Canadian Forest Service Publications

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. 2014. Dimitrov, D.D.; Bhatti, J.S.; Grant, R.F. Ecological Modelling 291:96-108.

Year: 2014

Issued by: Northern Forestry Centre

Catalog ID: 35649

Language: English

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Available from the Journal's Web site.
DOI: 10.1016/J.ecolmodel.2014.07.020

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Forest productivity declines along the transition zones (ecotone) occupying the elevational gradient fromupland boreal forest to peatlands. This decline is associated with water table rise and increasing peat depthfrom 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 froman upland black spruce forest down to a poor forested fen over an organic–mineral soil gradient withpeat depth increasing from 60 cm to 160 cm. Model output was compared with field-derived tree carbonstocks, 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 overa 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 themodel vs. ∼6600 to 800 g C m−2 observed along the ecotone.

Plain Language Summary

Canada’s boreal forest has large areas where upland forests transition to forested peatlands, and then to unforested peatlands. As trees and mosses (that create peatlands) grow they take up and store carbon from the atmosphere (carbon sequestration), so understanding what controls their rate of growth is important to understanding their role in carbon sequestration. We know that the rate at which trees grow slows down in the transition from upland to peatland, but we don’t why. Phosphorous is an important nutrient for tree growth and we propose that the decline in growth might be due to a decline in the amount of phosphorous available to the trees. We used an ecosystem computer model that lets us study the complex interactions of trees and mosses with phosphorous and water in a transitional area in central Saskatchewan. Results from the model suggest that as the peat layer becomes thicker and the water in the peat layer comes closer to the surface in the forested peatland, it prevents the trees’ roots from coming into contact with the soil, which contains the phosphorous that trees need to grow. The results are important because this will help us do a better job of estimating the amount of carbon that is stored by these forests; this will also help us learn about the role of carbon in greenhouse gases in Canada. Our findings are important for studying the natural processes underlying forest productivity across forested peatlands in boreal landscapes.