Water competition among the coexisting Platycladus orientalis, Prunus davidiana and Medicago sativa in a semi-arid agroforestry system

Abstract

Agroforestry has been widely used for the ecological restoration construction of semi-arid regions of the Chinese Loess Plateau. Investigating plant water sources is crucial for understanding the key ecohydrological processes of plant root water uptake and the water relations between coexisting plants. Clarifying the plant water use strategies can help to assess the sustainability of the vegetation restoration in agroforestry systems and provide references for the vegetation management. Thus, we investigated the water use characteristics of plants in an agroforestry system consisting of Platycladus orientalis, Prunus davidiana and Medicago sativa. The δD and δ18O of plant stem water and soil water in the 0–300 cm layer were measured. The MixSIAR model and proportional similarity (PS) were used to quantify the proportion of soil water absorbed by the plants in each layer and the hydrological niche overlap of plants. The results showed that the combined effects of rainfall recharge and evapotranspiration (ET) loss resulted in a decrease in soil water content (SWC) for the agroforestry system with increasing soil depth, and the SWC of the deep layer (80–300 cm) had minor variability (6.1–7.1%) throughout the study period. During the study period, due to the deep soil desiccation, P. orientalis, P. davidiana and M. sativa mainly utilized water from shallow (0–80 cm) soil layers, with a contribution rate of 65.4 ± 8.3%, 64.9 ± 7.7% and 73.0 ± 0.2%, respectively. The three coexisting plants relied on the unstable shallow soil water (recharged by rainfall) while the water use proportion of the deep soil layer was consistently low. This implies that the three plants were less resistant to drought stress, thus posing a threat to the stability and sustainability of the agroforestry system. Moreover, even in July and September with the high SWC in agroforestry, the partitioning of soil water among the three coexisting plants did not increase with increased ET loss. The three coexisting plants showed strong competition for soil water use, as indicated by the high values (0.69–0.97) of PS. Overall, the hydrological niche overlap of the coexisting plants in the agroforestry system resulted in low ecosystem stability. Density reduction or selection of suitable species is required to achieve sustainable vegetation restoration.