4.2 Advanced leaf out and first flower dates
Warming significantly advanced leaf out and first flower dates for six of the eight species. Higher temperatures with climatic warming will primarily decrease the growing degree days required for a certain phenological event, and this is particularly critical in cold environment such as the Tibetan Plateau (Piao et al. 2015; Suonan et al. 2017). Open top chambers were installed year-round across the whole experimental period. During the growing season, increased soil temperature stimulated decomposition and nutrient cycling (Chenet al. 2015a; Estiarte & PeƱuelas 2015), which could facilitate plant growth through promoting increased soil N availability. At the beginning of the growing season, higher temperatures would prevent soil water from freezing and support both plant and microbial activities (Chenet al. 2017a; Suonan et al.2017). During the non-growing season, higher temperature may also have positive effects on leaf out, despite a certain period of chilling requirements may also be critical prior to initiation of leaf out (Marchin et al. 2015; Guo et al. 2019). This is because the mean soil temperature was quite low during winter at our study site (Chen et al. 2017b), and thus it is less likely that OTCs raised soil temperature enough to break the winter chilling requirements threshold (Suonan et al. 2017).