Hydraulic conductivity and freezing thaw embolism
We estimated the efficiency of water transport during winter by the
specific hydraulic conductivity (Ks_max) and the
percentage of loss conductivity (PLC) as a proxy for freezing-thaw
embolism. We collected stems from 10 individuals per species (n= 160),
with an approximate stem diameter of 5-10 cm and an approximate height
of 3 meters, these were different individual to whom growth rate was
measured but with similar phenotypic characteristics. All samples were
collected during winter, in a sampling period range of 5 to 7
consecutive days, and before dawn to avoid the xylem tension. To avoid
the “open vessels effect” (Choat et al., 2010), we transported the
samples in water to the place where we conducted the measurements. We
cut the samples under water to a length equivalent to the maximum length
of vessels of the species.
The specific hydraulic conductivity corresponds to the water flux rate
mobilized by a driving force through a branch, normalized by the length
of the segment (Tyree & Ewers, 1996), calculated as:
KS = F L / (ΔP Ax)
KS = Kg s-1 m-1MPa-1 Equation 1.
where F is the water flow mobilized through the segment expressed in
mass (kg s-1), L is the length of the segment (m), ΔP
is the pressure difference between the ends of the segment
(MPa-1) and Ax is the xylem
transversal area (m-2), which makes the measurements
of water transport comparable between segments of different sizes.
Ks was measured using a field system based on a pressure
drop flowmeter (see detailed description in Tyree et al., 1993, 1994;
Brodribb & Field, 2000).
Once we determined the specific hydraulic conductivity in natural winter
conditions (Ks_field), then we flushed out the
potential embolism in the sample by refilling the segment with KCl
solution for 10–15 min (Sperry et al., 1987) and we carried out a new
measurement of Ks_max, which represent the maximum
water transport capacity of the branch segment. Then, we calculated PLC
as:
PLC = [(Ks_max–Ks_field)/Ks_max] Equation 2.