6.4 Relationship between ΨL regulation
and vulnerability to hydraulic failure
The most anisohydric species in
our study possessed xylem that were more vulnerable to embolism than the
more isohydric species. This pattern was consistent at both P12 and P50,
where Q. alba (mean ΨL interquartile range 0.71\(\pm\) 0.04 MPa) embolism thresholds were consistently greater than the
more isohydric L. tulipifera (mean ΨLinterquartile range 0.36 \(\pm\) 0.02 MPa) and A. saccharum (mean
ΨL interquartile range 0.37 \(\pm\) 0.10 MPa),
respectively. Across all stands, Ψsafety were
smallest and often negative for Q. alba.The average slope of the
regression between Ψsafety and ΨLinterquartile range were -3.80 (\(\pm\) 0.32) and -4.92 (\(\pm\) 0.40)
for Ψsafety,P12and Ψsafety,P50, respectively and was consistently
negative across 100 bootstrapped simulations (Fig. S4). Age-independent
analysis of variation in Ψsafety across the aridity
gradient revealed that the lowest Ψsafety often occurred
in the more arid regions of our study (e.g., MO and NC_E);
nevertheless, the largest differences were associated with species (Fig.
S5 & Table S2). Overall, the degree of isohydricity was strongly linked
to hydraulic safety across species and sites
(R 2 = 0.57, p = <0.001 andR 2 = 0.61, p = <0.001 for
Ψsafety,P12 and Ψsafety,P50,
respectively), such that increasingly anisohydric behavior promoted
greater risk for hydraulic damage (Fig. 7).
Estimated native embolism patterns were highly similar to
Ψsafety (Fig. 8a). Q. alba had greater estimated
native embolism than L. tulipifera and A. saccharum under
field conditions (Species effect; Fndf,ddf =
162.559, p = 0.001, Table 4). We detected some intra-species
differences associated with stand age across regions (Species*Age*Region
effect; Fndf,ddf = 3.347, p = 0.039, Table
4). However, these differences were often inconsistent across regions
(e.g., greater estimated native embolism with increasing stand age forQ. alba in IN and lower estimated native embolism with increasing
stand age for Q. alba in NC_W, Fig. 8a) and may be due to
non-overlapping ΨL sampling periods within
chronosequences. Regardless, spatio-temporal effects (e.g.,
Species*Age*Region effect) were substantially more marginal than the
large species effect (Table 4).
Overall, increasing
ΨL interquartile range across species and stands were
strongly associated with a greater extent of estimated native embolism
(R2 = 0.67, p = <0.001, Fig.
8b & Fig. S6). This relationship additionally coincided with a lower
magnitude of hydraulic conductivity. For example, in NC_E, estimatedin-situ Kstem was 0.30 (\(\pm\) 0.133) kg
m-1 s-1 MPa-1 forQ. alba while estimated in-situ Kstem for L. tulipifera and A. saccharum were 1.12 (\(\pm\)0.15) kg m-1 s-1MPa-1 and 0.62 (\(\pm\)0.04) kg m-1s-1 MPa-1, respectively.