Effects of climatic factors on leaf trichome density across a large geographic gradient
We investigated spatial variations in leaf trichomes in oriental oak and evaluated their associations with environmental factors and other functional leaf traits across Eastern Asia. Our results revealed that differences in latitudinal and longitudinal factors drive changes in climate such as temperature and precipitation, which inevitably direct or indirect impact the spatial patterns of leaf trichome traits (Fig. 2). Our results indicated that the density of trichomes was reduced with temperature, which did not align with the study of Skelton et al. (2012), who found that there was positive relationship between temperature and trichome density, but did with an investigation by Pérez-Estrada et al. (2000). This was likely the result of a reduced trichome density via temperature increases that reduced the thickness of the boundary layer; thus, lowering the water diffusion resistance to cool leaf temperatures (Gasparini et al. 2020). Since temperature and precipitation were coupled in our study area, the temperature was higher and accompanied by greater precipitation (Du et al. 2021); thus, plants at low latitudes (high temperature and precipitation) were induced to lower their trichome density to enhance the diffusion of water to maintain an optimal water vapor balance and leaf temperature.
The effects of water limitations on the density of leaf trichomes have been widely investigated in previous studies (Kessler et al. 2007, Cach-Pérez et al. 2016, Amada et al. 2019), most of which indicated that trichome densities increase under restricted water conditions, as demonstrated by our results (Fig. 3). On one hand, dense trichomes can collect dew droplets above stomatal pores (Fernández et al. 2014) that reduce the water potential gradient between the leaf interior and ambient air (Konrad et al. 2015), which can store water and delay the leaf drying process. Conversely, higher trichome densities under drought conditions can increase the leaf boundary layer thickness, which can reduce the plant transpiration rate and improve water use efficiencies (Bickford 2016). The transpiration rate was decreased with minimal reductions in photosynthetic rates, thereby improving the plant WUE, as demonstrated by previous studies of Arctotheca populifolia(Ripley et al. 1999) and Mallotus macrostachyus (Kenzo et al. 2008). In general, our results suggested that the trichome density possessed highly adaptive variation to environmental changes, which may contribute to higher plant tolerances against future drought pressures.