Asymmetric warming reduces the strength of selection pressure of moderate grazing on reproductive phenology in alpine plants

Both warming and grazing already affect the reproductive phenology of alpine plants. However, their effects have mostly been studied in isolation, and their interaction is still unclear. In this study, an asymmetric warming (average + 1.2 °C during daytime and + 1.7 °C during nighttime and + 1.5 °C during summer and + 2.0 °C during winter) with moderate grazing experiment was conducted for four years to determine their individual and interactive effects on the onsets and durations of reproductive phenophases for fifteen alpine plant species on the Qinghai-Tibetan Plateau. Individual warming and grazing simultaneously advanced the average start dates and ending dates of budding, flowering and fruiting by 5.3-6.2 days, and further resulted in smaller effects on their durations for most plant species. The interactions between warming and grazing on them varied with plant species and year, which advanced by average 12.1 days for all plant species. The effects of grazing on the temperature sensitivity of the start dates of reproductive phenophases (average by -8.5 days °C-1) were greater than that of warming alone (average by -3.4 days °C-1) and warming with grazing (average by -5.5 days °C-1) for most of the alpine plant species. There were significant effects of the previous phenological events on subsequent reproductive phenophases. Therefore, our results suggested that both warming and grazing advanced reproductive phenophases through altered soil temperature and soil moisture and carry-over effects of previous phenological events on subsequent phenological events. Warming reduced the temperature sensitivity of the start dates of reproductive phenophases to grazing, suggesting that it depressed strength of selection pressure of grazing on the onsets of reproductive phenology in alpine plants.