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.