Effect of thawing regime on growth and mortality of frozen-stored Norway spruce container seedlings planted in cold and warm soil

Abstract

One-year-old frozen-stored Norway spruce (Picea abies (L.) Karst.) container seedlings were planted in a controlled environment providing an air temperature of ∼22 °C and soil temperature of 9 ± 1 or 18 ± 1 °C. At planting the root plugs were either frozen or had been thawing for 4 days at 9 °C. During a 5-week growing period, in both cold and warm soil the root growth and height growth were less in frozen-planted seedlings than in thawed seedlings. In addition, frozen-planting delayed bud burst and increased mortality. Soil temperature, however, had no effect on bud burst or mortality. Low soil temperature retarded root growth of seedlings thawed before planting but resulted in both retarded root growth and height growth if root plugs were frozen when planted. These results indicate that planting Norway spruce seedlings with frozen root plugs constitutes a considerable risk for successful forest regeneration at soil temperatures normally prevailing in Fennoscandia in spring or early summer especially if the soil is dry at the time of the planting.