Black Spruce (Picea mariana) Water Use Response to Moisture Deficits on a Permafrost Peat Plateau

English Abstract:

Large-scale permafrost degradation is occurring rapidly over the discontinuous permafrost regions of the Canadian subarctic. Within this region, peatlands are particularly vulnerable as permafrost controls the hydrology, carbon cycling, and the health and distribution of vegetation. This study examines the degree to which P. Mariana responds to water stress in relation to changing frost and water table depth, soil tension, and soil moisture. These findings will aid in advancing our understanding of the processes governing forest browning and the hydrological modeling of cold regions. Three P. Mariana were instrumented with 5 Heat Ratio Method sap flow sensors (SFS) on a peat plateau at Scotty Creek, NWT in the summer of 2014. Tensiometers were placed at a depth of 15 cm and at distances of 20 and 100 cm from the focal tree along SFS monitored roots. Volumetric soil moisture over the top 5 and 20 cm was measured every 10 cm along four transects that intersected the instrumented roots. Frost table depth was also measured along these transects. Two water table wells were installed near the bog and in the plateau interior. Diurnal patterns of soil tension, sap flow, and water table depth were observed within an overall drying trend throughout the summer. A correlation existed between soil water tension and sap flow velocity. Complex feedback processes were observed between tension, frost table depth and sap flow. Increasing frost table depth resulted in decreased water table height, as well as decreased surface soil moisture, causing drought stress in P. Mariana and reducing transpiration. Large precipitation events and proximity to bogs or fens may also temporally induce waterlogging. A conceptual model of P. Mariana response to water deficits was developed and compared to the predicted response.