Late termination of freezer storage increases the risk of autumn frost damage to Norway spruce seedlings

Abstract

Over the last few years it has become increasingly common in artificial forest regeneration to extend the planting period by using freezer-stored seedlings for early summer plantings. Developmentally, however, planted freezer-stored seedlings lag behind seedlings planted earlier in the spring. As freezer-stored seedlings also start hardening later, they are more susceptible to early autumn frosts, especially in years when the thermal growing season ends and the first autumn frosts come earlier than usual. By means of computer simulations with a simple temperature sum model and long-term air-temperature data from three locations in Finland, we examined the effect of the freezer-storage termination date on the risk of autumn frost damage to the seedlings. The long-term simulations revealed a drastic effect of year-to-year variation in the thermal conditions during the growing season on the occurrence of autumn frost damage. Such results provide crucial information complementary to those obtained in field experiments, which are always restricted to a relatively short time period. Together with earlier field data, the present results suggest that at an average regeneration site in central Finland, the planting of seedlings whose storage has terminated on 15 June and 22 June involve autumn frost damage every tenth and every fifth year, respectively. The sensitivity analysis revealed that the temperature sum requirement of maturation has a great effect on the risk of autumn frost damage, thus pinpointing the need for experimental studies addressing this ecophysiological trait of the seedlings.

Damage caused by an exceptionally warm and dry early summer on newly planted Norway spruce container seedlings in Nordic boreal forests

“…In these cases, the planting of seedlings was delayed, and storage durations were lengthened by at least three weeks, due to the wait for site preparation. Hänninen et al (2009) and Luoranen et al (2022a) have clearly shown that the risk of damage other than drought increases if freezer-stored seedlings are planted in July. For these freezer-stored seedlings, the growing season is too short for growth and hardening processes to be completed before the first fall and winter frosts (Hänninen et al 2009).…”

“…Hänninen et al (2009) and Luoranen et al (2022a) have clearly shown that the risk of damage other than drought increases if freezer-stored seedlings are planted in July. For these freezer-stored seedlings, the growing season is too short for growth and hardening processes to be completed before the first fall and winter frosts (Hänninen et al 2009). In addition, during the summer months, the storage duration of seedlings in closed packaging cannot exceed 1-2 days (Luoranen et al 2019).…”

“…This may explain why the freezer-stored seedlings in our experiment had less drought damage than other types of seedlings, and why fresh seedlings packed in boxes had more drought damage in early June than other seedlings. In a warming climate with an increased probability of early summer droughts, the planting of freezer-stored seedlings might be the safest option if the freezer storage ends in the middle of June (Hänninen et al 2009), and the seedlings are planted at the end of June.…”

The effects of planting practices and growing environment on the early performance of boreal tree seedlings

“…The planting period of freezerstored Norway spruce seedlings should end no later than mid-June (Luoranen et al 2005). Planting freezer-stored seedlings after that can increase mortality and autumn frost damage, and reduce growth (Luoranen et al 2005;Hänninen et al 2009;Luoranen et al 2021). Overall, Norway spruce summer plantings can be successful if the recommendations regarding seedling types, storage methods, and durations are carefully followed.…”