Influence of competition on root architecture and root anchorage of young hybrid poplar plantations on waste rock slopes

Root systems are crucial to tree and soil stability in terms of mechanical support, but are rarely considered in revegetation studies. This study aimed at determining the effect of planting density and herbaceous hydroseeding on the root architecture and mechanical resistance to uprooting of planted hybrid poplars on waste rock piles. The study took place at an open pit gold mine in the Canadian boreal zone where a hybrid poplar plantation was established in 2013 on soil-covered waste rock slopes (33%). Four treatments were randomly distributed in three replicated blocks: 4 × 4 m, 2 × 2 m, 1 × 1 m between trees and 2 × 2 m + h, corresponding to trees spaced at 2 × 2 m with hydroseeding. Lateral uprooting tests of four-year-old hybrid poplars were conducted in the direction of the slope. Maximum resistance to the uprooting force was observed in the 2 × 2 m treatment. Root systems were more symmetrical, deep, branched, and had higher root diameters in the 2 × 2 m and 4 × 4 m treatments. In contrast, the treatments with greater intra- or interspecific competition showed shallower, asymmetrical root systems with smaller roots. Therefore, competition significantly reduced tree anchorage.