7.1 Why were embolism thresholds invariant with climate
and stand age?
Although P50 was impacted for some
species by a combination of forest age and region, species was the
predominant factor explaining variability in embolism vulnerability.
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 studies (Beikircher & Mayr, 2009; Awadet al ., 2010). However, surveys of hydraulic traits across
species’ ranges have found more ambiguous patterns (Martínez-Vilaltaet al ., 2009; Wortemann et al ., 2011; Charra-Vaskouet 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).
Although we found little intra-species variation in stem anatomy across
age class and sites, modifications of other traits may explain howQ. alba, L. tulipifera , and A. saccharum establish
dominance across diverse climate ranges. These acclimations may include
modifications to leaf:sapwood area ratio (Addington et al., 2006;
Martínez-Vilatla et al ., 2009), root:leaf area ratio (Sperryet al ., 2002), fine root turnover (Meier & Leuschner, 2008), or
vulnerability of root tissues (Alder et al., 1996; Wolfe et
al ., 2016).
These species may also rely on morphological changes to alleviate
emerging hydraulic constraints as they mature. As canopies grow in
height, greater xylem tension and pathlength resistance restricts
hydraulic transport to canopy leaves (McDowell et al ., 2002;
Novick et al ., 2009). To cope with these constraints, stem
embolism resistance often increases with 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.