Characterisation of the <i>Pinus banksiana</i> root system on analogues of a cover with capillary barrier effects

To control acid mine drainage, a cover with capillary barrier effects (CCBE) can be constructed over mine wastes. This cover relies on a fine-grained material (moisture retaining layer; MRL) that maintains a high degree of saturation (≥85%). However, the roots of deep-rooted plants on CCBE can penetrate the MRL and extract water, thus impacting the performance of the cover. Furthermore, as cover systems are expected to perform for hundreds of years, CCBE design must anticipate long-term ecosystem changes that will influence cover performance. However, data representative of the rooting of future ecosystems at reclaimed sites can be difficult to acquire by conventional methods. Using natural analogues of cover systems can help to obtain this information. In this paper, the root systems of nine Pinus banksiana (61 to 89 years old) were studied in sand over silt layers (used as CCBE analogues) in the boreal zone of Quebec, Canada. The maximum rooting depth (MRD), coarse root colonisation intensity along the soil profile, and root length densities (RLDs) were characterised in both material layers. The MRD ranged from 25 cm to 160 cm (depending on the distance of the trenches from the trees), with some roots able to penetrate 120 cm into the MRL. However, root colonisation of the MRL was mostly concentrated in the surface sand layer, with rooting occurrences and densities > 90% in the top 50 cm of sand. These data are critical to optimise the thickness of the coarse layer to protect the MRL from root intrusion and to predict the long-term performance of CCBEs.