Soil Freeze–Thaw Cycles Can Advance or Delay Spring Green-Up across Northern Hemisphere

Despite global warming，spring green-up has advanced across much of the Northern Hemisphere, though it still shows little change or even delays in some regions. A new study published on Dec. 29 in Global Change Biology reveals that soil freeze–thaw cycles (FTCs) as an overlooked process, can reshape the timing of the start of the growing season (SOS) each year.

The SOS is important because it sets the timeframe for photosynthesis and carbon uptake. It also influences water and energy exchanges at the land surface. These shifts can affect regional carbon-cycle estimates, especially in high-latitude and high-elevation environments.

FTCs occur when soils repeatedly oscillate around 0°C, alternating between freezing and thawing. This process has mixed effects: repeated freezing can stress roots and increase nutrient losses, while thawing can boost microbial activity, mobilize nutrients, and replenish soil moisture. This process could advance or delay SOS within a specific biome.

To test this mechanism across the pan-Northern Hemisphere, researchers from the Xinjiang Institute of Ecology and Geography of the Chinese Academy of Sciences combined multi-source remote-sensing datasets from 2002 to 2022. They integrated freeze–thaw dynamics into a cross-ecosystem phenology analysis. They used pixel-level models to capture nonlinear responses and to disentangle FTC effects from other climate drivers.

They found that, on average, SOS advanced by about 1.9 days per decade across the terrestrial ecosystems of the Northern Hemispheres (≥30°N). However, more than 28% of vegetated areas experienced a stable or even later growing season. This persistence was most pronounced in boreal forests, tundra, and cold high-elevation regions.

“The impacts of FTCs are not uniform but biome-specific,” said MA Xiaofei, the study's corresponding author. “In boreal forests, higher FTC frequency tended to coincide with earlier SOS—up to ~6.5–7 days. In contrast, frequent FTCs in deserts and xeric shrublands were linked to delayed SOS.”

By elevating FTCs from a “background” fluctuation to a quantifiable driver, this study offers a clearer pathway for incorporating spring phenology into land–vegetation models. This approach could improve carbon-cycle projections in regions where the onset of spring is difficult to predict.

Read the full article: https://doi.org/10.1111/gcb.70675

Contact

LONG Huaping

Xinjiang Institute of Ecology and Geography

E-mail: longhp@ms.xjb.ac.cn

Web: http://english.egi.cas.cn