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.