Impact of soil water content on the overturning resistance of young Pinus Pinaster in sandy soil

Background and aims: The tree resistance to uprooting is crucial to face wind damage in temperate forest. Tree anchorage varies considerably with site conditions, species, and tree age. Only few studies have focused on the influence of the site soil properties on the tree anchorage. With ongoing climate change, the soil hydrologic conditions are changing in Europe due to higher precipitations during winter, with possible higher risk of wind damage in forests. Methods: This study investigates the role of soil hydrology on tree anchorage of Pinus pinaster in sandy soil with a combination of field experiments and simulations. Tree pulling experiments until root-soil system failure were performed for 12 Pinus pinaster of 14 years-old growing in podzol to measure the tree resistance to uprooting M-c for two contrasted soil water conditions. In addition, simulations were conducted to analyze how M-c changes during the progressive wetting of the layered soil. For that purpose, a new model was developed for M-c. This model also includes a sub-model for the shear mechanical strengths of the sandy soil layers and their variation with soil water content. The model was calibrated with different data sets: (1) the M-c-data obtained from the tree pulling experiments performed on 14 years-old Pinus pinaster; (2) the 3D root system architectures of the pulled trees; and (3) the soil shear mechanical strength as function of the soil water content measured in laboratory by direct shear tests and soil water retention curve measurements. After calibration,-calculations were performed when simulating a progressive soil wetting by water table increase or by water saturation front progression. Results: Field-data and simulations show that M-c depends little on soil water content outside the domain of complete soil saturation. Close to saturation, simulations show that M-c decreases drastically up to 40 % of its value. This is specific to sandy soil whose mechanical strength is mainly due to the capillarity forces between grains. As illustrated by simulations, the anchorage resistance results from two components. The first friction component slightly increases with soil water content. The second suction component decreases little with soil water content and drops down at saturation when all the interstitial water in the soil porous network merges. Conclusions: This loss of anchorage resistance at full saturation may increase considerably the risk of wind damage of forest growing in sandy soil as floods increase with climate change in Europe.