Soil moisture and water redistribution patterns in white oak (Quercus alba) saplings and trees in fragmented urban woodlands

In the midwestern United States, models predict extended summer heatwaves and increasingly frequent and prolonged drought conditions. In the Chicago region, the potential for large-scale mortality of white oak trees (Quercus alba) coupled with the ongoing decline of white oak sapling recruitment are major concerns for researchers and practitioners. In this study, we determined the sources of water used by mature white oak trees and saplings in three qualitatively different sites within a remnant oak forest in Chicago during the 2021 drought. We investigated soil moisture dynamics (volumetric water content, VWC) and water isotope composition of leaf tissues (δD, δ¹⁸O), rainwater, and groundwater. These data were linked to sapling height (proxy for biomass) and ectomycorrhizal (ECM) functional types. We predicted that: (i) mature oak trees use deeper water sources and conducted hydraulic redistribution (HR), and (ii) mature trees shared water with saplings during dry periods via long-distance ECM functional types. Soil moisture decreased progressively from June to October (spring to fall), with August and September having the lowest moisture (<20 % VWC). Following rainfall recharge, temporal patterns of soil moisture showed gravity drainage and then ongoing stair-stepwise drawdown consistent with plant evapotranspiration. Leaf δD and δ¹⁸O values in mature trees and saplings were consistent with water uptake from rainfall and subsequent enrichment via evapotranspiration. In two sites, mature trees and saplings demonstrated distinct δD: δ¹⁸O slopes, with mature trees more enriched than saplings. In the third site, mature trees and saplings δD: δ¹⁸O slopes overlapped but here, the ECM community was dominated by contact-type ECM and sapling height increased with distance from the mature oak. Our findings indicate that HR was not a component of site ecohydrology, and future climate conditions may present increasing challenges for white oak recruitment as both mature trees and saplings compete for limited rainfall-derived soil moisture.