Precipitation Regulates Phenological Responses of Annual and Perennial Forbs to Warming and Degradation in an Alpine Meadow on the Qinghai-Xizang Plateau.

Alpine grasslands on the Qinghai-Xizang Plateau are increasingly threatened by climate warming and degradation, which promote an increase in annual plants in the community. However, how they shape the plant phenophases of annual and perennial plants remains unclear. In particular, there is limited understanding of whether and how ambient climate regulates their responses to warming and degradation. We conducted a three-year warming experiment across moderately (MD) and severely degraded (SD) grasslands to determine the responses of phenophases of an annual and a perennial forb. We found that neither warming nor degradation significantly affected the onsets and durations of phenological sequences for the annual plant due to the offsetting of a direct advancing effect by an indirect delaying effect via decreased plant richness, which varied with the year. Warming effects on the onset of leaf-out and end of leaf-coloring of the perennial plant varied with degradation and year, whereas SD advanced the onset of leaf-out and prolonged its total activity period relative to MD under warming. This suggested that warming could benefit the growth of perennial plants in the degraded alpine grasslands. Precipitation regulated the responses of phenophases of both annual and perennial plants to warming and degradation via decreased plant richness and/or increased their relative coverage in the community. Our findings indicate that attention should not only be paid to the life history characteristics of plants but also to the regulatory roles of vegetation status and ambient climate when evaluating the impact of climate change on plant phenology.