Seedling, planting site and weather factors affecting the success of autumn plantings in Norway spruce and Scots pine seedlings

Due to the lack of labor and the mechanization of planting work, seedlings should be planted during the entire period of unfrozen soil, from early spring to late autumn. In experimental studies, it is difficult to investigate all the risk factors related to different planting windows. In Finland, the winter of 2015-2016 was harsh for seedlings. First temperatures were above zero, then they suddenly dropped to <−30 °C without any protective snow cover. These conditions offered a good opportunity to study the success of autumn plantings and clarify which factors best predict the field performance of autumn planted conifer seedlings. In the survey, 22 regeneration sites planted with Scots pine (Pinus sylvestris L.) and 71 with Norway spruce (Picea abies (L.) Karst.) seedlings were inventoried in the summer of 2016. Scots pine seedlings were more sensitive, and the regeneration result was good only in a few sites. With Norway spruce, the results were better although both good and poor results were found. For Norway spruce, we modeled the probability of severe damage and leader growth damage separately for summer (July and August), September and October plantings. The most important factors affecting the probability of damage was the quality of the planting and the planting spot as well as the soil type. If a planting hole was open or a seedling was planted in a planting spot other than a mineral-covered mound, the risk of damage was high. In fine and course textured soils, as well as medium coarse soils with a high risk of frost heaving, the probability of damage was also high. Low precipitation after planting and during the following spring also increased the probability of leader growth damage. Planting in the summer and early September exposed seedlings to leader growth damage, especially when the seedlings were planted in sites with a high risk of frost damage in autumn and spring. The results indicate that the planting of Scots pine seedlings in the autumn should be avoided. Norway spruce seedlings can be planted even in October when the work quality is good and suitable sites are selected for plantings.