Spring and autumn phenology of hybrid aspen (Populus tremula L. x P. tremuloides Michx.) genotypes of different geographic origin in hemiboreal Estonia

Background: Increasing demand for renewable energy resources and the need to mitigate climate change have raised interest in short-rotation forestry with fast-growing deciduous trees like hybrid aspen (Populus tremula L. x P. tremuloides Michx.) in northern Europe. Given that climate warming has already considerably extended the growing season in this region, northward transfer of genotypes could improve forest plantation productivity and enable more efficient mitigation of climate change. We studied the spring and autumn phenology of hybrid aspen genotypes of different geographic origin (European P. tremula parent from 51 degrees to 60 degrees N and North American P. tremuloides parent from 45 degrees to 54 degrees N) 3 and 6 years after planting in a progeny trial established in Estonia at 58 degrees N. Findings: The effect of geographic origin on spring and autumn phenology of hybrid aspen was evident at the age of 3 and 6 years. Geographic origin did not affect spring phenology. However, hybrids with P. tremula parents of northern origin, with bud-burst occurring some days later, were able to unfold and develop full-sized leaves faster than genotypes with early bud-burst. The main differences between different geographic origins appeared in the autumn of year 6, when genotypes of northern origin (60 degrees N) started autumn defoliation significantly earlier than those of southern origin (51 degrees to 57 degrees N). The genotypes of southern origin (55 degrees 53' to 57 degrees 31' N) had a period from bud-burst to defoliation 27 days longer than that of genotypes of northern origin (60 degrees 22' N). The interval between spring and autumn phenological processes showed significant positive correlation with current annual height growth for both study years. Conclusions: Hybrid aspen genotypes from 55 degrees to 57 degrees N responded well to northward transfer, having a longer leafy period and greater height increment than southward transferred genotypes. Northward-transferred genotypes were apparently better adapted to climate-change-induced extension of the growing season at higher latitudes.