Leaf temperature and leaf trichome
In all field sites across the elevation gradient, leaf temperature was higher than the air temperature in daytime (Figure 2b,f, and S4) where leaf trichomes were partly responsible for this increase (Figure 3). Thicker leaf trichomes decreased the sensible‐heat and latent‐heat fluxes through higher leaf‐trichome resistance (Meinzer & Goldstein, 1985). While some studies argued or suggested that leaf trichomes could increase heat dissipation through increasing turbulence in the boundary layer (Schreuder et al., 2001) or through increasing the effective heat‐transfer surface area like cooling fins (Baldocchi, Singsaas, Pimentel, Portis, & Long, 1983; Defraeye, Verboven, Ho, & Nicolai, 2013), our study suggested that the dense leaf trichomes in M. polymorpha had greater effects as heat resistance rather than as heat dissipation.
In both field measurements and simulation analyses, the effects of leaf trichome on leaf temperature were largest in the coldest and driest site (Figure 2, 4g) where the amount of leaf trichomes was largest (Table 3; Joel et al. 1994; Tsujii et al. 2016). The magnitudes of these effects tended to be greater in the field measurements than in the simulation analyses: for example, leaf trichomes with 0.5-mm thickness can increase leaf temperature over 3 °C in the field conditions (Figure 3) but ca. 1.8 °C in simulation analyses (Figure 6d). The difference of these effects between the actual conditions and the simulation is possibly due to other factors that we did not consider in the simulation analyses, such as, roles of leaf trichomes other than diffusion resistance including their heat storage. Regardless other factors that may influence the leaf temperature, the similar trends between the actual values and simulated results indicate that major effects of leaf-trichome resistance on the gas-exchange rates are covered by our model.
The largest effects of leaf trichomes on the sensible‐heat flux and leaf temperature in the coldest and driest site were attributable not only to the largest trichome thickness but also to the smallest leaf size (Table 3; Figure 4g and 5c). A smaller leaf, i.e., a smaller characteristic leaf dimension, have lower boundary‐layer resistance (Eqs. 6-7; Nobel, 2009; Schuepp, 1993). Since sensible‐heat resistance is considered as a sum of the leaf‐trichome and boundary‐layer resistance (Eq. 2), the relative effects of leaf trichomes is greater in smaller leaves for a given trichome thickness (Figure 6b). As seen in alpine ecotypes ofM. polymorpha , many alpine plants typically have small leave (Halloy & Mark, 1996; Körner, Neumayer, Menendez-Riedl, & Smeets-Scheel, 1989; Tsujii et al. 2016; Wright et al., 2017); therefore, having leaf trichomes in alpine regions may be particularly beneficial as increase in leaf temperature with leaf trichomes can be more pronounced in smaller leaves.