5.6 Data processing and analysis
We investigated differences in P12 and P50 across species and stands (hypotheses 1 and 2) with a two-way ANOVA, where species and stand age were fixed factors and region was a blocking factor. We compared vessel density and lumen area between Q. alba and L. tulipiferastems with a two-way ANOVA, where species and stand age were fixed factors. We removed region as a blocking factor because there was no significant region or region interaction effect at p = 0.05. The relationships between xylem anatomy (e.g., vessel density and vessel lumen area) and embolism thresholds (e.g., P12 and P50) were assessed with linear regression within and across species.
We analyzed the relationship between embolism thresholds and isohydricity (hypothesis 3) in two ways. First, we compared embolism thresholds between isohydric and anisohydric species using a two-tailedt -test. Second, we investigated this relationship in the context of a hydraulic safety margin (Ψsafety) (MPa). Safety margins to P12 (Ψsafety, P12) (MPa) and P50 (Ψsafety, P50) (MPa) were calculated using the following equation (Domec & Gartner, 2001; Delzon & Cochard, 2014):
\(\Psi_{\text{safety}}=\ \Psi_{L,min}-\Psi_{\text{thresh}}\) (3)
where ΨL,min (MPa) is the minimum ΨL of a given species in a specific stand, and Ψthresh,(MPa) is mean embolism threshold (e.g., P12 or P50) for the same species in the same stand. A negative Ψsafety suggests a high level of xylem embolism, while a positive Ψsafety suggests a window of safety from critical levels of xylem dysfunction (Johnsonet al ., 2016). We then performed a linear regression between Ψsafety and ΨL interquartile range across species and stands.
All ANOVA analyses were performed at the α = 0.05 level and were followed by a Tukey post-hoc test for significant main effects. Significant interaction terms were assessed by pairwise comparison of least square means.