The transition zones (ecotone) between boreal forests and peatlands: Ecological controls on ecosystem productivity along a transition zone between upland black spruce forest and a poor forested fen in central Saskatchewan

Forest productivity declines along the transition zones (ecotone) occupying the elevational gradient from upland boreal forest to peatlands. This decline is associated with water table rise and increasing peat depth from upper to lower topographic positions. We hypothesize that in boreal transition zones to poor fens, phosphorus limitations to plant growth are imposed by low pH, rising water table and increasing depth to mineral soil in the waterlogged peat. These phosphorus limitations, together with O2 limitations to root growth, cause sharp decline of productivity down to the fen. The ecosys model was applied to test these hypotheses in a boreal transition zone in central Saskatchewan, Canada. This zone extended from an upland black spruce forest down to a poor forested fen over an organic-mineral soil gradient with peat depth increasing from 60 cm to 160 cm. Model output was compared with field-derived tree carbon stocks, leaf area indexes, and moss net primary productivities at upper, middle, and lower undisturbed ecotone positions. Model results suggest that root contact with mineral soil through shallow peat over a deep water table at the upper ecotone sustained phosphorus uptake and plants did not experience phosphorus limitations. However deep peat and shallow water table towards the fen prevented root contact with mineral soil, imposing phosphorus limitations to plants growing in the poor, acidic peat, causing productivity decline by ∼75-80%, and tree biomass decline from ∼6900 to 2100 g C m−2 in the model vs. ∼6600 to 800 g C m−2 observed along the ecotone.