Induced response to herbivory in stinging hair traits of Japanese nettle (Urtica thunbergiana) seedlings in two subpopulations with different browsing pressures by sika deer

Evolutionary theories predict that natural selection favors inducible defense when the risk of predation is unpredictable. In this context, the magnitude of the induced defense in populations experiencing intermittent herbivory is predicted to be larger than that in populations experiencing constant herbivory when there is genetic differentiation between populations. To test this prediction, we conducted a clipping experiment to investigate induced response to shoot damage by the stinging hair traits of Japanese nettle (Urtica thunbergiana) seedlings. For this purpose, we studied two nettle subpopulations, one under constant browsing and another under intermittent browsing by sika deer in Nara Park, central Japan. The clipping experiment demonstrates that both subpopulations exhibited induced defenses in response to the clipping of the shoot apex as the number and length of stinging hairs increased after clipping. The subpopulation experiencing intermittent browsing exhibited smaller trait values and larger induced defenses, indicated by the number of stinging hairs on the upper leaf surface and the length of stinging hairs on both leaf surfaces compared with the subpopulation experiencing constant browsing. These results are consistent with the prediction and suggest that genetic differentiation of the induced defense between subpopulations is caused by adaptation to the herbivory regime. We discuss other plausible factors affecting the magnitude of the induced defense of the nettle subpopulations.