Microclimate, soil moisture and forage yield vary spatially within a temperate tree-based intercropping system: From competition to facilitation

Tree-based intercropping (TBI) can improve water conservation in agroecosystems and yield stability in the face of climate change. Greater understanding is required regarding mechanisms determining spatial and seasonal variation in moisture availability and agricultural yields in TBI systems. Our two-year study assessed spatiotemporal dynamics of soil hydraulic properties, microclimatic conditions, soil moisture and forage performance (yield, nutritive value) with increasing distances from tree rows (0, 4, 12, 20 m) within a TBI system (50 trees ha−1; 10- to 11-years-old) and in agricultural controls. TBI rows included hybrid poplars that were inter-planted with high-value hardwoods. In each block (n = 3), the TBI system and control were divided in two: with vs without root-pruning (0.75 m depth using a sub-soiler). Increased soil organic matter and infiltration rate, and decreased bulk density were measured in the TBI compared to the control, but only under trees (0 m). We observed increased soil moisture near the centre of the alleys (12 and 20 m) during both extremes of water availability, likely due to measured wind speed reductions. Under high potential evaporation, atmospheric evaporative demand near the alley centres was lower than in the control. Moisture, light availability and forage yield close to tree rows (4 m) were lower than those near alley centres and in the control. Tree root pruning increased soil moisture and yield at 4 m from the tree row, but not completely to control levels, indicating that forage was limited by light and water availability at the tree-crop interface. Unlike alfalfa, yield of grasses was not negatively influenced by tree competition. Our study suggests that TBI systems can potentially improve hydrological responses of agroecosystems to extreme weather events. This potential of TBI systems appears highly useful given expected increases in the frequency and intensity of moisture deficits and extreme rainfall events.