Frost controls spring phenology of juvenile Smith fir along elevational gradients on the southeastern Tibetan Plateau
Impacts of climatic means on spring phenology are well documented, whereas the role of climatic variance, such as occurrence of spring frosts, has long been neglected. A large elevational gradient of forests on the southeastern Tibetan Plateau provides an ideal platform to explore correlates of spring phenology and environmental factors. We tested the hypothesis that spring frost was a major factor regulating the timing of bud-leaf phenology by combining 5 years of in situ phenological observations of Abies georgei var. smithii with concurrent air temperature data along two altitudinal gradients. Mean lapse rate for the onset of bud swelling and leaf unfolding was 3.1 ± 0.5 days/100 m and 3.0 ± 0.6 days/100 m, respectively. Random forest analysis and conditional inference trees revealed that the frequency of freezing events was a critical factor in determining the timing of bud swelling, independent of topographic differences, varying accumulation of chilling days, and degree-days. In contrast, the onset of leaf unfolding was primarily controlled by the bud swelling onset. Thus, the timing of bud swelling and leaf unfolding appear to be controlled directly and indirectly, respectively, by spring frost. Using space-for-time substitution, the frequency of spring freezing events decreased by 7.1 days with 1 °C of warming. This study provides evidence for impacts of late spring frosts on spring phenology, which have been underappreciated in research on phenological sensitivity to climate but should be included in phenology models. Fewer spring freezing events with warming have important implications for the upward migration of alpine forests and treelines.
KeywordsSpring phenology Bud swelling Leaf unfolding Conifer Spring frost Abies georgei var. smithii Altitudinal gradient
We appreciate the great support from the Southeast Tibet Station for Alpine Environment, Observation and Research, CAS.
E.L. designed the research; all authors analyzed data and wrote the paper.
This work was supported by the National Natural Science Foundation of China (41525001, 41661144040) and the International Partnership Program of Chinese Academy of Sciences (131C11KYSB20160061). AME’s participation in this project was supported by the Chinese Academy of Sciences (CAS) President International Fellowship Initiative for Visiting Scientists (Grant No. 2016VBA074).
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