changes within a few decades. Given the importance of shallow bedrock for ecohydrological and biogeochemical processes, the broader impacts of woody plant encroachment on weathering rates and permeability warrant further investigation.

Impacts of Woody Encroachment on the Fate of Soil Co2 in Grassland Watersheds

“…Dodds et al, (2023), did not observe a change in stream flow and attributed this to woody encroachments' ability to permanently change soil structure and preferential flow paths. Consistent with observations made in Edwards Plateau, that as woody plants die the leave behind decaying roots that increase preferential flow (Leite et al 2023).…”

“…Studies in Edwards Plateau include effects by woody encroachment on subsurface porosity, groundwater recharge and changes to stream flow. One of these studies concluded that in Edwards Plateau bedrock below woody plants has much higher permeability and can increase over time (Leite et al 2023).…”

“…Coupled with these changes in soil structure, woody plants have the potential to alter soil CO2 levels and thereby affect subsurface chemical weathering (Macpherson et al 2008;Leite et al 2023). Woody roots have different transpiration rates compared to grasses and may also differ in terms of root exudates that feed soil microorganisms.…”