Water sources for <i>Robinia pseudoacacia</i> and <i>Pinus thunbergii</i> under different topsoil thicknesses on granite slopes in central southern Shandong Province, China.

Understanding the changes in water absorption sources of typical tree roots under different soil development thickness conditions, clarifying the adaptability of typical tree species to shallow soil habitats, are of great significance for sustainable construction and management of artificial forest ecosystems. This study focused on <i>Robinia pseudoacacia</i> forests (topsoil thickness 27 and 20 cm) and <i>Pinus thunbergii</i> forests (topsoil thickness 20 and 10 cm) in the granite hilly area of central southern Shandong Province under different soil horizon thicknesses. We measured the stable hydrogen and oxygen isotope compositions (δ²H and δ¹⁸O) of xylem water, soil water, groundwater, and precipitation, and quantified the changes in water source composition of the two species under different top soil thicknesses by the MixSIAR model. The results showed that during the drought period, the soil moisture content of the two forest types was 0.018-0.108 cm³·cm^-3. <i>R. pseudoacacia</i> and <i>P. thunbergii</i> mainly utilized groundwater, with the rates exceeding 50%. Under the condition of thinner soil horizons, both species had higher utilization rates of groundwater. During the wet and relatively wet periods, soil moisture content was 0.053-0.194 and 0.032-0.124 cm³·cm^-3, respectively. <i>R. pseudoacacia</i> and <i>P. thunbergii</i> mainly utilized soil water, with rates over 70% and 60%, respectively (except for <i>P. thunbergii</i> which still relied mainly on groundwater in July 2021). Under the condition of thinner soil horizons, the two tree species had higher utilization rates of soil water in the 0-40 and 0-20 cm layers, respectively. Under the same condition of soil thickness, <i>R. pseudoacacia</i> had a higher utilization ratio of groundwater than <i>P. thunbergii</i> in the drought period, while <i>R. pseudoacacia</i> had a higher utilization ratio of soil water than <i>P. thunbergii</i> during the wet period. At the beginning of the wet period, <i>R. pseudoacacia</i> used soil water as its main water source, while <i>P. thunbergii</i> mainly utilized groundwater. Therefore, <i>R. pseudoacacia</i> is more sensitive to changes in soil moisture in thinner soil horizons, while <i>P. thunbergii</i> has stronger adaptability to thinner soil conditions in the granite hilly areas of central and southern Shandong.