Abstract:

Aims The frequency and intensity of exceptional climatic events such as warm spring have increased significantly over the past few decades and exerted a significant impact on the spring phenophases of plants. However, the influence of exceptional climatic events on the thermal requirements of spring phenophases is still unclear, which limits the predictive accuracy of the future phenological changes. Here we aim to demonstrate how the first leaf dates of woody plants and their associated thermal requirements change under exceptional climatic conditions and how exceptional climatic conditions affect the ability of the growing degree model to predict leaf unfolding date. Methods Using data on the first leaf date of 39 woody species at Xi’an Botanical Garden from 1963 to 2018 and the corresponding meteorological data, this study firstly classified each year into the cold year, normal year and warm year. Subsequently, we analyzed the phenological change in the years with abnormal climate compared to the years with normal climate. Second, three kinds of algorithms were used to calculate the thermal requirements of the first leaf date for each plant, and the difference in the thermal requirements between years with abnormal climate and normal climate was compared. Finally, the error of the traditional growing degree day model in the simulation of the first leaf date in exceptional climatic conditions was assessed. Important findings For all plant species, the first leaf date was earlier in warm years than that in normal years with a mean advance of 8.6 days, and it was later in cold years with a mean delay of 8.2 days. In warm years, the thermal requirement of the first leaf date (257.5 degree days on average) was significantly higher than that in normal years (195.1 degree days on average, p < 0.05) for most species. However, in cold years, the thermal requirement (168.0 degree days on average) was lower than in normal years (not statistically significant) for most species. In cold years, the ancient group delayed by more in first leaf date and showed smaller changes in thermal requirement than the young group, but there was no significant difference in warm years.There were no significant differences in changes of first leaf date and thermal requirement among different life forms. The high temperature in the previous winter caused plants to receive less chilling, and thus reduced the thermal requirement in the following year. The first leaf date of woody plants simulated by the growing degree day model was 4.1 days earlier than the observed date in warm years, and was 3.0 days later than the observed date in cold years. Therefore, when predicting the future phenological changes, it is necessary to consider changes in the thermal requirement under exceptional climatic conditions; otherwise, it will overestimate the promotion effects of climate warming on the leaf unfolding date.

Key words: climate change, phenology, first leaf date, thermal requirement, Xi’an;