Overwinter Storage of Planting Material: History
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Overwinter storage of planting material, i.e., its storage over one period of dormancy, is becoming an increasingly important and costly part of nursery production. Planting stock is usually left over the winter in soil (bare-rooted plants) or at a holding area (containerized plants), and then lifted up and planted in the spring. Due to climate change, the period for spring forestation is shortening, which, with the growing volume of forest nursery production and the time−consuming nature of spring work, leads not only to autumn forestation but also to the storage of planting material lifted up in autumn. Prior to storage, the plants are prepared for dispatch from the nursery and placed under conditions designated for this purpose until the time of spring forestation.

  • European beech
  • freezer storage
  • Norway spruce
  • overwinter storage
  • root electrolyte leakage

1. Introduction

In nursery practice, a common way of overwinter storing of containerized planting stock is to leave it on the holding area, while bare-rooted plants are usually left in soil (beds). The plants must be protected from frost [1]. Containerized plants are deposited on the surface of the holding area and the free sides are covered by strips of rubber [2]. Another possibility of eliminating the effect of frost is use of special packaging–Styrofoam [3] or overwintering of plants in natural snow or in artificial snow produced by snow cannons [4][5]. Bare-rooted plants are prepared for overwintering, i.e., they are released from bunches and the roots are covered with soil [1].
For overwinter storage of plants, appropriate spaces (e.g., caves, cellars) are places with a stable temperature of up to +6 °C and relative humidity above 80% [6]. These conditions are provided by specially air-conditioned storage areas that allow storage of plants at temperatures from 0 °C to +3 °C and at high relative humidity in the range of 93% to 98%, depending on the method of protection of the seedlings against desiccation [7]. Storage of planting stock at temperatures above the freezing point, however, poses a risk of intensive development of fungi and mold on the stored plants [8]. In recent years, it has become standard operating procedure in many foreign nurseries, particularly in the countries of Northern Europe and North America [9][10], to store planting stock in freezers. Landis et al. [8] indicate that the planting material may be stored in a state of complete dormancy over the winter period in a freezer with a maintained air temperature of −2 to −3 °C. According to Ritchie [10], the freezing of planting stock is, in practice, limited to a period of 6−8 months. Luoranen et al. [11] states, however, that planting stock of Norway spruce can be safely frozen for 8−9 months.
For overwinter storage in freezers, plants must be in complete dormancy and are therefore lifted up as late in the autumn as possible [12]. Dormant plants show the necessary frost resistance, which can also be supported by adjusting the temperature conditions before storage. It was found that acclimatization of planting stock in early September to October, with a temperature of +1 °C to +5 °C, accelerates the development of frost resistance and improves hardiness to freezing temperatures during storage. According to Němec [13] in Central Europe, dormant plants are suitable for storage when lifted up from mid-October to mid-March. Dormancy is determined by various methods, e.g., with the help of dormancy meters: calculating chilling sums or using the bud break test according to Landis et al. [1], and, for accurate verification of the date for storage of planting stock, it is possible to use the N−Sure method or the ColdNSure test [2].
During storage at freezing temperatures, it is necessary to control the air temperature and humidity [2]. The temperature should be measured not only in the storage space, but also inside the packaging because the plants are still respiring and the temperature in the bag or box may be a few degrees higher. Kooistra [9] stated that for this reason the temperature in the storage space should be 1–2 °C lower than desired temperature in the packaging that the plants are in. Radoglou, Raftoyannis [14] recommend a monthly inspection of the plants, particularly for possible frost damage. In addition, frost can cause desiccation, which Garriou et al. [15] pointed out.
When in overwinter storage, respiration and desiccation of plants must be reduced to a minimum, preferably by closing them in storage containers [2]. Indeed, Vitra [16] point out that overwinter storage in freezers can cause desiccation stress in the planting stock and a reduction in the supply of carbohydrates for maintaining its respiration. These physiological changes may cause a decline in chlorophyll fluorescence, which is directly related to photosynthetic activity and a slowdown in growth and a decline in the hardiness of seedlings after planting [17].

2. Overwinter Storage of European Beech and Norway Spruce Planting Stock

Overwinter storage of planting stock has become a normal part of growing technology in many tree nurseries. Use of the latest method–the storing of plants in freezers–will undoubtedly continue to rise. Fundamental to its successful use is the choice of optimal storage temperature and the regular monitoring of plants.
Plants intended for storage at freezing temperatures must be in a state of complete dormancy [1]. Janda et al. [18] indicates that, at the onset of dormancy in plants, there is a gradual accumulation of carbohydrates and other osmotically active substances. In addition, the amount of water in the plant cells decreases, which leads to division of the central vacuole [19]. Temperatures lower than 0 °C led to the formation of ice crystals both inside and outside the cells. If crystals form inside the cells, it leads to irreversible damage to the cell structures [20]. Conversely, ice generally forms outside (i.e., in the inter−cellular spaces) when the cells are exposed to temperatures of about −1 °C to −3 °C. However, if the plants are not hardy, severe dehydration of the cell content and mechanical damage to the cell walls and plasma membranes can occur [20][21].
REL, height increment and mortality of plants proved to be the best indicators of the plants’ response to possible damage (frost damage and/or desiccation) during overwinter storage in the current research. The plant height and the root collar diameter were not affected by the higher initial variability and mortality (i.e., the smaller and weaker plants often died first). The results showed that both bare-rooted and containerized beeches and spruces grew best and achieved the lowest losses after storage at a temperature of −3.4 °C to −1.7 °C (measured directly on the plants inside the bag). In addition, Wesoly, Chabowska [22] confirm that freezing temperatures are better for the storage of planting stock, since there is the risk of fungal infestation at higher storage temperatures. Dumroese et al. [23] also indicate the most suitable storage temperature to be in the range of −2 °C to −4 °C. Suitability of the storage temperature down to −5 °C was also proven in testing conducted by Repo [24]. In addition, it was confirmed that overwinter storage at −5 °C does not affect the metabolic activity of mycorrhizal fungi.
In the current research, lower temperatures from −6.6 °C to −8.4 °C for storing both types of planting stock and for both woody species were inappropriate. The tested plants were found to have low vitality of fine roots (a high REL value), shorter increments and unacceptable mortality at the end of the growing season after planting. Wang, Zwiazek [25] also note that very low storage temperatures reduce the growth potential of roots and cause delayed sprouting of plants after planting. Results of the current research showed that only the containerized spruce possessed greater durability during shorter storage (approximately 1 month) at these temperatures and, after planting, were able to thrive and grow. Containerized spruce even showed comparable vitality of the fine roots after storage at higher temperatures (from −3.4 °C to −1.7 °C). The current result of the inspection of the condition of frozen planting stock suggests that the REL and the restoration of the growth after planting appear to be reliable indicators showing the condition of the planting stock during and after freezing. It is also clear that containerized planting stock is more resistant to lower freezing temperatures than bare-rooted planting stock and that Norway Spruce withstands these temperatures better than European Beech.
The current research showed that the temperature limit for storing of tested plants by freezing is from −5.6 °C to −5.9 °C. The plants stored at this temperature generally showed comparable vitality of fine roots, as when stored at −3.4 °C to −1.7 °C. A slightly increased plant mortality of up to 30% and a comparable increment with the plants stored at −3.4 °C to −1.7 °C were recorded, but some containerized beeches and spruces even showed higher increment and lower mortality than those stored at −3.4 °C to −1.7 °C. Mena-Petite et al. [26] explain that the root ball provides protection against desiccation during storage. The unsuitability of storage temperatures below −5 °C was then indicated by Camm et al. [27]. Plants tested in this research and stored at approx. −6 °C can also be influenced by storage time and the conditions encountered after planting. Bare-rooted and containerized spruce and beech stored at −5.6 °C for one month and planted in soil in the tree nursery in the current research, showed zero mortality after planting but after two months of storage and planting in a clearing, their mortality increased to 20−30%.
The main benefit of storage of planting stock at freezing temperatures, according to Rikala [2], is the possible extension of the spring planting season. Frozen plants are, according to the author, extra resistant to drought and cold and, especially in dry conditions, demonstrate better growth than open-storage plants. Jacobs et al. [28] emphasize that plants stored in the freezer have proved to be highly resistant to the damage from spring frosts.
The open-storage method, based on the current results, is suitable for containerized beeches and bare-rooted spruces. Rikala [2] recalls that, in this way, mainly containerized root planting stock is stored. The current results may be affected by the unusually high air temperatures that were measured (without snow cover) during the 2017/2018 winter season [29]. Air temperature and the amount of precipitation during this winter may have greatly influenced the open storage of the planting stock and such negative impact could be mitigated by the overwintering of plants.
The storage of the plants in the current research through the winter in the cave (+2.2 °C), without the protection of the root system (bare-rooted plants) or only covered the root balls with foil (containerized plants), has proven to be inappropriate for overwinter storage of planting stock of both species of trees due to unacceptable mortality after planting. During storage, planting stock was already recorded as having a high REL value (i.e., low vitality of fine roots). This was probably due to a large variation in air temperatures (between +2 °C and +6 °C) and a lack of relative humidity (58−75%). Wilnen, Vaartaja [6] point out that the storage of plants in non-air-conditioned spaces (i.e., cellars) was only possible with roots stored in moist peat and with high relative humidity. These authors [6] recommended a stable temperature of up to +6 °C and a relative humidity of above 80%. The conditions of an air-conditioned warehouse meet these criteria where our tested plants achieved a high vitality of their fine roots (low REL), zero mortality and, usually, an increment comparable to that of plants stored at −3.4 °C to −1.7 °C. However, in these conditions, despite fungicide treatment, intensive fungal infestation of spruce was noted, as also Landis et al. [8] point out. Even though the infected plants survived and were able to grow after planting, the fungal infection can cause a higher mortality when storing a larger number of plants. Storage at temperatures of above 0 °C, with a high relative humidity may be an inappropriate way to store evergreen coniferous trees.

3. Conclusions

Overwinter storage of planting stock in an air-conditioned storage or freezers, while ensuring optimal conditions, is a safe way to store planting stock over the winter period and, at the same time, facilitates the spring expedition of plants from the nursery by preparing the planting stock. This study has confirmed some information known about overwinter storage of plants, compared its methods and also specified the optimal temperature values and limits which depend on the tree species, the type of planting stock and the length of storage period. From the results obtained from the testing of more treatments of three methods for the storage of both bare-rooted and containerized planting stock of Norway spruce and European beech, we can conclude that:
  • Overwinter storage of planting stock is possible within the temperatures range of −1.7 °C to −3.4 °C. After removal from the storage, the plants achieve a high vitality of the fine roots, minimal (sometimes even zero) mortality and a higher increment after planting.
  • Air temperatures of −5.6 °C to −5.9 °C are suitable for short 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. Only some containerized plants (Norway spruce) can be stored in such lower temperatures for a shorter time (the period tested was 1 month). Therefore, the temperature limit for plant storage will be probably affected by the time of storage.
  • Storage in an air-conditioned storage (+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.
  • Open storage is recommended only in suitable weather conditions with the careful overwintering of plants.
  • Plants cannot be stored if the temperature in the storage area varies considerably (tested from +2 °C to +6 °C) and if they are not protected against desiccation in the case of low relative humidity (tested 58−75%).
  • Containerized planting stock is more resistant to low freezing temperatures than bare-rooted planting stock.
  • Norway Spruce is more resistant to low freezing temperatures than European Beech.

This entry is adapted from the peer-reviewed paper 10.3390/f12091286

References

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