6.3 Diagnosing leaf water status and leaf hydraulic
strategy
Seasonal midday ΨL values were variable across species
and stands, but in general, Q. alba experienced a lower overall
midday ΨL and a broader range. Larger species-specific
declines in midday ΨL occurred in the more arid forest
stands (e.g., NC_E chronosequence and MO), while the smallest occurred
in NC 15yo (Fig. 7a). Leaf hydraulic strategy was primarily determined
by species (Fndf,ddf = 22.20, p =
<0.001), and no influence of age on mean ΨLinterquartile ranges were detected (age and age-species interactions
NS). The L. tulipifera and A. saccharum mean
ΨL interquartile ranges were indistinguishable, but
significantly lower than Q. alba (Fig. 7b); mean
ΨL interquartile ranges across stands were 0.709 MPa (SE
= 0.040), 0.358 MPa (SE = 0.022), and 0.372 MPa (SE = 0.098) forQ. alba, L. tulipifera, and A. saccharum , respectively.
Overall, Q. alba displayed more anisohydric behavior whileL. tulipifera and A. saccharum were more isohydric.
6.4 Relationship
between ΨL regulation and vulnerability to
hydraulic failure
The most anisohydric species (Q. alba ) in our study possessed
xylem that was more vulnerable to embolism than the more isohydric
species. This pattern was consistent at both P12 and P50, where Q.
alba mean embolism thresholds were significantly greater (p =
<0.001 for P12 and P50, respectively) than the more isohydricL. tulipifera and A. saccharum (Fig 8). Mean
Ψsafety,P12 values were −1.77 MPa (SE = 0.166), −0.35
MPa (SE = 0.078), and 0.63 MPa (SE = 0.319), and mean
Ψsafety,P50 values were −0.147 MPa (SE = 0.196), 1.94
MPa (SE = 0.143), and 2.634 MPa (SE = 0.351) for Q. alba ,L. tulipifera , and A. saccharum , respectively. Moreover,
the degree of isohydricity (as defined by seasonal ΨLinterquartile range) was strongly associated with risk of xylem
dysfunction such that ~60% of the variability in
Ψsafety across stands was explained by a tree’s relative
ΨL control (Fig. 9).
Across all stands, Ψsafety values were smallest and
often negative for Q. alba , suggesting that Q. albaoperated at lower proportional level of its maximum hydraulic function
relative to L. tulipifera and A. saccharum . Likewise,
estimated in-situ Kstem values were
consistently lower for Q. alba than other co-occurring study
species. For example, in the NC_E chronosequence, where
Ψsafety were often smallest, estimated in-situKstem was 0.30 kg m-1s-1 MPa-1 (SE = 0.133) for Q.
alba , accounting for a 67.1% reduction of Kmax .
By comparison, estimated in-situ Kstem forL. tulipifera and A. saccharum in these sites were 1.12 kg
m-1 s-1 MPa-1 (SE
= 0.148) and 0.62 kg m-1 s-1MPa-1 (SE = 0.04) (accounting for a 18.79% and 7.02%
reduction of Kmax , respectively).
7. Discussion
We tested three hypotheses to assess variability and coordination of key
plant hydraulic traits across ten deciduous forest stands. We found
little support for hypotheses 1 and 2; we observed that variation of
embolism vulnerability was better explained by species than
intraspecific factors. While we detected some region and age effects,
variation in vulnerability was principally determined by the large
species effect. Additionally, our results did not confirm hypothesis 3,
which predicted stricter ΨL regulation would be
associated more vulnerable xylem. Contrary to the prevailing paradigm,
we found that anisohydric Q. alba possessed stem tissues more
vulnerable to embolism than isohydric L. tulipifera and A.
saccharum . Moreover, we found that Q. alba had small or negative
Ψsafety, such that its anisohydric behavior likely
occurred with a substantial hydraulic cost.
7.1 Why were
embolism thresholds invariant with climate and stand age?
We found little variation in hydraulic vulnerability across climate and
age. This result, however, must be reconciled with the body of work
demonstrating vegetation’s capacity to acclimate xylem to pedo-climatic
conditions (Awad et al ., 2010; Durante et al ., 2011;
Gea-Izquierdo et al ., 2012). The clearest trends of acclimation
are often found in manipulation experiments (Beikircher & Mayr, 2009;
Awad et al ., 2010). However, surveys of hydraulic traits across
species’ ranges have found these patterns more ambiguous
(Martínez-Vilalta et al ., 2009; Wortemann et al ., 2011;
Charra-Vaskou et al ., 2012; Lamy et al ., 2014).
The similarity across climate observed here may be evidence that
acclimation reflects a broader set of morphological changes to the
whole-plant hydraulic architecture, rather than simple adjustments to
stem xylem traits (Lamy et al ., 2014). Modifications to other
drought-tolerant traits may therefore explain how Q. alba,L. tulipifera , and A. saccharum establish dominance across
diverse climate ranges. Such acclimation may include modifications to
leaf:sapwood area ratio (Addington et al., 2006; Martínez-Vilatlaet al ., 2009), root:leaf area ratio (Sperry et al ., 2002),
fine root turnover (Meier & Leuschner, 2008), or vulnerability of root
tissues (Alder et al., 1996; Wolfe et al ., 2016).
The absence of xylem acclimation
with age/height may similarly reflect a reliance on morphological
changes to alleviate emerging hydraulic constraints as forests mature.
As canopies grow in height, greater xylem tension and pathlength
resistance restricts hydraulic transport to canopy leaves (McDowellet al ., 2002; Novick et al ., 2009). To cope with these
constraints, species representing the extremes of tree height often show
strong patterns of increased stem embolism resistance with increased
height in the canopy, indicative of acclimation (Burgess et al .,
2006; Ambrose et al ., 2009). Although age effects on embolism
thresholds were minimal across stands, age also had little impact on
ΨL decline. Thus, age-related constraints may have been
mitigated through whole-plant adjustments that reduce damaging plant
water potential gradients, rather than increased xylem resistance.