“Thicketized oak woodlands reduce groundwater recharge”
Article available online Dec. 2022 in Science of the Total Environment. Access the article via the permanent web address (DOI). (https://doi.org/10.1016/j.scitotenv.2022.160811)
Highlights
• Woodland thicketization is a common phenomenon across the oak savannas (USA) with possible implications for groundwater recharge.
• Chloride and soil moisture were measured under different vegetation covers in a woodland pasture in Post Oak Savanna, Texas.
• Both sets of measurements showed substantial deep drainage in the open areas and no deep drainage in the thicketized woodland.
• Our results strongly suggest that thicketization is greatly reducing recharge in the Post Oak Savannas and the underlying Carrizo-Wilcox aquifer.
Abstract
Woodlands and pastures across the Post Oak Savannas (POS) in Texas have been undergoing thicketization over the last century via encroachment by understory shrubs such as Yaupon (Ilex decidua, Ilex vomitoria) and expansion of eastern redcedar (Juniperus virginiana). Because a large part of POS overlies the Carrizo-Wilcox (CW) aquifer – the third most important aquifer in Texas, there is a strong incentive to identify opportunities to increase groundwater recharge through land management.
The purpose of this research is to evaluate the influence of thicketization of post oak (Quercus stellata) stands on deep drainage (DD) in POS. We achieved this by, a) applying chloride mass balance on soil cores, and b) simultaneously monitoring soil moisture in a woodland pasture setting in POS. Four sites representing different vegetation covers were identified for sampling: 1) a thicketized oak woodland paired with an adjacent open site, 2) a woodland mosaic, 3) a pasture and 4) a pine-oak stand paired with an adjacent open site. A total of 24 soil cores to the depth of 260 cm were collected and (soil) pore water chloride concentrations at multiple depths were measured. Soil moisture was monitored at 21 locations, to the depth of 140–260 cm using a neutron moisture meter.
Negligible DD was estimated in the thicketized woodland, whereas most open locations recorded 3–18 cm/year and the woodland mosaic 0–1 cm of DD. Soil moisture data, collected from Jul-2020 to Jun-2021 also suggested higher deep drainage fluxes under open areas – with occurrence of sub-surface saturation only under the open areas and never under the woodlands. These results suggest that the thicketization in oak savannas is substantially reducing groundwater recharge.
Given the extent of thicketized oak savannas across United States, this could be impacting water budgets and groundwater recharge rates on regional scales.
Keywords: Woody plant encroachment; Ecohydrology; Chloride mass balance; Carrizo–Wilcox aquifer; Thicketization
Citation
Basant, Shishir, Bradford P. Wilcox, Chelsea Parada, Briana M. Wyatt, and Brent D. Newman. "Thicketized oak woodlands reduce groundwater recharge." Science of The Total Environment 862 (2023): 160811.
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