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.

Figure 1

(a) Location of our study sites within the Edwards Plateau of Texas and the experimental plots (black outline). (b) and (c) The experimental plots and immediate vicinity in 2020 (Google Earth image) and in 1989 (aerial imagery). In (b), white circles indicate points where bedrock permeability tests were performed and dashed lines indicate the locations of our trenches. Within the plots, dotted green lines delimit the zones covered by woody plants.

Figure 2

Profiles in the trench at the encroached site, at locations approximately 5 m apart and equally distant from the canopy dripline, in (a) an intercanopy zone and (b) a canopy zone. The knife is 20 cm long. (c) A 5-mm-thick root growing directly through the weathered limestone matrix. (d) Petrocalcic fragments between the Cr1 and Cr2 layers. (e) Wide fracture, filled with soil and woody roots, in the Cr2 layer. (f) Termite tunnels in the Cr2 layer.

Figure 3

(a) Box plots showing the median, inter-quartile range, 95% confidence interval, and mean (triangles) of weathered bedrock K_sat in the canopy and intercanopy zones of the encroached and non-encroached sites. The y-axis was log10-scaled for improved visualization. (b) Interpolation maps of K_sat for weathered bedrock in the encroached and non-encroached sites. The dotted black lines delimit the areas covered by woody plants and the red dots indicate the points at which the permeability tests were performed (see also Fig. 1b).

Figure 4

Bar graphs showing mean values of soil thickness (depth to the Cr1 layer), regolith thickness (depth to the Cr2 layer), and the maximum observed rooting depth by cover type (canopy vs. intercanopy) for the encroached and non-encroached sites. Error bars represent the 95% confidence interval.

Figure 5

Box plots showing matrix porosity (Φ) and rebound values (R) of rock samples (obtained in situ with a Schmidt hammer) from the Cr1 and Cr2 layers of the encroached and non-encroached sites, by cover type (canopy vs. intercanopy).