Sapflow measurements
In spring 2016, a network of Granier sap flow sensors (Granier 1985) was installed in 70 trees (30 Pinus sylvestris , 30 Picea abiesand 10 Betula sp .) to continuously measure tree-level transpiration, an integrated measure of whole tree hydraulic stress, in the three tree species that are dominant in Fennoscandian boreal forests. This technique includes a pair of thermocouple sensors that detect changes in the temperature difference (∆T) from the baseline (∆Tm at zero flow), which in turn reflects the flow rate of water through stems (Granier 1987). In addition, a field deployed Picarro L2131-i analyzer provides continuous, high temporal resolution isotopic measurements of tree xylem water. Taken together, these measurements provide a unique opportunity to test how changes in environmental conditions affects stand avaporation and tree-level transpiration across a range of temporal scales as well as directly compare the importance of transformations into other water balance components (i.e., streamflow) within a northern boreal headwater catchment.