Effect of growth conditions on wood properties of Scots pine (Pinus sylvestris L.)

Under lack or insufficient availability of soil nutrients the annual tree ring widths in heartwood were relatively small in heath and peatland sites compared to the annual rings of the same age of the trees growing in Myrtillus site type (Table 2). The mean annual ring widths in heath pine forest heartwood were close to or less than in trees growing in raised bog and drained raised bog pine stands; however, the proportion of latewood in annual rings was larger in heath pine forests. The proportions of latewood in heartwood and sapwood in heath stands even exceeded the corresponding values for Myrtillus site type stands at all the heights under study. In peatland stands, the proportions of late-wood were considerably smaller.

Unlike heartwood, the mean annual ring widths in sapwood in the three cross-sections of the tree stem were broadly similar in heath and Myrtillus site type stands. In heath pine stands in sapwood, however, the number of annual rings was greater than in the other site types. In tree stem cross-sections the number of annual rings in sapwood is normally not identical in different directions. With height, stem heartwood boundary increasingly shifts towards the latest annual rings.

At h_1/2, heartwood percentages had decreased by 5% in the heath pine stands, 8% in the raised bog pine stand, 4% in the drained raised bog pine stands and 4% in the pine stands on Myrtillus site type.

At h_3/4 heartwood contained relatively more wood weaker in strength properties around the pith. There were many trees where the heartwood had not yet formed at that height, which accounts for the high variability of heartwood proportions at that height. In general, the variability in the heartwood percentage was higher between trees in individual stands than between stands.

It is common knowledge that there is a strong correlation between latewood percentage and oven-dry density. In our results, the determination coefficient in a stand (R²) as an indicator of correlation ranged from 0.70 to 0.90 both for sapwood and heartwood. There was a moderate linear correlation between annual ring width and density (R² = 0.40). The distribution of mean oven-dry density values on the tested samples in the study is presented in Table 2 and it shows similar results with latewood percentages.

Because of higher latewood levels in heath pine forests the heartwood and sapwood density there exceeds wood density in the corresponding parts of the stem in other stands. Although it is clear that there is normally less latewood in heartwood than in sapwood, heartwood density was higher than sapwood density at equal latewood percentages (Figure 1). The results show that the difference is variable in different site types. It was smallest in Myrtillus site type pine stands (1%), followed by the raised bog pine stand (5.7%), heath pine stands (8.2%), and drained raised bog pine stands (11.2%). The difference in drained raised bog pine stands is greater at smaller densities, which is contrary to Myrtillus site type pine stands (Figure 2), where the difference is greater at higher wood densities. In heath pine stands and the natural raised bog pine stand the difference in density between heartwood and sapwood was constant, which did not change with variations in latewood proportion.

Figure 1

Figure 2

Significant differences were observed between the stands under study in wood bending strength, compression strength and hardness. If we take the heartwood bending strength of the heath site type pine stands as 100%, then the bending strength of samples taken from the Myrtillus site type pine stands is 85.4%, from the raised bog pine stand 69.9%, and from the drained raised bog pine stands 70.8%. The distribution of sapwood bending strengths by site types was roughly analogous: 100%, 90.2%, 66.7% and 74.8%, respectively. Smaller differences were observed in compression strength, and even smaller ones in wood along-the-grain hardness. All the strength properties studied in heath pines at breast height and stem relative heights (h_1/2, h_3/4) presented in Table 3 exceed the corresponding wood properties of trees from the other site types. However, the sample tree breast height diameters in heath pines and drained raised bog pines were similar (16.4 and 15.7 cm, respectively). The mean breast height diameter of pines from the raised bog stand was smaller than these (12.4 cm) while in the Myrtillus site type pines it was larger (22.6 cm).

Increased wood density does not have similar effects on mechanical properties. The effect of density was the strongest on bending strength and compression strength (Figures 3–4) and relatively weak on along-the-grain hardness (Figure 5).

Figure 3

Figure 4

Figure 5

Comparison of the bending strength in heartwood and sapwood at the same density showed that it was greater in sapwood in all the site types. At the same oven-dry density the bending strength was almost identical in heath and Myrtillus site type pines. The bending strength of wood originating from drained raised bog pine stands proved to be significantly smaller (Figure 3).

Study of along-the-grain compression strength at the same density yielded an identical result in three site types for both sapwood and heartwood (Figure 4). In contrast, along-the-grain hardness of heartwood and sapwood at the same wood density proved to be the greatest in wood from the drained raised bog pine stands (Figure 5).

Growth conditions in different stands can be best characterised by annual height growth or ring width (Mäkinen, 1998; Metslaid et al., 2011) and wood properties often, but not always, by latewood percentage and density. Thus, in younger stands annual ring width has a significant effect on wood density, modulus of rupture and modulus of elasticity (Mattsson, 2002). In older pines, annual ring width only has a limited value for determining wood properties; there, wood properties are characterised to a significant degree by latewood percentage (Seco & Barra, 1996; Wilhelmsson et al., 2002; Wimmer, 1991). With age, latewood percentage increases in pine. A number of authors have observed a very weak correlation between latewood content and growth site as well as between stand and single tree variables (Björklund, 1999; Björklund & Walfridsson, 1993; Metslaid et al., 2018; Uusvaara, 1974). Based on our results, it may be maintained that in trees of the same age latewood percentage in both heartwood and sapwood may depend on the growth site.

The transition from sapwood to heartwood is a steady occurrence, and heartwood boundary does not necessarily go past an annual ring (Yang & Hazenberg, 1991). The difference between heartwood and sapwood is primarily chemical and on the border of heartwood replacing of some elements occurs, which may influence bonds and the growth stress division between heartwood cells and thus affect wood properties (Bowyer et al., 2003; Werberg, 1930). In Scots pine trees growing in poorer conditions the share of heartwood was highly variable. Trees growing on more fertile sites, however, contained higher proportions of heartwood. In same-age stands of higher heartwood proportions on more fertile soils coincide with previous results from a number of authors (Bektas et al., 2003; Havimo et al., 2009). Based on our findings, differences at breast height may be as great as threefold. Compared to breast height values, heartwood proportions decreased towards the top.

Climate is a significant factor in heartwood development. Wider heartwood occurs in drier climate types (Climent et al., 2002). The amount of heartwood also depends on the geographical location, age and growth rate (Savva et al., 2008). Therefore, the differences in heartwood proportions between our dry and moist growth sites were something to be expected. Other tree species, too, have fairly significant differences in sapwood to heartwood ratios between growth sites (Bektas et al., 2003). Our results and the references given herein do not confirm the view of Kärenlampi & Riekkinen (2002) that heartwood content is independent of tree growth rate and tree size. Rather, one may agree that site conditions have little effect on heartwood formation on more fertile sites (Björklund, 1999; Mörling & Valinger, 1999). According to some studies, heartwood formation in Scots pine begins from 15 years of cambial age on (Björklund, 1999; Fries & Ericsson, 1998; Mörling & Valinger, 1999). This is a considerably earlier age than those found by other authors (Lappi-Seppälä, 1952; Werberg, 1930). The diversity in the results may be due to site moisture and site fertility if we compare trees of the same age in our research.

In addition to latewood percentage, density is another key characteristic of tree properties (Wilhelmsson et al., 2002). In pine, basic density decreases from stub to top (Lagana et al., 2008) and from pith to bark (Havimo et al., 2009). Significant is the correlation between basic density and site quality or cambial age (Oliva et al., 2006), as well as the dependence on the geographical location, age and growth rate (Konofalska et al., 2021; Mattsson, 2002). Results have been obtained to the effect that basic density is independent of growth rate or even negatively correlated with annual ring width, and is only dependent on cambial age (Kärenlampi & Riekkinen, 2004). This is at variance with the increase in wood density in Finland from north to south (Hakkila, 1979; Kärkkäinen, 1985), which indirectly points to the impact of growth conditions, which are poorer for pine towards the north. However, wood of equal density values contains very different proportions of latewood in different geographical regions (Zvirbul et al., 1976).

It is well known that silvicultural methods influence wood formation. Drainage of peat bogs leads to a number of changes in wood properties. Wood formed before drainage has greater density than wood formed after drainage (Varhimo et al., 2003). After forest drainage, wood basic density and annual ring latewood content decrease and the variability in all wood properties is higher than on mineral soils (Rikala, 2003). Raised bog pine forests drainage results in an increment that is relatively small under Estonian circumstances. Accordingly, the effect on wood density is small, and the present study did not ascertain any decrease in it.

One of the factors influencing density is cell wall thickness. Cell wall thickness is directly correlated with the length of the growth period (Antonova & Stasova, 1993) and with factors of influence during the time of wood cell and tissue formation (Wodzicki, 2001).

The difference in density during the period of heartwood formation cannot be caused by wood macroscopic structure, since cell wall thickness remains the same in the process of heartwood formation (Crivellaro & Ruffinatto, 2021). However, the process sees the accumulation in heartwood of various substances, the quantities of which are influenced by growth conditions. Among these, consideration is given primarily to resins, dyes, tannins and other substances (Bowyer et al., 2003). The process of heartwood formation is accompanied by the accumulation of Ca into heartwood. As there is relatively little Ca in raised bog soil, it is certain to affect the accumulation of Ca in heartwood.

Our results harmonise with those of Aleinikovas and Grigaliūnas (2006), according to which wood bending and compression strengths were the highest on poor sites with normal moisture, and significantly different from corresponding values for fertile sites with normal moisture.

Improvement in growth conditions after drainage may reduce basic density and latewood percentage. However, as the period of intensive growth in a raised bog is relatively short compared to tree age, then, according to our previous studies, reduced density ultimately has relatively little effect on wood mechanical properties (Pikk et al., 2004). Changes in wood properties after forest drainage on mineral soils and swamps are similar in Finland; however, the variability of the results from peatlands is higher and the wood from there is clearly poorer in quality (Rikala, 2003). The same conclusion can be drawn from data collected in the present study.

The findings obtained cast doubt on the hypothesis that the wood of stunted swamp pines has the greatest strength. Do the findings coincide with those showing that wood obtained from fresh forests and from fresh mixed forests containing deciduous trees is the best (Spława-Neyman, 1994)?

Our test results had large variation and showed statistically weaker relations of density with wood hardness than bending strength and compression. We found that although the most important factor affecting mechanical properties was wood density, there were other important factors (annual ring width, proportion of heartwood, proportion of latewood) influencing strength properties.

From the perspective of practical wood utilisation, a reduction in along-the-grain mechanical properties is of importance. Percentage wise, the reduction in strength properties from breast height (h_1.3) up to ¾ of tree height (h_¾) per one metre was the highest for sapwood bending strength in natural raised bog pines (5.0%), followed by drained raised bog pines (4.0%), heath pines (3.2%) and Myrtillus site type pines (1.9%). The reduction in sapwood compression strength ranged from 3.2% to 1.5% and in hardness from 0.8% to 2.0%. At this point, it should be noted that the reduction in strength properties from breast height to half-tree height is smaller than toward the top from there on, to ¾ of tree height. The poorer the tree nutrition conditions, the greater the variability in mechanical properties in the same tree stem.