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.