Leaf temperature and environmental conditions
In all sites, leaf temperature was usually higher than the air temperature in daytime, but lower than the air temperature in the night due to radiative cooling (Figure 2a,c,e,g, and S4). While the leaf temperature varied greatly along time, the leaf temperature tended to be consistently higher in intact pubescent leaves than in trichome‐shaved leaves at the 2400, 1800, and 1280 m sites. In the 700 m site where leaf trichomes are least abundant, glabrous leaves were used for the measurement and thus leaf temperature did not differ between shaved and non-shaved leaves (Figure 2). The amount of leaf trichomes was largest and its effect on leaf temperature was largest in the 2400 m site on average ca. 2.0 °C increase (Table 3; Figure 2b). In the sunny morning, leaf temperature was sometimes lower in the intact pubescent leaves than in trichome‐shaved leaves (Day 1 at the 2400 and 1280 m sites, Day 3 at the 1800 m site; Figure 2b,d,f) because the dense and wooly leaf trichomes in M. polymorpha prolonged the duration of leaf wetness (Amada, unpublished data), and thus increased latent‐heat flux. Relative humidity was usually nearly 100% at night and decreased to ca. 60% at daytime at the 1800, 1280, and 700 m sites (Figure 2c,e,g). On the other hand, at the 2400 m site, relative humidity often showed the opposing diurnal pattern; that is, relative humidity reached ca. 20% at night and increased at daytime (18 days in August 2017). This was probably due to the cool‐dry‐nighttime drainage winds originating above the trade-wind inversion while in the daytime upslope winds bring moist marine‐surface air (Nullet, Juvik, & Wall, 1995).
When we compared leaves with different thickness of leaf trichomes in a given site (1800 m site), the difference in leaf temperature between intact and trichome‐shaved leaves was significantly larger in leaves with higher trichome thickness (Figure 3).