Leaf heat tolerance of R. ferrugineum. Measurements were performed on Mt. Patscherkofel (1960 m a.s.l.) using different methodical approaches with regard to the controlled heat exposure. Measurements took place on 27 July 2012 and the following assessment procedures were used: 1. Heat exposure of detached leaves (ex situ, in darkness), 2. In situ heat exposure using the Heat Tolerance Testing System (in darkness and under natural solar irradiation). A. Viability assessed by the visual estimation method (VEM). LT50 of excised leaves (hatched box) and LT50 determined in situ and in darkness (sprinkled box) did not differ significantly (exception: 30 July 2012). LT50 determined in situ under natural solar irradiation (white boxes) was always significantly higher. B. LT50 assessed by the chlorophyll fluorescence method (CHFL) showed no significant differences when detached leaves (hatched box) or attached leaves in situ in darkness (speckled box) were compared. LT50 determined in situ under natural solar irradiation (white boxes) was also not different except for 9 Aug 2012. Boxes: median, upper and lower quartile. Whiskers: maximum, minimum. Significant differences between mean values of heat tolerance based on ANOVA and the Duncan - test are indicated by different letters (P < 0.05).