Unexpected Vulnerability to High Temperature in the Mediterranean Alpine Shrub Erysimum scoparium (Brouss. ex Willd.) Wettst

Current understanding of the effects of extreme temperature on alpine evergreens is very limited for ecosystems under Mediterranean climate (characterised by a drought period in summer), despite being exceptionally biodiverse systems and highly vulnerable under a global change scenario. We thus assessed (i) seasonal change and (ii) effect of ontogeny (young vs. mature leaves) on thermal sensitivity of Erysimum scoparium, a keystone evergreen of Teide mountain (Canary Islands). Mature leaves were comparatively much more vulnerable to moderately high leaf-temperature (≥+40 and <+50 °C) than other alpine species. Lowest LT(50) occurred in autumn (−9.0 ± 1.6 °C as estimated with R(fd), and −12.9 ± 1.5 °C with F(v)/F(m)). Remarkably, young leaves showed stronger freezing tolerance than mature leaves in spring (LT(50) −10.3 ± 2.1 °C vs. −5.6 ± 0.9 °C in mature leaves, as estimated with R(fd)). Our data support the use of R(fd) as a sensitive parameter to diagnose temperature-related damage in the leaves of mountain plants. On a global change scenario, E. scoparium appears as a well-prepared species for late-frost events, however rather vulnerable to moderately high temperatures.