Insert Table 1
Determination of drought
tolerance
Drought tolerance was determined by vulnerability to xylem cavitation
(P50) using the centrifuge method to induce cavitation
in the xylem (Cochard et al., 2013, 2005). This advanced centrifuge
technique creates centrifugal force that generates tension in the branch
xylem vessels to induce cavitation in branch segment, thereby allowing
measurement of xylem percentage loss of conductivity at set points of
tension. Straight stems, 27 cm in length and with 6 mm basal diameter,
were sampled and cut under water from the remaining 40 cm-long branch
segments, placed on the custom-built rotor and spun at different
velocities. To control for the artefact associated with the centrifuge,
initial measurements were obtained at lower pressures (-0.5 MPa = 2378
rpm) that did not induce cavitation (López et al., 2019). The percent
loss of conductance (PLC) at negative xylem pressure (tension) was
automatically recorded through a step-wise increase (1000 rpm each) atca. 2 min stabilization time (Zhang et al., 2017) until 90-95%
loss of conductivity was attained. At each new xylem pressure (tension),
hydraulic conductance (Kh) was measured from 30 repeated measures. The
PLC was computed as PLC = 100 × (1 − Kh/Kmax). The dependence of PLC on
xylem pressure was used to generate vulnerability curves for each
species and 50 % loss of conductance (P50) were
obtained from slope of the curve using the fitplc R package
(Duursma & Choat, 2017).
Statistical
Analysis
Stem P50 difference between species were tested using a
linear model (lm ), while differences between biome, life
histories traits (resprouting ability and leaf forms), as well as
interactions, were determined using a linear mixed effect model
(lme4 ) R package (Bates et al., 2015) with species as a random
variable. Residuals of models were inspected; appropriate
transformations were conducted and extreme outliers were removed where
necessary. ANOVA for mixed effects models was undertaken using Kenward
Roger degrees of freedom approximation. Linear mixed effects model with
species as random effect was used to explore predictors of cavitation
resistance (P50). Posthoc Tukey tests were undertaken
using the emmeans R package (Lenth, 2020) to determine which
species and life histories are significantly different.