MODELS OF AGE AND WEATHER EFFECTS ON NUMBERS, WIDTHS AND COARSENESS AND GROWTH OF YOUNG NORWAY SPRUCE

Annual growth, fibre and wood properties are under strong influences from genetics, age and weather. They change dynamically, particularly at young ages. Most genetic research and tree improvement programs are based on data from this dynamic phase of the life of trees, affected by differences in weather among sites and years. In the presented work, such influences were studied for young Norway spruce trees, and modelled at the detail of annual rings and their compartments earlywood (EW), transitionwood (TW) and latewood (LW), focusing on tracheid numbers, widths and coarseness (biomass/unit length, expressing biomass allocation at cell level), and radial growth. Increment cores were sampled at age 21 years from almost 6000 Norway spruce trees of known genetic origin, grown on two sites in southern Sweden (Chen et al., 2014), and analysed with SilviScan. Trees of different longitudinal growth reach breast height at different ages, meaning that rings with same cambial ages are not formed same years, but influenced by different weather conditions. Therefore, the trees were divided into classes on at what age they reached breast height (Lundqvist et al. 2018). The mean developments for the traits were calculated for all class and studied versus both cambial age (CA) and total tree age (TA) for comparison. General additive mixed models (GAMMs) with an autoregressive element and a hierarchical structure in the random model part were fitted to model the influences of age and local weather.  CA and TA were compared as independent variables to model age related influences. After comparison of different model structures and sets of independent variables, six input variables were used to estimate the trait variations in relation to their averages: Age (TA or CA), temperature sum across the vegetation period (GDD) and precipitation sums for four equal length parts of the period (Psum1-4). Site and family influences are being analysed separately in genetic studies. The highest R 2 values were obtained for number of tracheids formed radially per year (0.53 using TA), radial tracheid width in earlywood (0.44 with CA), ring width (0.43 with TA) and tracheid coarseness (0.42 with CA). The models are illustrated in Fig. 2 by influences on numbers of tracheids radially in rings, EW, TW and LW, according to the models. On average 120 tracheids were formed radially per year; At tree age 5 years on average 120 more than that average, in total 245, at age 15 years 40 less tracheids, in total 80, a very large relative variation. Superimposed on this was a positive GDD effect from -15 to +15 tracheids across the span of GDDs, 10% of the extreme events at each end excluded, similar effects of precipitation from mid-June to late July (Psum2), each effect corresponding to ±0.3 to ±0.4 mm in ring width, but smaller precipitation effects during the rest of the year. The adverse was observed for radial tracheid width: Limited relative variation and dynamics closer related to CA were observed, meaning that ring width was largely governed by the number of tracheids formed. The decreasing intensity of tracheid formation was associated with increasing width and coarseness. The study of young Norway spruce trees showed that the number of cross-sectionally formed tracheids at breast height was mostly related to the total age of the tree. Trees that reached breast height late had also the competitive disadvantage of forming thinner rings from the start. The cross-sectional dimensions and biomass allocated were in turn stronger related to cambial age. Weather related influences of temperature and precipitation had a lesser but significant contribution. The results indicate that this type of models can be used to harmonise data from experimental sites with systematic differences in weather and between years, to refine data prior to evaluations to follow, maybe also for estimation of effects on different genotypes from climate change scenarios. Acknowledgement The work was performed within the Swedish Strategic Research Program Bio4Energy. Thomas and Stefan Seifert additionally want to express their gratitude to the EU funded RISE project “Care4C”, which provided an excellent platform for scientific discussion.