Covariation between Tleaf and
Tcrit impacts thermal safety margin
Across all species we found a correlation between Tleafand Tcrit, supporting other studies that suggest thermal
tolerance (either Tcrit or T50) broadly
acclimates to maximum Tleaf (Bison & Michaletz, 2024;
Geange et al., 2021; Perez & Feeley, 2020; Tserej & Feeley, 2021).
This represented a + 0.4 °C increase in Tcrit per 1 °C
increase in plant averaged Tleaf measured around midday,
which is comparable to values reported with mean annual temperature in
other studies (O’Sullivan et al., 2017; Slot et al., 2021; Zhu et al.,
2018). We expected that this trend would also be evident within-species;
however, we did not observe a significant positive correlation between
Tleaf and Tcrit for any of the measured
species. This is similar to Kullberg et al. (2023), where only one of
seven species had a significant correlation between T50and max Tair, and none with T50 and
modelled Tleaf. While this could reflect the importance
of other variables besides Tleaf as drivers of variation
in thermal tolerance (Lancaster & Humphreys, 2020), it could also be a
result of a relatively low sample size compared to the variation of
these traits and may require further study across an even larger
gradient of Tleaf.
The strength of within species covariation between Tleafand Tcrit is important as it will determine patterns in
the thermal safety margin, and thus how susceptible different
provenances are to reaching critical leaf temperatures. While we
observed differences across provenances in Tleaf and
Tcrit, no provenance-differentiation was observed when
considering these traits together through their thermal safety margins.
This highlights that if attempting to consider vulnerability of
provenances to heat stress for climate-matching of seed sources, an
assessment based on either Tleaf or
Tcrit alone could misinform practitioners, resulting in
a priority of one provenance over another. Although our findings suggest
provenance-level variation in thermal safety margins may not be a high
priority for climate matching of seed sources within species,
provenances do differ in other traits such as survival and growth rates
(Middleby et al., 2024b). Climate-matching for provenance selection
should therefore consider suites of leaf and whole plant traits and
their potential trade-offs, rather than thermal safety margins alone.