Ecohydrological advantage of young apple tree-based agroforestry and its response to extreme droughts on the semiarid Loess Plateau

Abstract

Agroforestry is considered to be climate-smart; not only can it be profitable for smallholders, but also it represents a sustainable form of intensive agriculture that can help buffer the effects of extreme climate. However, its feasibility remains under dispute in semiarid regions, especially where there can be extreme droughts. We therefore investigated how intercropping with annual bioenergy crops, soybean (Glycine max) and canola (Brassica rapa), affects ecohydrology in young apple trees, and how the trees respond to droughts of varying degrees on the semiarid Loess Plateau of China. A monoculture orchard was used as a control, and the droughts were controlled by reducing natural precipitation by 15% (moderate drought) and 25% (severe drought). We found that, compared with the monoculture, the agroforestry system increased soil water storage (SWS) in the 80-280 cm by 5-8%, apple tree's daily water use (Q) by 76-118%, and transpiration per unit leaf area (TrL) by 33-71%; it also promoted tree growth. Both drought levels affected soil water availability and water use. Compared with agroforestry without an enforced drought, moderate drought conditions reduced SWS in the 80-180 cm by 11-15%, the Q value by 19-24%, and TrL value by 13-17%, and these apple trees still had higher Q and TrL levels than monoculture trees suffering no drought; and severe drought conditions caused the apple trees to absorb soil water from deeper soil layers, and reduced SWS in the 80-280 cm by 16-18%, Q by 50-60% and TrL by 12-38%. However, there were no significant difference in Q, TrL and aboveground growth parameters for trees between monoculture and severe drought treatments. These findings demonstrate that agroforestry has clear ecohydrological advantage to monoculture for young apple trees in semiarid regions.