Growing deep roots has opposing impacts on the transpiration of apple trees planted in subhumid loess region

Abstract

Both field observation and numerical model have demonstrated that trees can extend their roots to deep soil (>1 m in depth). However, little work has been done to assess how deep roots impact trees’ transpiration. This study hypothesized that when available water in deep soil is exhausted, roots in drying soil will inhibit forest transpiration. To test this hypothesis, two irrigation schemes—shallow soil irrigation (SI) and shallow soil plus deep soil irrigation (SDI)—were performed in September 2017 in a 23-year-old apple orchard. After irrigation (100 mm for SI and 500 mm for SDI), soil water within the top 3 m for SI and top 8 m for SDI were replenished to about field capacity. Measurements were conducted before irrigation in July, 2017 and after irrigation from April to July in 2018. The results showed that the two treatments had similar soil water status and sap flow density before irrigation (P > 0.05). Soil water storage from 1 to 18 m depth decreased more than 1200 mm after afforestation. However, after irrigation, sap flow density in the SI treatment was significantly smaller than that in the SDI treatment (P < 0.05). Sap flow density in the SI treatment only accounted for 60 ± 6% of that in the SDI treatment during the period with high water demand. Stomatal conductance in the SI plot was significantly smaller than that in the SDI treatment, while there was no difference in leaf area index between the two plots. These results demonstrated that growing deep roots has opposing roles: it promotes transpiration when there is sufficient available water in deep soil, but inhibits transpiration via decreasing stomatal conductance when available water in deep soil is exhausted. These findings improve the understanding of how deep roots impact apple tree water use in water-limited environments.