Do not delay planting of frozen stored seedlings for too long

The title of the article is Autumn Frost Hardiness in Six Tree Species Subjected to Different Winter Storage Methods and Planting Dates in Iceland. It was published in Volume 15 of the journal Forests. Co-authors are Erla Sturludóttir, associate professor at the Agricultural University of Iceland, Inger Sundheim Fløistad, a researcher at NIBIO in Norway, and Brynjar Skúlason, forensic geneticist and director of research at Land and Forest Iceland.

The introduction of the paper indicates that the method of using freezers to store forest seedlings during the winter may compromise their ability to attain an adequate temperature sum for survival during the first winter after planting. Consequently, the project aimed to identify the species best suited for overwintering in a freezer within a nursery setting, considering their autumn frost hardiness during the initial growing season post-planting.

In Scandinavia, forest seedlings are typically stored at -3°C during the winter months as opposed to being kept outdoors. This practice helps preserve the quality of the plants by preventing frost damage on new growth and roots, while also facilitating their transportation to foresters in the spring. Additionally, it enables the optimisation of various processes within the nurseries and during planting.

Frozen seedlings start growth later in spring

It is, however, important to note that while freezing can be an effective wintering method, it does have some drawbacks. Forest seedlings that are stored outdoors in nurseries receive a longer growing period than those stored in freezers. This is because frozen stored seedlings are still dormant in the freezer when spring arrives, resulting in a shorter growth period. In that way, the time required for seedlings to emerge from winter dormancy can impact their ability to develop sufficient autumn frost hardiness. A shortened growth period increases the risk of autumn frost damage, as seedlings may not have adequate time to complete the necessary growth processes before winter sets in. Furthermore, seedlings stored in freezers tend to continue growing longer into the summer, which can result in delayed frost hardiness development and an increased risk of autumn frost damage.

In this project the autumn frost hardiness of seedlings that had been stored outdoors or frozen stored was compared. The species in the project were downy birch, Siberian larch, the 'Hrymur' hybrid of European and Siberian larch, Sitka spruce, and two seed sources of contorta pine, Närlinge and Skagway. The seedlings were planted in a field experiment at four different times during the summer. On May 24, 2022, both frozen and unfrozen seedlings were planted in a field experiment in Teigur in Eyjafjörður. Frozen seedlings were subsequently planted on June 7, June 21, and July 5 of the same year. Outdoor stored seedlings planted on May 24 served as a control in the experiment.

Planting later resulted in increased damage

Freezing tolerance tests were conducted on September 12 and 26. The frozen stored seedlings exhibited lower frost hardiness the later they were planted compared to those stored outdoors. Frost hardiness varied among the species. The Hrymur hybrid demonstrated lower frost hardiness than the Siberian larch, while the Sitka spruce had lower frost hardiness than the Lutz spruce. Contorta pine and birch displayed the highest frost hardiness in September.

Figure 1. Mean initial height (blue section, n = 60) and annual growth of seven species/seed sources measured on June 13 (yellow), July 5 (red) and July 19 (green). Annual growth measurements were made on plants stored outdoors (n = 40). Different letters in blue sections indicate significant differences in initial height between species (p ≤ 0.05).

Different growth rhythms

The growth rhythm of the species accounts for their differences in frost hardiness. Early in the growing season, annual shoot growth was measured in the nursery to determine the average shoot height (see figure). The annual growth was measured again on specific dates in June and July for plants that remained in the nursery. Both contorta pine seed sources exhibited rapid growth at the start of summer, reaching 83%-84% of their annual growth by mid-June. This suggests that early growth cessation, and consequently the onset of frost hardiness, contributed to the contorta pine's superior frost hardiness in September. In contrast, both larch types and birch continued substantial growth through mid-July. Notably, birch has demonstrated excellent adaptation to the Icelandic summer and an ability to effectively utilise the entire growing season without compromising frost hardiness. Conversely, less adapted species like larch and spruce exhibited more growth later in the summer indicating late growth cessation, and subsequently impaired autumn frost hardiness.

This project highlights the crucial link between nursery production processes of forest seedlings and planting success in afforestation and emphasises the significance of incorporating this relationship into forestry planning to optimise outcomes.