Freeze-Thaw Dynamics Reshape Climatic Control of Spring Phenology Across Northern Ecosystems

Area of Science:

• Ecology
• Climate Science
• Remote Sensing

Background:

• Global warming advances spring phenology in northern ecosystems, but regional variations and nonlinearities are poorly understood.
• Temperature and precipitation are key drivers, yet the role of freeze-thaw cycles (FTCs) on the start of growing season (SOS) is often overlooked.

Purpose of the Study:

• To assess the impact of freeze-thaw cycle (FTC) frequency on start of growing season (SOS) dynamics across the pan-Northern Hemisphere using 20 years of data.
• To investigate the biome-specific and nonlinear ways FTCs modulate SOS and to identify shifts in climatic constraints on phenology.

Main Methods:

• Integrated 20-year satellite phenology data (2003-2022) with climate reanalysis data.
• Analyzed FTC frequency impacts on SOS dynamics across various northern ecosystems.
• Employed sensitivity analyses using ridge regression and generalized additive models.

Main Results:

• Despite an average SOS advancement of 1.9 days/decade, over 28% of vegetated areas showed stable or delayed trends, linked to heterogeneous FTC increases.
• Frequent FTCs advanced SOS in boreal forests (up to 7 days) but delayed it in desert and temperate forests (over 20 days) due to stress.
• FTCs explained up to 14.6% of SOS variability in temperate broadleaf forests, comparable to precipitation; however, sensitivity to FTCs declined over time.

Conclusions:

• Freeze-thaw cycles are dynamic regulators of phenology, capable of offsetting warming-driven advancements and creating regional disparities in SOS.
• Shifting sensitivities indicate a reorganization of climatic constraints, with biomes showing divergent trajectories in response to temperature, radiation, and FTCs.