Abstract:

The canopy seed bank serves as a critical adaptive mechanism for plants, enabling them to cope with environmental challenges, resist disturbances, and facilitate vegetation regeneration and ecosystem restoration. Understanding the effects of grazing on canopy seed banks is essential for managing alpine grassland ecosystems, where grazing practices and livestock combinations can significantly impact plant community structure and seed dynamics. In this study, we investigated the effects of different grazing livestock combinations on the canopy seed bank within an alpine grassland ecosystem in Xihai Town, Haibei Prefecture, Qinghai Province. Specifically, we examined the impacts of grazing regimes that included varying ratios of yak to Tibetan sheep, specifically 1꞉2, 1꞉4, and 1꞉6, as well as single-species grazing by yaks or Tibetan sheep alone. Our research aimed to clarify how these grazing combinations influence canopy seed bank density, species composition, seed dormancy, and seed quality. Results indicated that 1) 13 plant species within the study area maintained canopy seed banks, spanning six botanical families and ten genera. Of these species, seven demonstrated the capacity for seed persistence within the canopy seed bank for over five months. 2) Our findings highlight the significant influence of grazing practices on the density and diversity of the canopy seed bank within alpine grassland plant communities. Grazing consistently reduced canopy seed bank density, with notable decreases in the densities of both Poaceae (grass family) seeds and seeds from miscellaneous forb species. Among the grazing treatments, Tibetan sheep-only grazing and mixed yak-sheep grazing regimes exerted a more substantial impact on canopy seed bank density compared to yak-only grazing. Furthermore, grazing treatments led to a reduction in the species richness and diversity indices of the canopy seed bank, underscoring the susceptibility of seed bank diversity to grazing-induced pressures. 3) In addition to these density and diversity effects, our study observed that seeds stored within the canopy seed bank undergo qualitative improvements following prolonged dormancy. Specifically, after a five-month dormancy period, seeds of Carex alatauensis (a dominant sedge species in the study area) exhibited significant increases in both seed mass and germination rates. This suggests that while grazing may reduce the overall quantity of seeds in the canopy seed bank, dormancy can enhance the physiological quality of the remaining seeds, potentially benefiting species that rely on canopy seed banks for regeneration. 4) Moreover, the trade-off between seed quantity and quality in the canopy seed bank was found to be influenced by the specific combinations of grazing livestock. Under mixed grazing conditions involving both yaks and Tibetan sheep, Carex alatauensis displayed a tendency towards increased seed quality, as indicated by higher seed mass and improved germination rates. In contrast, Tibetan sheep-only grazing promoted an increase in seed quantity rather than quality. This observed trade-off suggests that the presence of different livestock species may drive distinct evolutionary or adaptive strategies in seed bank dynamics, which could have implications for species composition and community resilience in grazed ecosystems. Overall, our findings indicate that grazing exerts a significant, density-reducing effect on the canopy seed bank, while also prompting adaptive changes in seed quality among certain plant species. These results contribute to a broader understanding of how grazing influences plant reproductive strategies and canopy seed bank dynamics in alpine grasslands, with potential applications for ecological management and conservation. Based on our findings, we recommend implementing strict grazing controls before the green-up period in alpine grasslands, as this may promote vegetation recovery, enhance canopy seed bank quality, and improve the stability of plant populations. This research provides valuable theoretical support for the sustainable management of alpine grassland ecosystems, particularly in regions where grazing is a predominant land use practice.

Key words: alpine grassland, canopy seed bank, moderate grazing, grazing livestock combination, seed germination, trade-off analysis

摘要：

植冠种子库是植物适应外界环境的一种机制，在抵抗外界干扰和植被更新恢复等方面具有重要意义。基于青海省海北州西海镇高寒草地，研究不同放牧家畜组合（牦牛藏羊1꞉2、1꞉4、1꞉6、牦牛/藏羊单牧）对高寒草地植物群落植冠种子库的影响。结果表明，1）研究试验区具有植冠种子库的物种13种，分属6个科、10个属，其中7种宿存时间超过5个月。2）放牧会降低群落植冠种子库的密度，且主要降低禾本科植物种子与杂类草植物种子密度，藏羊单牧与混合放牧对植冠种子库密度的影响大于牦牛单牧，同时放牧处理会降低群落植冠种子库丰富度指数和多样性指数。3）植冠种子库经5个月宿存后，共有物种矮生嵩草（Carex alatauensis）的植冠种子库种子质量提高、萌发率提高。4）矮生嵩草植冠种子库数量与质量权衡策略受不同放牧家畜组合影响，表现为牦牛与藏羊混牧条件下更倾向于增加种子质量，而单一藏羊放牧则偏重种子数量提升。综上所述，放牧会显著降低植冠种子库密度，种子经宿存后整体质量得到提升。建议在返青期前严格控制放牧，以促进草地的植被恢复和种群稳定。该研究为高寒草地的生态管理提供了理论依据。

关键词: 高寒草地, 植冠种子库, 中度放牧, 放牧家畜组合, 种子萌发, 权衡分析