Southeast Climate Adaption Science Center Honolulu, Hawaii, United States
Abstract: While outplanting populations is often considered essential to the persistence and recovery of critically rare plants, outplanted populations often fail due to low recruitment, driven by low survival and growth of seedlings. Alien plant species are often considered a threat to native plant populations broadly and reintroduced populations specifically and control of alien plants is one of the most common management actions in those populations. However, alien plants can simultaneously have positive and negative effects across the demographic rates of reintroduced populations. Even if positive effects of alien plants are less common than negative effects, if the positive effects are focused on a key demographic rates, such as those that drive recruitment, removing alien plants could result in failed restoration outplanting efforts. I performed an outplanting of the critically endangered endemic Hawaiian shrub Schiedea adamantis. S. adamantis is restricted to fewer than 15 wild individuals and this outplanting currently represents the largest population (~300 mature plants) of the species. I performed factorial alien plant removal experiments around seedlings in 3 years in which significant recruitment occurred with 5 different removal timing treatments—initial complete removal, complete and continued removal monthly, two month delayed removal and continued removal monthly, and 4 month delayed removal. I performed annual censuses of the outplants and monthly censuses of seedlings in the removal experiment for 6 months to assess the growth, survival, and recruitment of outplants and recruits. I performed demographic rate regressions for annual growth, survival, and reproduction (fruiting and recruitment) and constructed Integral Projection Models of population growth for each year in each population at different levels of alien plant removal, applying different rates of seedling growth and survival to compare weeding removal treatments.
Weed removal treatments had significant effects on both survival and growth of seedling recruits. Removal of alien neighbors had a strong negative effect on survival of recruits across each treatment. Removal of alien neighbors had a significant negative effect on growth of recruits initially, no effect after 2 months, and a significant positive effect on growth after 3 months, indicating that alien plants had initially positive effects on recruits in general but eventually had negative effects. Integral projection models showed that these effects of alien plants drive a difference between population growth or decline, indicating the need to consider both positive and negative effects of alien plants when managing rare plant populations.
