Cold storage of evergreen seedlings affected by global warming
Due to global warming, conifer seedlings are more often exposed to desiccation during snow-free winters. Overwinter cold storage allows to improve their survival, provided that optimal temperature and storage time are considered.
In recent years, overwinter storage of planting material in freezers has become a standard operating procedure in nurseries in Northern Europe and North America.  Due to climate change, winters without or with only thin or intermittent snow cover are predicted to become more common.  Usually, planting stock is left in the soil over winter or at a holding area before being replanted in spring. Until recently, newly planted conifer seedlings were safe under the snow cover. Unfortunately, the risk of winter damage is likely to increase.
One of the factors causing winter damage is winter desiccation. It occurs in a snowless or low-snow late winter or early spring when there are freezing night temperatures, above-zero daytime temperatures, and a high vapour pressure deficit combined with frozen soil. Exposure to strong sun radiation and warm temperatures increases transpiration in seedling shoots, which can lead to desiccation. The consequence can be that stems, needles, and buds dry out.
For instance, in the spring of 2020, extensive seedling damage was observed in Central Finland on seedlings planted the previous year. The winter of 2020 was also exceptionally warm and rainy, with thin snow cover in Southern and Central Finland. Under these conditions, the risk of winter damage was high. According to a survey on a sample of nine nursery companies in Finland, winter damage was observed in 46% of seedlings. The damage was determined to be serious (death or withering) in 31% of the seedlings affected by winter damage. 
Overwinter storage of planting stock in air-conditioned storage or in freezers ensures optimal storage conditions and facilitates replanting in spring. A team of researchers in the Czech Republic tested several methods of storing planting stock of Norway spruce and European beech.  Their findings include the following:
- Overwinter storage is possible within a temperature range of −1.7°C to −3.4°C. After removal from storage, the plants show high fine root vitality, minimal (sometimes no) mortality and a higher increment after planting.
- Air temperatures of −5.6°C to −5.9°C are suitable for short-term storage (the period tested was 1 month). Two months and longer storage can lead to a slight rise in plant mortality, lower vitality of the fine roots and an increment comparable to that during the storage from −1.7°C to −3.4°C.
- After overwinter storage at air temperatures of −6.6°C to −8.4°C, the plants show low vitality of the fine roots and unacceptably high mortality. Nevertheless, some tree species are more resistant to low freezing temperatures depending on storage duration.
- Large variations in storage temperature are harmful to plants (tested from +2°C to +6°C), as is low relative humidity (tested 58-75%).
- Storage in an air-conditioned warehouse (+2°C, 100% relative humidity) can be recommended for European Beech, but in the case of Norway Spruce, despite fungicide treatment, there is a disproportionate occurrence of fungal infestation.
 Pantová P, Houšková K, Mauer O. Overwinter Storage of European Beech and Norway Spruce Planting Stock: Effect of Different Methods and Temperature Conditions. Forests. 2021; 12(9):1286. https://doi.org/10.3390/f12091286
 Luoranen, J., Riikonen, J., & Saksa, T. (2022). Factors affecting winter damage and recovery of newly planted Norway spruce seedlings in boreal forests. Forest Ecology and Management, 503, 119759. https://doi.org/10.1016/j.foreco.2021.119759
(image source: https://pxhere.com/fr/photo/816524)