At high latitudes, the climate has warmed at twice the rate of the global average. As a consequence, northern forest plants are experiencing environmental conditions that are highly variable compared to in the past.
The increase in temperature does not concern only the growing season. In fact, temperatures have increased during winter months in Finland even more than those during the growing season (1).
Although ice encasement is common in the Arctic, it is expected to be more frequent in northern boreal ecosystems where it creates a new situation for both plant and soil microorganisms.
Changing snow cover
Although air temperatures are undoubtedly increasing, changes in soil temperatures might not follow the same trend as they depend on the quantity and quality of the insulating snow that covers the ground for a large part of the year.
Typically, snow cover protects the soil from deep frost during the coldest months (soil temperature remains near to 0 °C under thick insulating snowpack in fine-grain soil), and from steep temperature amplitude (more frequent freeze-thaw cycles) in late winter and early spring. More frequent warm spell, rain-on-snow events and freeze-thaw cycles may cause complete snow melting, compaction of snowpack, formation of ice layers above and within the snow layer, and ground ice encasement.
Solid ice is nearly impervious to respiratory gases and continuing soil microbial activity and minimal root respiration has the potential to induce severe hypoxia and accumulation of soil gases under the snowpack. Although ice encasement is common in the Arctic, it is expected to be more frequent in northern boreal ecosystems where it creates a new situation as both plant and soil microorganisms are adapted for overwintering under aerobic conditions.
Such impacts of winter climate change and their consequences on growth and winter survival of forest tree seedlings is largely unexplored.
New overwintering conditions
Lower insulating properties of compacted snow and ice will lead to deeper soil frosts and the presence of slow-melting ice layer will delay soil thawing in the spring. Such changes will have important consequences on plants as desiccation stress might be dramatic, especially for seedlings or small saplings with limited internal water reserves. So, although climate warming has advanced the biological spring, the situation might be different in high northern latitudes.
In parallel, more frequent warm spells have the potential to transiently stimulate plant metabolism. Consequences might differ depending on the conditions: in presence of frozen soil, increased transpiration in aboveground tissues would lead to high desiccation stress, while in an anoxic environment under ice encasement conditions the low energetic efficiency of anaerobic respiration could rapidly decrease the C and N reserves, especially in belowground tissues.
Such impacts of winter climate change and their consequences on growth and winter survival of forest tree seedlings is largely unexplored. Given that the snow-covered period can last more than half a year in boreal ecosystems, this creates a major gap in the understanding how climate warming will affect boreal ecosystem functioning.
Winter climate change research
To understand the effect of changing snow conditions on soil microclimate as well as on the survival and growth of conifer seedlings, we subjected forest experimental plots in Rovaniemi area to several different snow manipulation treatments (2).
Our study shows that among ice encasement, snow compaction or absence of snow, ice encasement appeared to be the most harmful winter conditions to outplanted Scots pine and Norway spruce seedlings. These effects could influence forest regeneration with important implications for boreal forest ecology and the associated economy such as forest regeneration costs.
A parallel experiment of snow manipulation was performed in controlled conditions in dasotrons in Luke Joensuu (collaboration with Timo Domisch, Tapani Repo). Results also showed that a missing snow cover or a flooded and frozen soil can be detrimental for small tree seedlings and that ice encasement significantly decreased photosynthesis and starch content in stems and roots of pine (5).
At that stage of our study, many questions remain and new questions are arising such as:
- What is the most damaging winter stress? As shown in our study, overwintering conditions changed in many ways and identifying which of the environmental factors had the most important influence on seedling survival and growth remains a key question.
- How does the development stage of seedling after its response to adverse snow conditions: in relation to their larger C and N reserves, are older seedlings or saplings more resistant to adverse winter conditions?
- In a larger scale, what is the effect of different snow conditions in forest ecosystem functioning: (nutrient cycling, soil microbiome, plant-soil interactions,…)?
- Although further studies are needed, our results suggest that denser snowpack or lack of snow cover due to winter warming could have a major impact on forest ecosystems. Understanding the physiological mechanisms behind the observed seedling decline and death are needed in order to find tools for mitigating the effect of ice encasement on boreal forest ecosystem functioning.
- Mikkonen S, Laine M, Mäkelä HM, Gregow H, Tuomenvirta H, Lahtinen M, et al. (2014) Trends in the average temperature in Finland, 1847–2013. Stochastic Environmental Research and Risk Assessment. 12/17:1–9.
- Academy research project funding: Winter in changing climate: effects of snow conditions on plants, soil and their interactions in the boreal forest (decision 267092), 2013–2017, project leader: Pasi Rautio, Luke Rovaniemi.
- Martz F, Vuosku J, Ovaskainen A, Stark S, Rautio P. (2016) The Snow Must Go On: Ground Ice Encasement, Snow Compaction and Absence of Snow Differently Cause Soil Hypoxia, CO2 Accumulation and Tree Seedling Damage in Boreal Forest. PLoS ONE. 2016 06/02;11(6):e0156620.
- Vuosku J, Männistö M, Stark S, Ovaskainen A, Suokas M, Saravesi K, et al. (2016) The influence of snow conditions on the structure of bacterial and fungal communities in boreal coniferous forests. In: Plant and Microbe Adaptation to Cold, 22–25 May 2016; Seattle, USA.
- Domisch T, Repo T, Martz. F, Rautio P. (2016) Forest tree seedlings may suffer from predicted future winters. In: EGU General Assembly, 17–22 April 2016; Vienna, Austria.