Sap flow responses to seasonal thaw and permafrost degradation in a subarctic boreal peatland

Abstract

Key message 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. Abstract 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 physio- logical 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 exam- ined, vapour pressure deficit was the strongest predictor of diurnal sap flow, while seasonal patterns were driven lar- gely 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 dif- ferences 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.