Characteristics of fine root system and water uptake in a triploid Populus tomentosa plantation in the North China Plain: Implications for irrigation water management

The form and water uptake characteristics of the fine root system in a 5-year-old triploid Populus tomentosa plantation were investigated to make recommendations related to irrigation water management for P. tomentosa plantation. Fine roots for analysis were collected from 2106 soil cores taken around eight trees. Soil moisture, trunk sap flow and evaporation were measured concurrently for four months in two experimental plots using time-domain reflectometry, thermal dissipation sensors and micro-lysimeters, respectively. Nearly half (44%) of fine roots corresponded to 0.2–0.5 mm diameter. Generally, lateral root distribution was even, however, the vertical root profile showed an unusual pattern (nearly an 'S' shape). Dense fine roots occurred in surface soil and nearly one third (28%) of total fine roots occurred below 100 cm depth, indicating the plantation had developed a dimorphic root system. With increasing distance from the tree, root distribution tended to be shallower. Mean fine root diameter was significantly larger (P < 0.05) below 120 cm, probably due to soil texture change or/and anoxia. Root water uptake in the 0–20 cm layer contributed 58% of that within the 0–90 cm soil layer, suggesting surface roots played the major water uptake role in shallow soil (<90 cm). On average, P. tomentosa extracted 57% of transpired water from deep soil (>90 cm), implying deep roots can contribute significantly to the water relations of mature P. tomentosa plantations. This functional significance of deep roots might be determined by their high length density and relatively large diameter. Based on these results, three irrigation management strategies were recommended: (1) irrigation schedules should be devised based on periodic measurement of the depth to water table; (2) water should be mainly provided to and maintained in the surface 40 cm soil; and (3) water should be applied to the zone within 1 m from the tree.