Changes in snow cover affect tree and forest floor processes in mature boreal forest

Introduction Rising global temperatures lead to milder winters with higher minimum temperatures and shorter periods of sub-zero conditions, but more frequent freezing and thawing in many boreal regions. Additionally, snow cover duration has decreased in recent decades in the northern hemisphere, and the overall amount of seasonal snow has declined (Pulliainen et al. 2020). This impacts the relationship between soil and air temperatures since snow acts as an insulator, helping to prevent deep soil freezing. Soil temperature and frost depth are crucial for the functioning of high-latitude ecosystems, as they influence tree water uptake, photosynthesis, and growth (e.g. Jyske et al. (2011), Lintunen et al. (2020)). For dwarf shrubs on the forest floor, snow’s insulating effect is even more critical in cold climates (Campbell et al. 2005). Soil temperature experiments are usually carried out in controlled environments such as laboratories or greenhouses, often focusing on shrubs or tree seedlings. However, some previous field studies have been conducted in mature boreal forests (e.g. Bergh and Linder (2001), Jyske et al. (2011)). In this study, we blocked snow from reaching the forest floor for two consecutive winters in a mature boreal forest to examine its effects on soil temperature, forest floor respiration, tree hydraulics, stem and root growth, as well as the growth of dwarf shrubs. Methods The experiment was conducted at SMEAR II station (Hari and Kulmala 2005) in southern Finland. The core of the SMEAR II stand with long-term tree-measurements was our control site and the snow removal treatment was done in its close proximity by preventing snow from falling to the ground with four shelters with total area of 360 m2. Each shelter was constructed around 2-3 study trees so that the tree canopies reached above the shelters. The shelters were removed for the snowless periods. The studied species were Scots pine, Norway spruce, silver birch, and a dwarf shrub bilberry. We measured air temperature, snow depth, soil temperature, soil moisture and root temperature at the control and treatment sites. Forest floor respiration was measured monthly with a dark manual chamber. Tree sap flow was measured with constant heat-dissipation probes and sap pressure from birches with pressure transducers. We measured stem diameter growth from tree cores and pine fine root growth with root ingrowth bags. We also measured coarse root hydraulic conductivity, bark osmolality and frost damages. Regarding dwarf shrubs, we measured diameter growth of the stem, elongation growth, fine root growth, leaf mass, leaf area, and the percentage of dead ramets. Results and discussion The absence of snow cover led to lower soil temperatures, which in turn reduced forest floor respiration during winter and spring. At the same time, the sensitivity of respiration to temperature appeared to increase, possibly due to the exposure of forest floor vegetation to cold air. The lack of snow also caused bilberry mortality, but the surviving plants grew taller and developed larger leaves, likely as a compensatory response to biomass loss. Tree hydraulics were also affected, with reduced water uptake in spring and a delayed start of the sap pressure season in birch. Pine and birch showed a tendency for reduced growth under snow exclusion, whereas spruce exhibited increased growth. However, previous studies have shown that the growth responses may have a time lag (Repo et al. 2021). Coarse root traits, such as water content and cellular frost damage, remained unaffected by the treatment. This study adds to our understanding of how changing snow cover influences springtime tree and forest floor processes in mature boreal forests. However, it also highlights the need for further research on mature trees to fully grasp the long-term impacts.