Modelling tree water uptake and water use efficiency as affected by cover thickness in reclaimed open pit mining landforms

Restoration of open pit mining landforms in to upland boreal forests strongly depends on the soil moisture availability as determined by cover thickness. A comprehensive mathematical model, ecosys was used to determine the effect of different cover thicknesses on soil water storage capacity, plant water uptake, water-use and thereby on plant productivity using three reclaimed covers (35 cm, 50 cm and 100 cm) in a 17-years-old reclamation site in the Athabasca Oilsands Region north of Fort McMurray, Alberta. The modelled soil moisture availability, plant water uptake, above ground carbon biomass and leaf area index outputs as determined by cover thickness followed the same trend as independent measurements, showing that ecosys was able to simulate plant water relations in three cover thicknesses in the reclaimed site. Available soil water content (ASWC) increased with cover thickness such that only the 100 cm cover was able to provide sufficient water for plant growth during drier periods while 35 cm and 50 cm (shallow) covers reached permanent wilting point. Lower available water holding capacity in 35 cm and 50 cm covers vs. 100 cm cover caused lower tree water uptake, CO₂ flux and consequently lower biomass gain to be modelled in these covers over the reclamation period. A positive linear relationship was derived between modelled transpiration and NPP and the water use efficiency increased from 35 cm to 50 cm and 100 cm covers due to decreased evaporation from increased leaf area index with increasing cover thickness. The greater aspen growth with increased cover thickness also contributed to increase water use efficiency in 100 cm cover vs. 35 and 50 cm covers. The current study demonstrated the importance of ensuring sufficient cover thickness to avoid water limitation in reclaimed lands and the value of using a terrestrial ecosystem model such as ecosys in forecasting short and long-term hydrological patterns, net primary productivity and thereby land capability restoration in reclamation projects.