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.