Leaf onset in the northern hemisphere triggered by daytime temperature

Area of Science:

• Ecology
• Climate Science
• Plant Phenology

Background:

• Recent global warming has accelerated spring leaf onset across the Northern Hemisphere.
• Current Earth system models struggle to accurately simulate this phenomenon using daily mean temperature (Tmean).

Purpose of the Study:

• To investigate the role of daytime (Tmax) versus nighttime (Tmin) temperatures in driving interannual variations in leaf unfolding dates (LUDs).
• To propose an improved parameterization for phenology models based on daytime temperature.

Main Methods:

• Analysis of in situ observations of LUDs in Europe and the United States from 1982-2011.
• Comparison of temperature sensitivity using Tmax, Tmin, and Tmean.
• Validation using satellite-derived vegetation green-up data across the Northern Hemisphere (>30 °N).

Main Results:

• Interannual LUD anomalies are more strongly correlated with Tmax than with Tmin or Tmean.
• A 1°C increase in Tmax advances LUD by 4.7 days in Europe and 4.3 days in the US, exceeding Tmean sensitivity.
• Satellite data confirm the dominant role of Tmax in spring vegetation green-up.

Conclusions:

• Daytime temperature (Tmax) is a more critical driver of spring leaf onset than previously modeled using Tmean.
• Incorporating Tmax into phenology modules can enhance the accuracy of Earth system models.
• This research provides a new framework for understanding and modeling plant phenology in response to climate change.