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

Abstract

With climate change, drought and warm growth conditions in the spring and early summer are predicted to become more common in Nordic boreal zones in the future. Such conditions occurred in Finland in June and July 2021, offering a good opportunity to study the field performance of seedlings and which factors, including the weather, regeneration site, and operational or seedling level factors, affected the field performance of the seedlings. In the survey, 65 regeneration sites planted with Norway spruce (Picea abies (L.) Karst.) container seedlings from May to July 2021 were randomly selected from different parts of South and Central Finland (60–64°N), and several variables were assessed in sample plots from these sites in September–October 2021. On average, 6 % of the seedlings died, and 26 % were damaged during the first growing season. In most cases, the damage was caused by drought. The most important factors affecting drought risk were the planting period and packaging and storage methods of the seedlings: in freezer-stored seedlings, the probability of drought damage was <0.20 for all planting periods, except for July; when the seedlings overwintered outdoors and were delivered to the forest in open trays, the risk of damage was <0.25 when they had been planted in May and early June, but in late June, it was > 0.60. When the seedlings had overwintered outdoors and were then packed in closed packaging, the risk was >0.20, regardless of the planting period. A low previous-year height of the seedlings (especially in the outdoor stored seedlings), precipitation a week before planting, and a low average temperature two weeks after planting reduced the risk of drought damage. When the average air temperature was below 15 °C (i.e., during the May plantings), the shade of the nearby stand slightly reduced the drought risk, but in higher temperatures, shade did not affect it. The effect of the pre-planting precipitation was stronger in coarse-textured and peat soils than in medium-coarse soils. The risk of drought damage was lowest when the freezer-stored seedlings were planted in good quality mineral soil mounds, deep enough in medium-rich (MT type) and medium-coarse soils. It is impossible to completely prevent drought damage in newly planted Norway spruce seedlings, but by optimizing operational choices and measures, the development of full density fast-growing young Norway spruce forests can also be ensured in a warming climate.