Trees are Groundwater Stakeholders Too

Une conférence de Steven P Loheide (University of Wisconsin - Madison, USA) dans le cadre des Darcy Lecture 2026

Groundwater dependent ecosystems depend on groundwater to thrive. Groundwater dependent ecosystems include wetlands and riparian forests that border streams and are well recognized in arid regions where lush vegetation may only exist where shallow groundwater is accessible to plant roots. However, in humid, temperate regions it’s often assumed that forests do not rely on groundwater because precipitation is typically sufficient to meet the plant water demand. We tested this assumption by quantifying groundwater's influence on tree growth and transpiration in northern humid forests with sandy soils. Time-series of groundwater levels show that groundwater levels fall during the daylight hours when transpiration occurs and recover during nighttime periods in some of the observation wells we monitored. We used these diurnal groundwater fluctuations to quantify groundwater consumption and found that northern Wisconsin’s forests consume groundwater when and where it is within 3m of the land surface. Furthermore, we analyzed tree growth response by coring trees and measuring annual tree ring increments. We found that trees in regions with shallow groundwater had up to twice as much growth as indicated by tree rings compared to regions where groundwater was deeper than 5m. Finally, we employed remote sensing techniques that compared vegetation indices during wet and dry periods and mapped the degree of groundwater influence across the study area. Counter to conventional wisdom, this research demonstrates that shallow groundwater subsidizes evapotranspiration even in humid forests and enhances forest productivity. Recognition of forests as groundwater users is important in guiding sustainable water and forest management decisions in the region.

More about [Steven P Loheide](https://www.ngwa.org/events-and-education/groundwater-lecture-series/2026-darcy-lecturer)