In a new study, mortality of Russian larch1 seedlings, due to a damaged root system, occurred not only in the first year but was still ongoing after two growing seasons. These results emphasise the importance of ensuring the quality of forest seedlings before planting.

The research paper was published in the Scandinavian Journal of Forest Research 26 April 2023, titled Root frost tolerance, seasonal variation in root growth, and field performance of one-year-old Russian larch seedlings with simulated root freezing damages. First author is Rakel J. Jonsdottir, PhD student at the Icelandic Agricultural University and scientist at the Icelandic Forest Service – Skógræktin, co-author Inger Sundheim Fløistad, Senior Research Scientist at NIBIO, Norway.

Planting healthy seedlings with optimal growth potential is essential for proper growth and survival in forest regeneration. Assessing the seedling quality prior to planting is therefore important. In this Icelandic study, effects of root damage induced with artificial freezing in young Russian larch seedlings were examined using the root growth capacity method (RGC).

Frost tolerance of roots varied during the winter, and root growth in undamaged seedlings fluctuated, indicating seasonal variations in growth rhythm. The LT50 value for root frost tolerance was −13.9°C in late January, but already at −10.6°C (LT10) root damages were severe. After one growing season, shoot elongation was significantly lower in seedlings frozen to −9°C, −13.5°C, and −15.5°C by 23%, 54%, and 72%, respectively, compared with undamaged seedlings. Control seedlings and seedlings frozen to −9°C achieved 100% survival after the first growing season. Survival in seedlings frozen to −13.5°C and −15.5°C was 85% and 27%, respectively. After the second growing season, survival decreased in all frost-damaged seedlings.

The ongoing mortality of Russian larch seedlings in the study demonstrates the long-lasting effects of planting seedlings with damaged root systems. The fluctuation in root frost tolerance of young Russian larch seedlings during winter emphasises the need for care when seedlings are moved to outdoor storage.

1Russian larch = Larix Sibirica var. sukaczewii