Research on the mechanical and physical properties of wood is commonly carried out on either small clear specimens or structural-sized boards. The first approach was more frequently utilized in the past, while the latter is more commonly used nowadays. However, there is very little information on how the two approaches relate with one another. This study aimed to quantify the relationships between the mechanical [modulus of elasticity (MOE) and bending strength] and physical properties (density) of both specimen sizes. A total of 1376 structural-sized boards from three different species (Douglas-fir, Norway spruce and Sitka spruce) were tested in bending, after which a small clear specimen was extracted from the undamaged portion of each board and re-tested in bending. Prior to destructive testing, all boards and clear specimens were evaluated using non-destructive technology. Poor-to-moderate relationships were found between all measured mechanical and physical properties of structural-sized timber and small clear specimens. In both specimen sizes, the properties correlated with one another within the same specimen size, as well as across the two sizes. The strength of correlations appears to be somewhat species dependent. Relatively good relationships were identified when comparing the mean tree values of the properties examined, suggesting either method can be used for a tree-level comparison. The non-destructive evaluation of specimens was shown to reflect the measured properties moderately well, with the relationships changing significantly depending on which measured property was being predicted.

Field measurements were carried out to assess the feasibility of ultrasound velocity and damping for the non-invasive testing of standing trees. A total of 87 trees of European beech ( Fagus sylvatica L.) and 68 trees of Norway spruce ( Picea abies Karst.) were measured in the field, felled and assessed individually for the presence of red heartwood or butt rot. The field assessment of these internal structural defects at the stump level (SL) of the trees was compared with the ultrasound measurements recorded at two tree heights [(i.e. at the SL and 0.5 m above the stump level (ASL)] and in several directions, all perpendicular (PP) to the tree stem. Lower ultrasound velocity and higher damping were found in both species with the presence of internal defects in both the radial and tangential directions of the tree stem. The diameter at breast height (DBH) had a varying effect on both ultrasound velocity and damping. A binary logistic regression was used to test the potential of ultrasound velocity and damping to predict the presence of internal defects. Both the approaches offer similar levels of prediction accuracy (0.72 and 0.76 in beech, and 0.83 and 0.82 in spruce). Due to the significant reduction in measuring time when using ultrasound damping only, this principle is recommended for the detection of red heartwood in beech trees and butt rot in spruce trees.

In Slovenia, the National Forest Inventory (NFI) is used to monitor the state and development of forests at the national level. In this paper, we analysed the diversity of tree and shrub species and assessed the presence of non-native tree species in Slovenian forests. According to the literature, Slovenia hosts 71 autochthonous tree species, of which 60 were found among the trees measured on the 2 x 2 km NFI density plot grid. The highest average number of trees and shrubs, as well as the Shannon-Wiener index of species diversity, were observed in sample plots located in the Pre-Alpine Ecological Region (H’ = 0.85), while the lowest values were found in the Alpine Ecological Region (H’ = 0.58). The total growing stock of trees and shrubs in Slovenia is 330.7 m3/ha ± 2.1%, with the growing stock of small trees and shrubs (less than 10 cm DBH for trees and shrubs) estimated at 9.7 m3/ha ± 5.9%. Common beech (Fagus sylvatica) (31.9%) and Norway spruce (Picea abies) (28.2%) dominate the growing stock of tree and shrub species, while common hazel (Corylus avellana) (85.0%) represents the highest proportion of shrub species. Ten different non-native tree species were identified across 138 NFI plots, with the highest distributions in the Sub-Mediterranean and Pre-Pannonian Ecological Regions. Black locust (Robinia pseudoacacia) dominates the growing stock of non-native tree species. Among small trees (those with a DBH of less than 10 cm), common beech represents 32.4%, with Norway spruce and manna ash (Fraxinus ornus) each contributing more than 10.0%. The growing stock of small shrub species (shrubs with a DBH of less than 10 cm) is dominated by common hazel (Corylus avellana) with 78.3%, while more than 2% is also represented by cornel (Cornus mas), old man’s beard (Clematis vitalba) and elderberry (Sambucus nigra). The results show that the combination of a panel system, a large number of sample plots (3027) and an improved coding system enabled the first comprehensive and objective assessment of tree and shrub species diversity, as well as the presence of non-native species in Slovenian forests.

Key message: Studied beech provenances showed different patterns of inter-annual variability in mean vessel area and ring widths, indicating influence of intraspecific variability and diverse environment on hydraulic conductivity and carbon storage potential. International provenance trials of ecologically and economically important tree species are crucial to deciphering the influence of environmental factors and intraspecific variability on tree growth and performance under climate change to guide assisted gene flow and assisted migration of tree provenances and species. In this context, we compared inter-annual trends in tree-ring widths (carbon sequestration potential) and vessel characteristics (conductivity optimisation) of four beech provenances in two international provenance trials, one in Slovenia (Kamenski hrib, a core beech growing site) and one in Hungary (Bucsuta, a marginal beech site) in 2009–2019. We found different patterns of inter-annual variability in mean vessel area and tree-ring widths among provenances and sites, pointing to diverse genetic background and environmental influence on these two wood-anatomical traits. The average values of the vessel area varied less between provenances at Kamenski hrib than at Bucsuta. Weather conditions differently affected tree-ring width and mean vessel area. Furthermore, the length of the period of response of vessel area to the analysed weather conditions differed in summer and winter periods. The differences in the mean vessel area within the tree ring were more pronounced in the weather-wise extreme years, regardless of the provenance. Consistent with previous studies, we confirmed that site conditions affect the climate sensitivity of trees, which is more pronounced at marginal sites or in extreme years. The findings on how different environmental conditions affect the radial growth of young beech trees of different origin are very important for future forest management.