Sap flow responses to seasonal thaw and permafrost degradation in a subarctic boreal peatland
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We demonstrate the negative impacts of ground thaw on sap flow in black spruce. Climate warming is accelerating permafrost thaw; our study may inform observed warming-related productivity declines in subarctic forests.
Many of Canada’s northern boreal peatlands are situated on discontinuous permafrost. Here, permafrost provides the physical foundation on which forests persist. However, climate warming is leading to increased rates of permafrost thaw resulting in both increased active layer thickness (ALT) as well as shrinkage of the area underlain by permafrost due to lateral thaw at plateau margins. Such changes to the substrate likely impact growth and physiological processes of the trees. Despite this, little is known about the role of active layer development, and changes to it, including thaw of the underlying permafrost on tree water relations. Here, we measured sap flow in black spruce from a peatland experiencing rapid permafrost thaw and quantified drivers of changes in sap flow. Time series analyses revealed that of the environmental drivers examined, vapour pressure deficit was the strongest predictor of diurnal sap flow, while seasonal patterns were driven largely by energy inputs; however, the environmental drivers of importance did not change as a function of ground thaw conditions. To understand the implications of changing permafrost and active layer conditions, we quantified differences in sap flow between trees in interior positions of permafrost plateaus and trees on degrading plateau edges. We found >65 % reductions in sap flow in edge trees, attributable to reduced root function at waterlogged edges. Seasonal thaw processes also influenced sap flow. Paired measures of ALT indicated a negative linear relationship with sap flow that was stronger for interior trees. Greater ALT, which corresponds with deeper frost and water tables reduced sap flow by up to 60 % and is likely attributable to drying in surface soil layers where rooting occurs. Climate warming will accelerate permafrost thaw, which our data suggests will drive decreases in the productivity of black spruce-dominated subarctic forests. These findings may thus enhance our understanding of widespread reductions in productivity in boreal forests in northwestern North America.
KeywordsEvapotranspiration Ecohydrology Discontinuous permafrost Mackenzie River Basin Northwest Territories Tree growth and productivity Plant–soil feedbacks
Author contribution statement
Sap flow data were collected by RP and JLB. Microenvironmental data were collected by WLQ and MH. RP and JLB performed all data analyses. All authors contributed to the development of ideas and to the writing of the manuscript. JLB and WLQ funded collection of field data.
Funding for this research was provided by the Natural Sciences and Engineering Research Council, the Canadian Foundation for Innovation, and the Canadian Foundation for Climate and Atmospheric Sciences. We are grateful to A. Downey from ICT International for exceptional technical and scientific support and B. Rosado and two anonymous reviewers for helpful comments on the manuscript. We thank R. Connon for assistance in the field. We are grateful to the Aurora Research Institute for their assistance in obtaining a research license (License Number 15005). We thank the Denedeh Resources Committee, Dehcho First Nations, Fort Simpson Métis Local#52, Liidlii Kue First Nation and the Village of Fort Simpson for their support of ongoing research at Scotty Creek. In particular we thank Allan Bouvier and Allen Bonnetrouge of the Liidlii Kue First Nation, and Chief Stanley Sanguez of the Jean-Marie River First Nation for continued support. We are grateful for the support provided through a Partnership Agreement between Wilfrid Laurier University and the Government of the Northwest Territories.
Conflict of interest
The authors declare that they have no conflict of interest.
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