Mechanism on the influence of winter precipitation on spring phenology in boreal forests

Understanding the responses of spring vegetation phenology to the variations in climate variables is important for improving our knowledge about biosphere-atmosphere interactions. Several recent studies have suggested that the start date of the vegetation growing season (SOS) can be delayed after wet winters in high latitudes. However, the underlying mechanisms are not yet clear due to the scarcity of observation sites as well as complex interactions between various climate variables. In this study, we investigated the impact of the interannual variation in winter precipitation on the SOS over the boreal forests for the period 1982-2005. Two model experiments were conducted: a control experiment with observed precipitation and sensitivity experiment with the perpetual precipitation of 1982. The effects of winter precipitation on the SOS are examined by comparing the differences between results of two model experiments for wet- and dry winters. After wet winters, the SOS was delayed by 2.7 days over 70.1% of the boreal forests than after dry winters; this accounted for 42.5% of the interannual variation in the SOS. The SOS delay was related to the decrease in growing degree-days (GDD) based on soil temperature, suggesting that the amount of heat applied to vegetation is significantly affected by winter precipitation. The GDD decrease is caused by both an increase in snowmelt heat flux and a reduction in absorbed solar radiation, which are proportional to the amount of winter precipitation and the ratio of short plants to tall trees, respectively. Our results provide a mechanical basis for the winter precipitation-SOS relationship, suggesting that an increase in winter precipitation can alleviate a strong advancing trend in spring vegetation phenology in conjunction with continuous global warming, even for temperature-limited ecosystems.