| Summary: | Understanding how soil water source is used spatiotemporally by tree species and if native species can successfully coexist with introduced species is crucial for selecting species for afforestation. In the rocky mountainous areas of the Taihang Mountains, alien <i>Robinia pseudoacacia</i> L. has been widely afforested into the native shrublands dominated by <i>Ziziphus jujuba</i> Mill var. <i>spinosa</i> and <i>Vitex negundo</i> L. var. <i>heterophylla</i> to improve forest coverage and soil nutrients. However, little is known about the water relation among species, especially seasonal water use sources in different microsites. We selected the soil and plant xylem samples of two opposite microtopographic sites (ridge and valley) monthly in the growth season to analyze isotope composition. The proportions of water sources were quantified by the MixSIAR model and compared pairwise between species, microsites and seasons. We found that deep subsoil water at a depth of 40–50 cm contributed up to 50% of the total water uptake for <i>R. pseudoacacia</i> and <i>Z</i>. <i>jujuba</i> in the growing season, indicating that they stably used deeper soil water and had intense water competition. However, <i>V. negundo</i> had a more flexible water use strategy, which derived more than 50% of the total water uptake from the soil layer of 0–10 cm in the rainy season, but majorly captured soil water at a depth of 30–50 cm in the dry season. Therefore, high niche overlaps were shown in <i>V. negundo</i> with the other two species in the dry season, but niche segregation was seen in the rainy season. The microtopographic sites did not shift the seasonal dynamic of the water source use patterns of the three studied species, but the water use niche overlap was higher in the valley than in the ridge. Taken together, the introduced species <i>R. pseudoacacia</i> intensified water competition with the native semi-arbor species <i>Z. jujuba</i>, but it could commonly coexist with the native shrub species <i>V. negundo</i>. Therefore, our study on seasonal water use sources in different microsites provides insight into species interaction and site selection for <i>R. pseudoacacia</i> afforestation in the native shrub community in rocky mountainous areas. It is better to plant <i>R. pseudoacacia</i> in the shrubland in the valley so as to avoid intense water competition and control soil erosion.
|
|---|
