Leaf temperature
To explore the question I in Introduction, we conducted leaf-temperature measurements in the field conditions. To determine the effect of leaf trichomes on leaf temperature across the elevational gradient, we measured leaf temperature of both intact and trichome‐shaved leaves at the four elevational sites (700, 1280, 1800, 2400 m). We excluded the 100 m site for this measurement because the studied individuals in this site were tall and the sun‐exposed leaves were out of reach (>4 m). We selected three individuals with typical leaf traits in each site, i.e., glabrous individuals in the 700 m site and pubescent individuals in the other sites. The mean trichome thickness of the three individuals was 0, 0.278, 0.433, and 0.716 mm at 700, 1280, 1800, and 2400 m, respectively. We selected two sun‐exposed leaves per individual and carefully shaved the leaf trichomes on either one of these two leaves by using a rubber thimble. These two leaves per each individual were close to each other and their microenvironmental conditions can be assumed to be almost same. In August 2017, we measured diurnal variations in leaf temperature of these leaves with 0.2‐mm diameter copper/constantan thermocouples connected with a data logger (0.1 °C precision; ADL12, AsOne, Japan). The thermocouple was attached to the lower surface of the leaf. For intact leaves with leaf trichomes, the thermocouple was inserted beneath the trichome layer and in contact with the lamina. During this measurement, we also measured the air temperature and humidity (TR‐72WF‐H, T&D Co. Ltd., Nagano, Japan) every ten minutes at each site.
To determine the association between trichome thickness and leaf temperature, we measured leaf temperature of both intact and trichome‐shaved leaves collected at the 1800‐m site where the variation in the amount of trichomes was largest (Table 3; Tsujii et al., 2016). We studied ten randomly selected individuals at the 1800 m site. One pair of opposite leaves that were fully exposed to sun was selected per individual, and the leaf trichomes on either one of the paired leaves were carefully shaved by using a rubber thimble. We confirmed that this trichome‐shaved treatment itself did not directly affect the physiological activities such as stomatal conductance and transpiration rate through damaging stomata and cuticle layer (Figure S2). At least two days later, we measured leaf temperature on the upper surfaces of intact and trichome‐shaved leaves using a radiation thermometer (0.1 °C precision; 62 MAX+, Fluke Corp., USA) between 11:30 and 12:30 for three times each on three sunny days in July 2016 (total nine times for each leaf; air temperature 22.0 ± 1.0 °C, PPFD > 2000 μmol m⁻2s⁻1) when the solar angle was nearly vertical which minimized leaf direction effect between the paired leaves. We used the averages of nine data for each pair of leaves to analyze the association between leaf temperature and trichome thickness. After the measurements, the measured leaves were collected and used for the measurements of trichome thickness, as mentioned above.