Anisohydric Species Acclimated Hydraulic Traits to High Temperature
while Isohydric Species Increased Dieback During Drought in Temperate
Tree Seedlings
Abstract
Climate warming is projected to increase the frequency of climate
extremes. The physiological response of tree species to these changes
has not been well characterized. Particularly, how hydraulic parameters
may adjust with temperature and what are the impacts during a severe
drought in mesic forest tree species have limited study. Further, the
pattern of responses to drought and recovery has been linked with
isohydry-anisohydry strategies, but we do not know if these strategies
are still relevant in the hotter and drier future. To fill this
knowledge gap, we built a factorial experiment with ambient (AT) and
high temperature (HT; ambient +4.5 0C) for two growing seasons, with six
weeks of the initial acclimation period, four weeks of drought, and six
weeks of recovery during the first year on seven tree species.
Throughout, we followed water relations, leaf gas exchange, stem water
potential, and hydraulic conductivity. Seedlings were acclimated to
prolonged HT by adjusting key hydraulic traits and increasing anisohydry
strategies, but these traits were not coordinated, as some isohydric
species had surprisingly high stem water potentials at 50% loss of
xylem conductivity (P50) and some anisohydric species had negative
safety margins. In addition to this, anisohydric species reduced stem
water potential during drought and delayed recovery, while isohydric
species experienced earlier reductions in photosynthesis and increased
dieback at HT. Our findings highlight that hydraulic traits may
acclimate to HT and reduce some of the additive effects during drought
in anisohydric species, but hotter drought intensified soil and
atmospheric drought and increased mortality in isohydric species. Thus,
acclimation of hydraulic traits to HT may help anisohydric species to
avoid some of the consequences but can’t compensate for the negative
effects.