Heat Deficit for Vegetation Leaf Senescence and Its Key Accumulation Process and Determinants at Northern Middle and High Latitudes During 2001–2022

Aim

Cold degree days (CDD) represent the heat deficit for vegetation leaf senescence in autumn and serve as a critical parameter in modelling leaf senescence. This study aimed to quantify the spatiotemporal patterns of CDD and its key accumulation processes and determinants.

Location

At northern middle and high latitudes (> 30° N).

Period

2001–2022.

Major Taxa Studied

Vegetation.

Methods

We estimate CDD as the cumulative sum of the difference between the daily mean temperature and a threshold temperature (12.75°C) during the period from midsummer to the end of the growing season. To identify its crucial metric, we employ a combination of grey relational analysis, random forest model and partial correlation analysis.

Results

The average CDD increases linearly with latitude at a rate of 5.9°C-days per degree. Higher latitudes exhibit larger CDD (> 300.0°C-days), longer accumulation periods (> 70 days) and faster accumulation rates (> 6.0°C/day), whereas lower latitudes show smaller CDD (< 60.0°C-days), shorter accumulation periods (< 30 days) and slower accumulation rates (< 1.0°C/day). Temporally, CDD tended to decrease from 2001 to 2022 with −1.3°C ± 4.0°C-days/year, largely attributed to climate warming. Precipitation frequency emerged as a significant climatic variable influencing CDD variations across > 46% of the study area, especially at high latitudes and on the Tibetan Plateau. While climate warming generally reduces CDD, an increase in precipitation frequency can counteract this trend and shape the relationship between precipitation amount and CDD. The effects of radiation and wind speed on CDD were less pronounced than those of precipitation frequency, with wind exerting a positive (cooling) effect that increases CDD accumulation and radiation producing a negative (heating) effect that decreases CDD accumulation.

Main Conclusions

This study highlights the critical aspects of the CDD accumulation process and emphasises the importance of incorporating precipitation frequency into CDD-based autumn phenology models across northern latitudes.