Early growth and phenological development of Finnish-origin curly birch clones under Estonian hemiboreal conditions

Curly birch (Betula pendula var. carelica) is a valuable tree form cultivated for its highly decorative figured wood. The successful establishment of curly birch plantations depends largely on the adaptation of planting material to local climatic conditions. This study evaluated the early growth performance and phenological characteristics of eight Finnish-origin curly birch clones under Estonian hemiboreal conditions. The study was carried out in a comparative clone trial established in southern Estonia in autumn 2012. A total of 360 cloned trees representing eight curly birch clones were monitored during the first two growing seasons after planting (2013–2014). Initial seedling height, annual height increment, root collar diameter and phenological characteristics were recorded. Differences among clones were analysed using linear models, Tukey's honestly significant difference (HSD) test and Pearson correlation analysis. Significant differences among clones were found in initial seedling height, annual height increment, root collar diameter and vegetation period length (p < 0.05). During the second growing season, clones Pertunmaa E-10524 (KL4) and Kerimäki E-8306 (KL2) produced the greatest height increment. Vegetation period length was positively associated with annual height increment, although the relationship was not statistically significant (r = 0.64, p = 0.084). The results demonstrate that Finnish-origin curly birch clones differ in both early growth performance and phenological development under Estonian hemiboreal conditions. These differences reflect variation in climatic adaptation among clones. Whether the observed differences persist during later stages of stand development and are reflected in stem quality and figured wood formation can only be determined through long-term monitoring.