Snow depth shapes aboveground but not belowground phenology during snowmelt, influencing carbon exchange in a northern boreal peatland

Snow cover exerts contrasting influences above- and belowground, with shallower snow exposing shoots faster to warmer air temperatures in spring while causing colder soils and deeper soil frost. In contrast, increased snow depth insulates soils, keeping them warmer, but isolates shoots from warm air. Given that temperature is a key driver of phenological progression both above and below, snow cover changes may produce diverging effects on above- and belowground phenology, impacting spring carbon dynamics.

Over two years, we tracked snowmelt and spring green-up in a snow manipulation experiment (i.e., snow reduction, snow addition, control) at the EcoClimate site in a northern boreal rich fen (66°22' N). Our findings revealed that snow reduction advanced snow-free conditions for shoots but caused colder soils and delayed peat soil thawing. Snow reduction accelerated shoot phenology, but net carbon exchange remained similar to the control. In contrast, snow addition did not affect shoot phenology but reduced ecosystem respiration. Root growth was absent across all treatments during the first seven weeks of observation up to mid-summer, but follow-up measurements showed an increase in root density one month later.

These findings demonstrate that substantial shoot growth can occur independently of root growth. While snow reduction and addition did not produce divergent phenological patterns between aboveground and belowground processes—due to the delayed onset of root growth—changes in snow cover influenced carbon dynamics in complex ways. This study highlights the intricate interplay between winter snow cover, spring phenology, and CO2 exchange in high-latitude ecosystems.