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

Abstract

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. × 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°to 60°N and North American P. tremuloides parent from 45°to 54°N) 3 and 6 years after planting in a progeny trial established in Estonia at 58°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°N) started autumn defoliation significantly earlier than those of southern origin (51°to 57°N). The genotypes of southern origin (55°53′ to 57°31′ N) had a period from bud-burst to defoliation 27 days longer than that of genotypes of northern origin (60°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°to 57°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.

An Analysis of Poplar Growth and Quality Traits to Facilitate Identification of Climate-Adapted Plant Material for Sweden

“…Our study confirmed the positive correlations between autumn phenology and growth reported in other studies [18,29,33,[51][52][53][54][55]. High heritability values for both growth and autumn phenology traits indicate that there is enough variability to adapt poplars to photoperiods at higher latitudes in Sweden.…”

Variation of growth and phenology traits in poplars planted in clonal trials in Northern Europe—implications for breeding

“…Besides a strong influence of genotype on growth in terms of high broad-sense heritability, there was also a generally stable clonal performance over different sites and a positive correlation between the late autumn foliage coloration and growth, suggesting that clones having green canopy late in the autumn generally perform better. The positive relationship between lifetime growth, measured as increase in height or weight, and late cessation of growth in autumn, i.e., the length of the growth period, is welldocumented in common garden studies and clone trials of several Populus species [11][12][13][14]. Savage and Cavender-Bares [15] showed that northern species of Populus and Salix exhibited slower growth rates than southern species only when grown under shorter summer day lengths than expected in their native range.…”

Consistent Poplar Clone Ranking Based on Leaf Phenology and Temperature Along a Latitudinal and Climatic Gradient in Northern Europe

“…We focused on leaf phenology and effective growing season duration and how it can change in the future, because the length of the effective growing season is proportional to annual height growth [5,[36][37][38][39]. Furthermore, there was no clear link between timing of spring phenology and leaf traits in selected clones in one site (Krusenberg); rather, there was a larger variability across individual trees and clones (SI, Section S4).…”

“…And higher light saturated CO 2 assimilation rate and lower specific leaf area were observed in Populus clones adapted to higher latitudes [40,41] and speculated to be an adaptation to shorter growing seasons [42]. Should this be the case, differences in effective growing season length could lead to smaller than expected differences in potential productivity, although the two are correlated [5,[36][37][38].…”

“…The constraints imposed by low temperatures typical of Northern Europe are being eased by climate change faster than elsewhere [27], thus potentially making clones developed for warmer climates more suitable to the new local conditions. Some assisted migration experiments showed that northward transfers of clones have potential benefits for growth [5,28], but it remains unclear whether existing clones are able to thrive in Northern environments, or whether new clones will be needed.…”