Efficacy of Scots pine forest stand renewal using mechanised direct seeding: Genetic and growth parameter analysis of seedlings

Abstract

Scots pine (Pinus sylvestris L.) is one of the most ecologically and economically important forest tree species in Latvia. In forest restoration, planting has traditionally predominated. However, in recent years, mechanised direct seeding has attracted increasing interest as a closer-to-nature, cost- and labour-efficient alternative. This interest is further enhanced by the improved availability of reproductive material from seed orchards. At the same time, an important practical problem is uncertainty about the origin of seedlings after direct sowing because Scots pine seeds are wind-dispersed and reforestation sites are frequently surrounded by mature pine stands. This aspect is particularly significant when improved seed material is used, because it directly affects the expected genetic gain. This study aimed to evaluate the effectiveness of mechanised direct seeding in the Scots pine stand regeneration using improved reproductive material from the Salaca pine seed orchard. Four forest stands in eastern Latvia, regenerated by mechanised seeding in spring 2014, were analysed. Seeding was carried out at an approximate sowing rate of 300 g ha^–1 after soil preparation with a disc trencher equipped with a seeder. Stand development was assessed in the seventh growing season after sowing by measuring seedling height and root collar diameter, and by conducting genetic analyses to determine seedling origin. In total, 367 seedlings were genotyped using 16 microsatellite (SSR) markers. Maternal assignment was performed to determine the proportion of seedlings originating from the Salaca seed orchard. On average, 87% of the analysed seedlings were assigned to maternal genotypes from the Salaca seed orchard; this proportion was similar across all four forest stands. The assignment results were further supported by higher relatedness and lower genetic differentiation (Fst) between the assigned seedlings and the seed orchard clones compared with background (unassigned) seedlings. Allelic richness in the assigned seedlings was slightly lower than in the background group; however, no statistically significant differences were detected. Seedlings originating from improved reproductive material tended to show greater height growth, particularly with increasing age, and a higher height-to-diameter ratio, but these differences were not statistically significant. The results demonstrate that mechanised direct seeding using improved Scots pine seed material is an effective forest regeneration method that ensures a high contribution of sown seeds to stand formation, enabling the transfer of genetic gains achieved in breeding programmes and reducing regeneration costs and labour requirements.

Keywords: Pinus sylvestris; forest management; genetic diversity; seed orchards; bio-based solutions; close-to-nature forestry