From Seed to Seedling: Constraints Imposed by Warming, UV-B, and Burial Depth on Alpine Cushion Plant Recruitment

Alpine cushion plants play keystone roles in regulating and sustaining various ecosystem functions, including enhancement and maintenance of community diversity and productivity. Therefore, their population dynamics may exert significant influences on alpine ecosystems. However, the mechanisms driving their population dynamics, particularly during critical early recruitment stages such as seedling emergence and subsequent growth, remain poorly understood. This study investigated how warming temperatures, intense UV-B radiation, and increased seed burial depth affect the emergence, growth, and mortality of seedlings from 11 cushion plant species in the alpine subnival zones of the Himalaya-Hengduan Mountains in southwestern China. Results revealed that deep seed burial (exceeding 1 cm) significantly suppressed seedling emergence in cushion species, with substantial loss of seed viability occurring within 3 months for seeds buried at greater depths. Warmed temperatures (> 20°C) accelerated emergence rates and enhanced seedling growth (increased height and leaf number), but also resulted in high seedling mortality rates (ranging from 60% to 100% in most species). In addition, intensified UV-B radiation consistently impaired seedling recruitment across multiple stages, leading to reduced emergence percentages, stunted growth, and increased mortality rates. These findings suggest that the coarse, mobile substrates characteristic of subnival zones exacerbate natural constraints on seedling emergence in cushion plants. While increased temperatures may promote emergence and growth, they can also result in significant mortality, thereby hindering future population recruitment and expansion. Furthermore, enhanced UV-B radiation due to ozone layer depletion poses additional threats to cushion populations. Nevertheless, the observed species-specific responses to these factors indicate that cushion species may exhibit diverse population dynamics under future climate change scenarios. This study provides essential insights into the potential trajectories of alpine cushion populations under climate change, offering valuable implications for the conservation of ecosystems and biodiversity.