Abstract: Responses of leaf senescence phenology (LSP) to climate change are closely linked to ecosystem carbon and nutrient cycling. The effects of climate change on LSP remain inconsistent, partly reflecting direct effects of ambient climate and the indirect regulatory role of biotic factors; however, the underlying mechanisms are poorly understood. We conducted a 7-yr field manipulative warming and precipitation alteration experiment to explore how combinations of abiotic and biotic factors affect community LSP (onset of leaf coloration, end of senescence, and senescence duration) in an alpine grassland on the Qinghai-Xizang Plateau. We found that increased precipitation significantly advanced LSP onset but did not affect its end, thereby prolonging its duration. Warming delayed LSP end without altering its onset or duration. Warming plus increased precipitation had no effect on LSP onset but delayed its end, thus extending its duration. Decreased precipitation and warming interactions had relatively weak effects on LSP. LSP also showed large interannual fluctuations. Precipitation frequency explained 72.9% and 44.3% of LSP onset and end variation, respectively. Total precipitation during the growing season explained 43.2% of variation in LSP duration. These results suggest that ambient climate exceeded manipulative climate-change effects on LSP in this alpine meadow. Furthermore, leaf-out onset, plant diversity, and productivity, as mediators downstream of ambient climate and treatments, were closely associated with LSP changes. Our findings highlight the need to consider ambient climate and biotic factors when investigating mechanisms of plant phenological responses using complex manipulative experiments.

Key words: warming, precipitation alteration, ambient climate, senescence phenology, alpine meadow, Qinghai-Xizang Plateau