Mixed-species plantations alleviate deep soil water depletion and facilitate hydrological niche partitioning compared to pure plantations

Abstract

Understanding the effects of different silvicultural patterns on soil water reservoirs and how silvicultural species adjust their water use strategies and plant function traits is critical to guiding future silvicultural activities toward the sustainable development of plantations. We continuously monitored soil gravimetric water content (SWC), soil and plant isotopes, leaf water potentials at predawn and midday (Ψpd and Ψmd), and plant morphological traits in pure Populus simonii tree plantations (PPs), pure Amorpha fruticose shrub plantations (PAf), and mixed P. simonii-A. fruticose plantations (Msf) during the 2021 growing season. The study showed that the average SWC of the deep layer (80–200 cm) was Msf > PAf > PPs. P. simonii in the Msf increased the water use proportion of the deep layer compared to that in the PPs, especially during the vigorous growth period of June-July, and shifted the major water use from shallow (0–40 cm) to deep layers on average. A. fruticose mainly used shallow soil water during the growing season. The leaf-level water use efficiency of the same species did not change in the Msf compared to that in the PPs or PAf. Based on the slope (σ) of the linear equation of Ψpd-Ψmd, P. simonii and A. fruticose exhibited isohydric (σ < 1) and an-isohydric (σ > 1) behavior under varying soil water conditions, respectively. The wood density was higher and specific leaf area (SLA) was lower in the PPs than in the Msf for P. simonii, while the ratio of stem sapwood area to leaf area was higher and SLA was lower in the PAf than in the Msf for A. fruticose. These findings reflected that mixed tree-shrub plantations alleviate deep soil water depletion compared to pure plantations, facilitate hydrological niche segmentation of co-occurring species, and reduce carbon investment in xylem and leaves to resist water stress and possibly devote more resources to growth.