Sylleptic over Proleptic Type of Free Growth in Young Norway Spruce Plantations: Stem Quality, Tree Height and Phenology Considerations

It is crucial for northerly Norway spruce to understand how seasonal warming and site conditions influence the intensity of free growth and what the effects of free growth on stem quality and adaptedness are. We studied the intensity of sylleptic and proleptic free growth in 660 6-to-9-year-old Norway spruce trees planted on normally irrigated and temporary overmoistured sites of variable fertility. We focused on the ability of individual trees to retain a type of free growth over three seasons and examined the associations between free growth, stem quality, and phenology traits. The results show that 23% to 50% of trees exhibited free growth, depending on the season. Mild and warm conditions in August and September tended to promote free growth. Among trees aged 6 to 9 years, 82% to 84% of those without free growth maintained this status over the following two seasons. While sylleptic growth decreased with age, proleptic growth increased. Over the seasons, individual trees were more consistent in maintaining proleptic growth than sylleptic growth. Trees on moist site types exhibited significantly more free growth than those in normally irrigated sites across all seasons. Trees with both sylleptic and proleptic free growth were significantly taller than those without free growth; however, sylleptic trees showed a markedly lower frequency of stem defects compared with those with proleptic growth. Free growth intensity was weakly associated with spring phenology and appeared to disrupt the well-established associations between phenology traits within the annual cycle. We conclude that selecting trees for overall height, particularly those with sylleptic free growth, may well exploit the benefits of free growth without significantly increasing the risk of autumn or winter frost damage.