Figure 3. Distribution of mean δD biases per tree species. The error bars
correspond to minima and maxima. Letters indicate significant
differences between species based on a Dunn-Sidák test (p <
0.05).
Deuterium biases and offsets against environmental
variables
Both pre-dawn LWPs (–1.97 to –0.18 MPa), RSWC (0.60 to 1.64 g/g) and
xylem water δD (–56.5 to –22.5\($\textperthousand$\)) varied over broad ranges, making it
possible to test the ability of these variables to predict variations in
δD biases and offsets (Figure 4). We found that neither δD biases nor δD
offsets were correlated with isotopic composition (Spearman; p=0.17 and p=0.78, respectively) (Figure 4a, 4d), pre-dawn
LWP (Spearman; p=0.73 and p=0.22, respectively) (Figure
4b, 4e), or RSWC (Spearman; p=0.38 and p=0.42,
respectively) (Figure 4c, 4f). The linear mixed-effect modelling
confirmed that xylem water δD, pre-dawn LWP and RSWC where not
significantly related to the δD bias. The model also found significant
differences between C. bella and all other species (Table 2). The
‘site’ random effect had very low (<10-5)
variance, implying that sites had a minimal effect on the δD bias. These
modelling results suggest that while CVD-induced δD biases were
independent of water content and water stress levels, inter-specific
differences cannot be ignored.