2.4 Sap flow
Two sap flow units (HeatPulser, Edwards Industries, Otaki, New Zealand) were installed 10 cm below each pair of dendrometers (base and top positions of all six trees). A drill guide was used to ensure axial alignment of the three Teflon probes from each unit. Measurement probes were placed to record heat velocity in the outer sapwood (1 cm depth); heater probes were fully inserted. Positions of all probes were fixed by tying the probe hubs to the stem. Care was taken that the probes were not installed in reaction wood or near old branches.
Sap flow data were logged every 15 min (CR1000, Campbell Scientific, Logan, USA). Sap flow units were installed in January 2014. Bark depth and stem diameter were measured once every month at each position to calculate the increase in sapwood area over the duration of the experiment. Hourly sap flow data are shown in Supporting Information (Fig. S3) and were calculated based on sapwood area measurements for the month of May. Macroscopic analyses of wood discs showed no signs of heartwood formation, so it was concluded that the entire sapwood area was conducting water (i.e. sapwood = xylem). Sap flow measurements were only used to calibrate our Stem Hydraulic Model and are not reported here in detail.