Differences in flowering phenology, architecture, sexual expression and resource allocation between a heavily haired and a lightly haired nettle population: relationships with sika deer

摘要

Japanese stinging nettles, Urtica thunbergiana, in Nara Park (660ha), central Japan, where several hundred sika deer Cervus nippon have been protected for 1200years, bear quite dense stinging hairs on leaves and stems compared to those in surrounding areas where the deer density is very low. Our previous studies have suggested that nettles in the park have evolved such a trait as a constitutive resistance against sika deer through natural selection. Here, we explored differences in plant architecture, flowering phenology, resource allocation pattern and sexual expression between a heavily haired and a lightly haired population with relationship to defence against sika deer. We raised seedlings from the two populations in a greenhouse and monitored relevant traits for three successive years. Individuals from the heavily haired population had earlier flowering, longer flowering period and lower growth rate than those from the lightly haired population. The former allocated more resources to inflorescences and leaves but less to stems and rhizomes than the latter. Most individuals from the heavily haired population were monoecious irrespective of age, while in the lightly haired population the proportion of females increased with age. These results indicate that the historically browsed population has evolved constitutive tolerance conferred by early and prolonged reproduction and that a resource allocation trade-off exists between defence and growth. Consequently, this study suggests that sika deer have exerted selection on U. thunbergiana for changes not only in defensive traits but also in resource allocation pattern and sexual expression.