The increased demand for wood to replace oil-based products with renewable products has lifted focus to the Baltic Sea region where the environment is favorable for woody biomass growth. The aim of this study was to estimate broad-sense heritabilities and genotype-by-environment (G×E) interactions in growth and phenology traits in six climatically different regions in Sweden and the Baltics. We tested the hypothesis that both bud burst and bud set have a significant effect on the early growth of selected poplar clones in Northern Europe. Provenance hybrids of Populus trichocarpa adapted to the Northern European climate were compared to reference clones with adaptation to the Central European climate. The volume index of stemwood was under low to medium genetic control with heritabilities from 0.22 to 0.75. Heritabilities for phenology traits varied between 0.31 and 0.91. Locally chosen elite clones were identified. G×E interactions were analyzed using pairwise comparisons of the trials. Three different breeding zones for poplars between the latitudes of 55° N and 60° N in the Baltic Sea Region were outlined. The studied provenance hybrids with origin from North America offer a great possibility to broaden the area with commercial poplar plantations in Northern Europe and further improve the collection of commercial clones to match local climates. We conclude that phenology is an important selection criterion after growth.

Poplar plantations harbor large potential as a renewable source of biomass for bioenergy and other industrial applications. The overall aim of this study is to analyze growth, phenology, stem form, and branching characteristics of 32 poplar clones grown in a trial in southern Sweden for their suitability to be grown as industrial feedstock. In a linear mixed model, performed for diameter at breast height and stem volume, the precision was improved by the use of two competition indices. The significance of phenology and quality characteristics for growth performance and ranking of poplar clones was evaluated through genotypic correlations, and multivariate hierarchical cluster analysis used to group the material. All traits showed moderate to high broad sense heritability. In general, higher stem volume was positively correlated with later leaf senescence, and uncorrelated with spring phenology. Selection efficiency for stem diameter and height was greatly improved between age 3 and 6 years allowing a better precision in selecting a subset of clones to be further tested in production plots and pilot plantations. Two commercial Populus maximowiczii Henry × trichocarpa Torr. & Gray cultivars performed best, while some intraspecific hybrids of P. trichocarpa are considered useful to genetically diversify commercial plantations in Southern Sweden (Belgian clones) or establish plantations in north-central parts of Sweden (Swedish clones). The cluster analysis emphasized growth traits and the grouping of the clones corresponded to their origin (or parentage). The results will facilitate decisions on the use of studied material in breeding, further testing and commercial deployment of poplar plantations in Sweden.

In Northern Europe, poplars (Populus) can provide biomass for energy and material use, but most available clones were developed for lower latitudes and are unlikely to be well adapted to higher latitudes, even under warmer climates. We thus need to understand how clones respond to climatic conditions and photoperiod, and how these responses can be predicted. We answer these questions exploiting leaf phenological data ofPopulusclones, grown in six sites across the Baltic region, in Northern Europe, for 2 years with contrasting climatic conditions. Regarding the effects of climatic conditions and photoperiod, within each site, higher temperatures advanced the timing and enhanced the speed of spring and autumn phenology, but reduced the effective growing season length. Across sites, latitude affected the timing of spring and autumn phenology, the speed of spring phenology, and the effective growing season length; clone affected only the timing of phenology. Regarding the predictability of clone response to growing conditions, the growing degree day (GDD) model could not predict spring phenology, because the growing degree day threshold for a specific phenological stage was not only clone-, but also latitude- and year-specific. Yet, this GDD threshold allowed a robust ranking of clones across sites and years, thus providing a tool to determine the relative differences across clones, independently of latitude and temperature. A similar, but not as strong, pattern was observed in the timing of spring and autumn phenological stages. Hence, while prediction of spring phenology remains elusive, the ranking of clones based on observations of their phenology in a single location can provide useful indications on the clones’ relative performance under different latitudes and climates.

Summary: Entering and exiting winter dormancy presents important trade-offs between growth and survival at northern latitudes and many forest trees display local adaptation across latitude. Transfers of a species outside its native range introduce the species to novel combinations of environmental conditions potentially requiring different combinations of alleles to optimize growth. We performed genome wide association analyses and a selection scan in a P. trichocarpa mapping population derived from crossings between clones collected across the native range and introduced into Sweden. GWAS analyses were performed using phenotypic data collected across two field seasons and in a controlled phytotron experiment. We uncovered 629 putative candidate genes associated with spring and autumn phenology traits as well as with growth. Many regions harboring variation significantly associated with the initiation of leaf shed and leaf autumn coloring appeared to have been evolving under positive selection in the native environments of P. trichocarpa . A comparison between the candidate genes identified with results from earlier GWAS analyses performed in the native environment found a smaller overlap for spring phenology traits than for autumn phenology traits, aligning well with earlier observations that spring phenology transitions have a more complex genetic basis that autumn phenology transitions.