Damage-induced response and seasonal variation in stinging hair traits of Japanese nettle (Urtica thunbergiana) seedlings in two subpopulations with different browsing pressure by Sika deer
We studied damage-induced response and seasonal variation in stinging hair traits of current-year seedlings of a Japanese nettle, Urtica thunbergiana, in two subpopulations with different browsing pressure by Sika deer to elucidate adaptive significance of phenotypic plasticity of the traits on the basis of field experiments in two sites; one with high browsing pressure (“browsed site”) and the other with low browsing pressure (“exclosure site”) in Nara Park, located in central Japan. We conducted clipping experiment, in which we clipped shoot apexes in spring and measured stinging hair traits produced after the clipping in the same year, and investigated seasonal variation in the traits in intact seedlings to evaluate the effects of browsing pressure on the magnitudes of phenotypic plasticity (plasticity due to clipping and seasons) by comparing the subpopulations for six traits: stinging hair number per leaf, stinging hair density, and stinging hair length for upper and lower surfaces of leaves.
The clipping experiment revealed that stinging hair density of upper surface and stinging hair length of lower surface exhibited smaller or reverse responses to clipping in browsed site compared with those in exclosure site; the magnitudes of plasticity due to clipping negatively correlated with browsing pressure for these traits. Nettle seedlings also exhibited seasonal variation; trait values in summer were significantly larger than those in spring for five traits in either site or both sites, and vice versa for one trait in one site. The magnitudes of plasticity due to seasons significantly differed between sites in several traits and correlated with browsing pressure positively for stinging hair density and negatively for stinging hair length for both surfaces. The negative correlations between the magnitudes of plasticity due to clipping and browsing pressure are consistent with an evolutionary hypothesis that constitutive defenses are favored over induced defenses when timing of predation is highly predictable. On the other hand, the positive correlation between the magnitudes of plasticity due to seasons and browsing pressure in stinging hair density suggests that not only browsing pressure but also seasonal change in growth rate may be involved in the plasticity due to seasons in the traits.