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(Amada, unpublished data), and thus increased latent‐heat flux.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).