Environmental factors affecting formation of lammas shoots in young stands of Norway spruce (Picea abies Karst.) in Latvia

Norway spruce is a relatively fast-growing tree species that is primarily regenerated by planting, which requires significant investments. The formation of lammas shoots (i.e., second flushing in late summer) have been proven to affect the quality and productivity of Norway spruce stands. The aim of our study was to assess the proportion of trees with lammas shoots in young stands of Norway spruce and to investigate the factors affecting their formation. Data have been collected at the end of 2011 in 102 three- to seven-year-old Norway spruce plantations randomly selected across different forest types in the central part of Latvia, and a subset of 21 stands were inventoried again at the end of 2012. In each stand, 20 sample plots were systematically established where trees with and without lammas shoots were counted and micro-environmental factors (moisture, competition, and browsing) were assessed on a three-grade scale. On average, the proportion of trees with lammas shoots was 6.5%. There was no significant effect of the stand age (from 3 to 7 years) on the proportion of trees with lammas shoots nor was there any age-related trend. The effect of forest type on the presence of lammas shoots was not significant. Micro-environmental factors had an important influence on the proportion of trees with lammas shoots. The proportion of lammas shoots in stands with no competition was significantly higher (14.5%) compared to stands with medium and high competition (6.0% and 2.2%, respectively). Similarly, a significantly higher proportion of trees with lammas shoots (11.7%) was observed in sites with normal moisture regime than in sites with slight or notable excess moisture, reaching 4.8% and 1.7%, respectively. Although the influence of browsing damage was not statistically significant, its trend was similar to that observed for competition and moisture regime. Overall, the proportion of trees with lammas shoots was highest in stands showing the most suitable micro-environmental conditions for Norway spruce.