Drought responses of profile plant-available water and fine-root distributions in apple (Malus pumila Mill.) orchards in a loessial, semi-arid, hilly area of China

The number of apple (Malus pumila Mill.) orchards has increased substantially in hilly regions of the Loess Plateau of China, as a significant element of the large-scale ‘Grain for Green’ ecological rehabilitation program that aims to conserve soil and water while improving the regions economic prospects. However, the long-term effects of the orchard expansion and the adaptive responses of apple trees to drought are not known. Thus, using a space-for-time substitution approach, we investigated plant-available water and fine-root distribution in the 0–8 m soil profile in apple orchards of various ages in a dry year (2015, 392 mm rainfall) and the following year with normal precipitation (2016, 500 mm rainfall). We found that plant-available water gradually decreased with stand age in the dry year, but increased in the normal year, especially in the 0–2 m soil layer. Fine root (<2 mm diameter) distribution and biomass increased with stand age and decreased with increasing soil depth in all treatment plots, predominantly in the 0–2 m layer. In all treatment plots, most of the soil layers in the deep soil (>2 m) had soil moisture storage deficit. In the dry year (2015), the apple trees increased both the average depth (D₅₀ and D₉₅ values) and biomass of their fine-root systems in response to water stress, relative to the normal year (2016). Thus, the apple trees extracted water primarily from the shallow (<2 m) layers in the normal year, but from deeper soil layers in the dry year, to sustain growth. The results of this study will help to guide land and agricultural water management in rainfed apple orchards in hilly regions of the Loess Plateau and similar dryland regions.