Stem physiological measurements
For each species, stem hydraulic conductivity was measured on eight stem
segments from different healthy trees in July 2018. The long (c.a. 1 m),
straight and sun-exposed terminal branches with diameter ranging from 6
to 8 mm were sampled at predawn.
The
end of samples was recut under water and transported to the laboratory
immediately. The samples were wrapped with black plastic bag and kept
the cutting end submerged in the water during transportation. Upon
arrival at the laboratory, a stem segment about 15 cm in length was
excised from each of the originally sampled branch. After peeling off
the barks and shaving both ends of the segments with a sharp razor
blade, segments were connected to a tubing apparatus consist with a 0.22
μm filter and a 50 cm hydraulic head to allow the degassed and filtered
20 mmol L-1 KCL solution to flow through the segments.
The stem native hydraulic conductivity (K h, kg m
s-1 MPa-1) was calculated as
follows:
\(K_{h}=\frac{J_{v}}{\left(\frac{P}{L}\right)}\) (6)
where Jv (kg s-1) is flow rate
through the segments and ΔP/ΔL is the pressure gradient across the
segment (MPa m-1).
The leaf-specific hydraulic conductivity (K l,
10-4 kg m-1 s-1MPa-1) and sapwood-specific hydraulic conductivity
(K s, kg m-1s-1 MPa-1) of each segment were
calculated as K h divided by leaf area (LA) and
sapwood area (SA), respectively. Branch segments used for the
conductivity measurements were perfused by Methyl blue dye under a
hydraulic head of 50 cm, the average value of dyed area of both ends of
segments was determined as SA. The transverse cross-sections were
scanned and the images were analyzed using ImageJ software (US national
Institutes of Health, Bethesda, MD, USA) to calculate areas of stained
xylems. Meanwhile, distal needles of each segment were scanned to
calculate leaf area (LA) and then oven-dried at 75 °C for about 48 h to
get constant dry mass to calculate leaf mass per area (LMA, g
cm-3).
The leaf area to sapwood area ratio (LA/SA, m2cm-2) of each species was calculated as LA divided by
SA. The segments used for hydraulic conductivity were then used to
measure wood density (WD, g cm-3) by the water
displacement method.
Stem hydraulic vulnerable curves were constructed using the centrifugal
force method (Alder, Pockman, Sperry, & Nuismer, 1997) on six stem
segments of 14.2 cm in length for each species. After measuring the
native hydraulic conductivity, the segments were fixed into a high-speed
centrifuge (Model 20K, Cence Instruments, Changsha, China) equipped with
a customized rotor (‘Sperry
rotor’).
The vulnerability to embolism of each segment was determined by
measuring the decrease of hydraulic conductivity in response to a
stepwise increase in xylem tension generated by
spinning.
Vulnerability curves were fitted using sigmoidal models (Fig. S1) and
the pressure at 50% loss of conductivity (P 50,
MPa) was calculated (Pammenter & Vander Willigen, 1998).