Salix psammophila afforestations can cause a decline of the water table, prevent groundwater recharge and reduce effective infiltration

Afforestation can reduce desertification and soil erosion. However, the hydrologic implications of afforestation
are not well investigated, especially in arid and semi-arid regions. China has the largest area of afforestation in
the world, with one-third of the world's total plantation forests. How the shrubs affect evapotranspiration, soil
moisture dynamics, and groundwater recharge remains unclear. We designed two pairs of lysimeters, one
being 1.2 m deep and the other one 4.2 m deep. Each pair consists of one lysimeter with bare soil, while on the
other one a shrub is planted. The different water table depths were implemented to understand how depth to
groundwater affects soil moisture and water table dynamics under different hydrological conditions. Soil
moisture, water table depth, sap flow, and rainfall were measured concurrently. Our study confirms that for
the current meteorological conditions in the Ordos plateau recharge is reduced or even prohibited through the
large-scale plantation Salix psammophila. Shrubs also raise the threshold of precipitation required to increase
soil moisture of the surface ground. For the conditions we analyzed, a minimum of 6 mm of precipitation was
required for infiltration processes to commence. In addition to the hydrological analysis, the density of root
distribution is assessed outside of the lysimeters for different water table depths. The results suggest that the
root-density distribution is strongly affected by water table depth. Our results have important implications for
the determination of the optimal shrub-density in future plantations, as well as for the conceptualization of
plant roots in upcoming numerical models.