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).