Decline of southernmost populations of <i>Trillium camschatcense</i> due to an increase of Japanese sika deer in Iwate Prefecture

<p>Abstract：Trillium camschatcense, listed as ‘vulnerable’ in the IUCN Red List, is a perennial plant growing in the understory of broad-leaved deciduous forests of northern Japan. While we have been monitoring the demography of three southernmost populations in Iwate and Akita Prefectures since 2013, there has been a rapid increase in the number of Japanese sika deer in Iwate. In this report, we analyzed changes in the populations during 2013–2022 to examine the impact of deer browsing. We found that the two Iwate populations have shown an increase in per capita herbivory rates since 2017. Infrared cameras suggest that the main herbivore responsible is the Japanese sika deer. Compared to reproductive plants that were not consumed by deer, consumed plants were more likely to regress to the three-leaf stage (nonreproductive stage) in the following season. Consequently, the proportion of the three-leaf stage markedly increased since 2019, and the number of reproductive plants has decreased to only three individuals each in 2022. In contrast, herbivory rates and the proportion of the three-leaf stage did not show notable changes in the Akita population, where the increase of deer is not as pronounced. In Iwate, many individuals that were previously in the flowering stage have not died and are still surviving in the nonreproductive stage, suggesting that we may currently be in the early stage of deer overbrowsing. Given that Trillium populations are highly susceptible to and among the first to be affected by deer browsing, their impact on understory vegetation may further increase in the near future.</p>