7.2 The perplexing case of Q. alba
Our finding that Q. alba had the most vulnerable xylem was
unexpected. Quercus species are often considered more drought
tolerant than many co-dominants, attributed to their morphological and
physiological adaptations that allow them to withstand soil moisture
deficits (Abrams, 2003). Our results complicate this perspective. We
found that Q. alba had particularly high P50 (consistent with
previous work: Maherali et al ., 2006; Kannenberg et al.,2020) but were also more anisohydric (consistent with previous work:
Thomsen et al ., 2013, Roman et al ., 2015; Meinzer et
al ., 2017). Additionally, we found that Ψsafety was
often negative for Q. alba , suggesting these trees were
remarkably vulnerable to drought.
The Ψsafety is widely used to characterize risk of
hydraulic dysfunction (Choat et al ., 2012; Delzon & Cochard,
2014; Johnson et al ., 2016); however, its use as a predictor for
drought-susceptibility must be carefully evaluated.
Drought-susceptibility is not explicitly determined by risk of xylem
dysfunction, but in the context of a plant’s ability to cope with
hydraulic damage (Meinzer & McCulloh, 2013). It is widely accepted that
refilling of embolized conduits occurs in many species, especially
ring-porous species (Brodersen et al ., 2010; Ogasa et al .,
2013; Trifilò et al ., 2019; Zeppel et al ., 2019).
Refilling could therefore explain how Q. alba tolerates drought
while possessing vulnerable xylem. Moreover, Q. alba bears only a
few hydraulically active sapwood rings (<10), with the newest
rings being the most efficient at moving water (Phillips et al .,
1996). Therefore, even without any xylem refilling, Q. alba could
potentially repair a 50% loss of conductivity in fewer than five years
just by the production of new yearly rings.
It is also not clear that Q. alba rely on the entire depth of
sapwood to actively conduct water (Cochard & Tyree, 1990). In a related
study from IN 85yo, Yi et al . (2017) found that Q. alba’sinner sapwood conducted a more significant fraction of water during
drought, with water transport largely restricted to outer rings during
well-watered periods. It is important to note that our methods, and
specifically the rehydration of branches to flush native embolisms,
permit an evaluation of the vulnerability of the entire sapwood depth
(not the portion thereof actively involved in water transport). Finally,
internal water storage can also play an important role in determining
the relationship between leaf gas exchange and stem xylem traits.
Ring-porous species are known to use smaller amounts of stored water
than diffuse-porous species because of their low number of active rings
(Köcher et al ., 2013). Additionally, Q. alba has much
higher wood density than either L. tulipifera or A.
saccharum, and species with greater wood density tend to have low
capacitance (e.g., Meinzer et al., 2008). Unlike L.
tulipifera and A. saccharum that bear large sapwood volume and
have low wood density, the small water storage capacity of Q.
alba cannot provide enough water to limit the rapid drop in water
potential due to stomatal water loss, which could also explain its
anisohydric behavior (Matheny et al ., 2015) .
More work will be necessary to disentangle the mechanisms contributing
to the perplexing hydraulic behavior of Q. alba , which are
particularly important to understand in light of the long-term and
ongoing decline of eastern Quercus species (Fei et al. ,
2011). Quercus decline has been attributed to numerous drivers
including fire suppression (Abrams, 2003), climate mesophication (McEwanet al ., 2011), and widespread failure of regeneration and
recruitment (Dey, 2014). Given the hydraulic behavior of Q. albaobserved in our study, water stress may also be an important factor
contributing to Quercus decline, particularly in the episodic
mortality events of mature individuals that are often preceded by
drought (Clinton et al ., 1993; Wood et al ., 2018).