The transition zones (ecotone) between boreal forests and peatlands: Modelling water table along a transition zone between upland black spruce forest and poor forested fen in central Saskatchewan

Close association between hydrology and ecosystem productivity in boreal transition zones requires that modelling ecosystem productivity in these zones be based on accurate modelling of water table dynamics. We hypothesize that these dynamics are driven by transfers of water through surface and lateral boundaries of transition zones, and that lateral transfers can be calculated from hydraulic gradients with external water tables at upper and lower boundaries. In this study we implement these hypotheses in the ecosys model to simulate water table dynamics along a boreal transition zone (ecotone) in central Saskatchewan, Canada, extending from upland black spruce forest down to a poor forested fen. Simulated water table depths were compared to measured values at upper, middle and lower ecotone positions during the dry year 2003 when peat was dried, the very wet year 2004 when peat was rewetted, and the hydrologically average year 2005 when peat remained wet. These hypotheses enabled ecosys to simulate declines in water table depth with declines in elevation along the ecotone that matched well those observed during each of the three years. Observed:expected plots of modelled vs. measured water table depths at all positions indicated reasonable goodness of fit with slopes (with respect to 1:1 line) and R2 of 0.92 and 0.53 in 2003-2005 period, 0.90 and 0.28 in 2003, 0.81 and 0.51 in 2004, 0.97 and 0.46 in 2005, confirming that our hypotheses enabled changes in water table depths along boreal transition zones to be properly modelled during successively dry, wet and normal years.