Factors affecting winter damage and recovery of newly planted Norway spruce seedlings in boreal forests

Abstract

In boreal and temperate forest zones, snowless winters and springs with varying temperature conditions are becoming more common with climate change. In the spring of 2020, extensive winter damage in Norway spruce (Picea abies (L.) Karst.) seedlings, which had been planted the previous year in Central Finland, was observed. In most cases, the probable reason was winter desiccation. This provided a good opportunity to study the regeneration site, seedling, and weather factors that affect winter damage and the recovery of seedlings from damage. In the study, systematic plot sampling was done in 60 selected regeneration sites where damage was known to have occurred. The prediction models for the probabilities of winter damage and the recovery of seedlings were fit to the data. The risk of winter damage was higher in seedlings packed in a closed package than in seedlings stored in open trays. The risk was especially high if seedlings packed in a closed package were stored for more than a week before planting in the middle of June or later. In open trays, the risk of damage was highest in seedlings planted in September, but even then, the risk was lower than in seedlings packed in a closed package. Long storage duration also increased the damage risk in seedlings stored in open trays and planted in September. Other factors that increased damage were coarse soil and the sample plot being on top of a hill. Factors reducing the risk were a fast chain from clear-cutting to planting, planting in good-quality mounds, a sample plot position on the north slope, and the shading of the forest edge on the southern side of a plot. Recovery of seedlings was weaker when seedlings were stored in a closed package and planted in the fall, in too shallow planting depth, or in humus-covered mounds. Recovery improved when seedlings were planted at a depth of at least 5 cm, or when the coniferous forest edge was on the southern or western side of a plot. Winter damage reduced seedling growth and induced the formation of multiple leaders. In practice, the most important factors to be taken into account were avoiding long storage duration and planting seedlings packed in a closed package after the middle of June. Good-quality site preparation and planting were also important for minimizing the risk of damage and improving recovery.