Effect of irrigation on tree vitality in a temperate forest in Germany

Over the past decades, extensive groundwater extraction has disturbed the water balance in the Hessian Ried region of Germany, resulting in a deepening of the groundwater table. Consequently, the forests in this area, which were initially reliant on groundwater, are now solely dependent on precipitation. The increasing frequency of summer droughts further jeopardizes the vitality of these forests. To address these issues, above-ground irrigation as an alternative to restoring the groundwater table could be pivotal. Implementing irrigation strategies may allow to revitalize trees and/or reduce mortality rates.

To assess the impact of irrigation on tree vitality, an experiment is conducted since 2021 in the municipal forest of Gernsheim, located in the Hessian Ried region, Germany, a region, where large forest areas are suffering from groundwater abstraction. The experiment involved designated control and irrigation plots, each in threefold replication covering 2500 m² per plot. Perforated pipes were installed at the irrigation plots on the ground, delivering water based on real-time field measurements of soil water content, matric potential, and precipitation regime. The irrigation plots received 186 mm, 505 mm, and 332 mm of additional water in 2021, 2022, and 2023, respectively.

Phenological observations indicated that irrigation prevented premature senescence of the foliage compared to control plots following a drought in 2022. Even in the following year, non-irrigated plots showed 10% less canopy cover. Additionally, leaf area index was significantly higher in 2022 and 2023 at the irrigated compared to the control plots, despite starting lower there in 2021. The negative impact of drought on tree growth was also significantly reduced by irrigation: the diameter increment of trees at irrigated plots during the drought year 2022 was similar to previous moist years for oak trees, while the increment was even higher for hornbeam trees. Tree mortality rates remained consistent at 3-5% in 2022 regardless of irrigation. However, tree mortality decreased to 0% in 2023 under the irrigation scheme, while it remained high at 10% in the control plots.

To conclude, forest irrigation effectively reduced premature senescence of tree foliage during the severe drought of 2022, and thus prolonged the growing period. Irrigation significantly contributed to preserving tree vitality and reducing tree mortality in the subsequent year. Given that trees might endure the adverse effects of drought for several years, long-term studies are necessary to determine to what extent forest condition can be revitalized over time with irrigation.