How tree morphology reflects ring properties in Norway spruce?
Authors: Pehkonen Mika , Holopainen Markus , Kukko Antero , Hyyppä Juha , Pyörälä Jiri 
Published: October 2022

We studied the relationships of wood anatomy (ring size and composition) with select morphological features (stem geometry and branching) in Norway spruce (Picea Abies H. Karst). We acquired the morphological modelling data by tracking the 1179 sawlogs of 479 trees from 14 even-aged, spruce-dominated stands in South-Eastern Finland to a sawmill, where X-ray log tomographic measurements were taken. We smoothed three measures of knottiness with continuous functions of log height and diameter, namely knot index (ratio of knot volume to log volume), knot size index (ratio of knot volume to number of whorls) and shoot lengths (whorl-to-whorl distances). The ring properties data came from a subset of 172 ring sample discs from variable heights in 51 of these trees. X-ray densitometric measurements were conducted to define the cambial age (CA), ring area (RA), latewood percentage (LWP), and ring density (RD) of each ring. We established reference mixed models for the ring properties like those frequently used in the literature. The reference models explained up to 80% of CA and RA, and around 44% of LWP and RD, with additional 6–20% explained by stand-level random variance. Next, we tested the relationships of the ring properties with the stem and knottiness features at individual ring-level. The tree morphology showed clear connection with the CA and RA: stem size, relative height, knottiness, within-crown positions, and apical shoot lengths explained 69% of CA, and 53% of RA, in addition to 12–16% stand-level variance. However, the tree morphology accounted for only 7% and 4% of the variation in LWP and RD, respectively, while the stand-level random variance accounted for another 22%–23% in these models. Based on previous literature, LWP and RD are largely affected by site fertility and annual weather conditions that control especially the timing and rate of early- and latewood production, and cell-wall thickness in latewood. Therefore, the statistical predictions of such subtle, intra-annual changes with the coarser morphological developments of crown and stem remain limited in accuracy. The analysis of our results revealed that increased crown vigour and longevity was reflected in increased earlywood production in juvenile wood. Therefore, the higher the growth rates in young trees, the higher the percentage of low-density juvenile wood in mature stems, especially on fertile and moist sites where trees favoured water uptake in the expense of structural support. We found limited possibilities to increase RD on the more fertile sites by increasing stocking density. The stem and knottiness features used here resemble those re-constructible from laser-scanners and photogrammetric sensors increasingly often used in both forested and industrial environments. The results here highlighted some of the possibilities and remaining knowledge gaps regarding the relationships of observable tree morphologies and the wood anatomy that require solving before feasible methods to, e.g., remote-sensing-aided wood property surveys are possible.