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ISHS Acta Horticulturae 991: IX International Workshop on Sap Flow

IMPACTS OF SEASONAL THAW AND PERMAFROST DEGRADATION ON PICEA MARIANA ROOT FUNCTION IN A SUBARCTIC BOREAL PEATLAND

Authors:   J. Baltzer, R. Patankar, A. Downey, W. Quinton
Keywords:   Picea mariana, discontinuous permafrost, climate warming, permafrost thaw, Northwest Territories
DOI:   10.17660/ActaHortic.2013.991.17
Abstract:
Boreal forests occupy latitudes that are expected to warm most dramatically over the coming decades, and evidence indicates that changes are already underway in these systems. Much of the boreal is underlain by permafrost, which can be expected to have important consequences for boreal forests as the climate warms. The southern margin of permafrost is especially susceptible to warming, since in this region, the permafrost is discontinuous, relatively thin (<10 m), warm and ice-rich. In the zone of discontinuous permafrost, permafrost forms the physical foundation on which trees develop, forming tree-covered peat plateaus. Trees in this system face two primary issues relating to ground ice: a) seasonal thaw of the active layer is not complete until July placing potential constraints on root function due to limited rooting space and low soil temperatures; and b) climate warming induced permafrost thaw is leading to ground surface subsidence, which destabilizes the trees, and waterlogging of soils, which may negatively impact root function. To investigate the impact of these cryotic processes on tree root function, we installed Heat Ratio Method (HRM) sap flow sensors on the roots of Picea mariana trees occurring on the edges of permafrost plateaus and in the interior of the plateau at the onset of active layer thaw. This allowed us to investigate seasonal changes in water uptake in response to increasing active layer thickness, which was remarkably stable throughout this period. In contrast, the location of trees in relation to degrading permafrost edges had marked impacts on root function. Specifically, trees on degrading edges had negligible sap flow rates that showed little or no diurnal pattern. However, when roots on the opposite side of the tree from the degrading edge were measured, sap flow rates comparable to the highest values from interior plateau trees were found. The relevance of these findings to permafrost thaw processes and local hydrology will be discussed.
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