Today, as always, the issue of the integrated use of wood resources, in particular, logging residues (Hakilla 1976; Ratajczak 2013), post-production waste (Gayda and Maksymiv 2007; Ratajczak et al. 2018) and post-consumer wood (Gayda 2016; Gayda and Kiyko 2020a, 2020b, 2021), remains relevant. Solving the problem of processing these additional resources will allow, to some extent, to save primary raw materials and reduce the load on the environment (Gayda 2013). A promising direction that will partially reduce the shortage of wood in Ukraine is the utilisation of stump-root wood (SRW) for the production of composite materials, including particleboard (chipboard), with the annual reserves of SRW averaging 2.2 million m3 (Nikishov 1985). The use of SRW is possible for no more than 40–50% the allotted area. The use of significant amounts of SRW on the site can cause irreparable harm to the environment: mixing of soil horizons, extracting a significant amount of organic matter from the soil, disturbing the microrelief of the site, destroying natural renewal and so on. The use of this raw material will make it possible to obtain additionally almost 500,000 m3 of boards, but today it is practically not used for the specified purposes. One of the reasons hindering the use of this raw material as the main structural component for particleboard is the lack of knowledge of its internal structure and properties. In order to apply SRW in woodworking and fill the scientific base with indicators of macro- and microstructure of stump-root systems of individual tree species, it is necessary to study the main characteristics that are crucial for the material use in the production of composite materials. Thus, the study of these issues and, on their basis, obtaining sound reference data on the internal structure of stump-root systems will solve the pressing problem of replenishing the raw material base of woodworking enterprises by using this additional wood resource for manufacturing particleboards (Nikityuk et al. 1993a, 1993b).
Scientists have been studying the structure and properties of wood of individual tree species, in particular, their roots, for over a hundred years. The work of Chernyaev (1864) is well-known. It is also necessary to note the work of Pislar (1936), who studied the anatomical structure of the roots of fruit trees. His studies were of a comparative anatomical nature, which allowed him to discover a number of anatomical features that can be used to distinguish not only genera, but even species of fruit trees. He was not engaged in the study of industrial species of trees. In coniferous species, the bulk of the root wood consists of early and late tracheids (Chavchavadze 1979). In addition, wood rays, wood parenchyma and resin ducts are found here. But in the structure of the roots, characteristic peculiarities are also observed. The roots do not have a pith; in the centre, there is a primary wood with one or more resin ducts. Heartwood is almost never formed in the roots.
In the studies of Perelygin (1954), it was noted that the wood rays in the root wood are wider and are located more densely than in the stemwood (SW). In the roots of spruce (
Some comparative studies of different parts of birch (
In his work, Howard (1974) noted the differences between the anatomical internal structure of root wood and SW, which manifested itself when they were used in a crushed form. But the research itself was not carried out by the author. A detailed catalogue for the identification of broadleaved species by microscopic characteristics of SW was presented by Wheeler et al. (1989). They did not conduct comparative studies on the internal structure of root wood and SW.
Thus, the internal structure of root wood has some differences compared to the structure of SW. Structural peculiarities are observed both at the macro- and microscopic levels, that is, in the structure of individual elements, in their arrangement and volume ratio. But, as a brief analysis of literature sources has shown, the structure of SRW, especially of those species that are mainly used for the production of composite materials, has not yet been sufficiently studied.
The aim of the study is to investigate the differences in the macro- and microstructures between SRW and SW.
The object of the study is the internal structure of SRW and SW of the main species.
The subject of the study is macrostructure: annual layers and microstructure: transverse dimensions of early and late tracheids and diameters of vessels in the SRW of coniferous species – pine (
Research objectives: 1) To establish the differences between the annual layers of the SRW and SW by determining the number of annual layers, the average width of the annual layers and the percentage of late wood. 2) To determine the sizes of tracheids in the SRW of coniferous species – pine (
As is known, wood particles are the main component of chipboard and have a significant effect on their properties. Therefore, their internal structure should be considered as an important characteristic of these reinforcing elements. Since the trunk and roots perform partially different functions in a growing tree, they obviously have a different internal structure that needs to be studied. To compare the internal structure of SRW and SW, microscopic studies were carried out. We analysed those elements of wood that can be distinguished under a microscope and which characterise porosity, that is, early and late tracheids in conifers, large and small vessels and libriform fibres in hardwoods. The macrostructure was studied with the help of a magnifying glass, in particular, the structure of annual rings, which are characterised by indicators such as the number of annual rings per 1 cm, the average width of annual rings and the percentage of late wood (GOST 16483.18:1972). The visibility of annual rings and sharpness of the transition from early to late wood within the annual ring were also studied. To compare the macro- and microstructures of the SW and SRW, model trees of spruce (
The results of comparative studies of the macrostructure of SRW and SW are given in Table 1.
Statistics of indicators of the macrostructure of SRW and SW
Wood | Indicators for pine | N, pcs | Maver | Mmin | Mmax | ± | V, % | P, % |
SW | average width of annual rings, mm | 41 | 1.30 | 1.10 | 1.45 | 0.019 | 9.23 | 1.44 |
number of annual rings in 1 cm, pcs | 41 | 7.60 | 6.45 | 8.46 | 0.025 | 2.11 | 0.33 | |
percentage of late wood, % | 23 | 25.00 | 21.23 | 27.84 | 0.211 | 4.04 | 0.84 | |
SRW | average width of annual rings, mm | 41 | 2.40 | 2.04 | 2.67 | 0.037 | 10.00 | 1.56 |
number of annual rings in 1 cm, pcs | 41 | 4.10 | 3.48 | 4.57 | 0.042 | 6.59 | 1.03 | |
percentage of late wood, % | 23 | 20.10 | 17.06 | 22.38 | 0.156 | 3.73 | 0.78 | |
Wood | Indicators for spruce | N, pcs | Maver | Mmin | Mmax | ± | V, % | P, % |
SW | average width of annual rings, mm | 41 | 1.90 | 1.67 | 2.13 | 0.067 | 22.63 | 3.53 |
number of annual rings in 1 cm, pcs | 41 | 5.20 | 4.57 | 5.82 | 0.153 | 18.85 | 2.94 | |
percentage of late wood, % | 23 | 30.40 | 26.72 | 34.02 | 0.540 | 8.52 | 1.78 | |
SRW | average width of annual rings, mm | 41 | 3.30 | 2.90 | 3.69 | 0.044 | 8.48 | 1.33 |
number of annual rings in 1 cm, pcs | 41 | 3.00 | 2.64 | 3.36 | 0.100 | 21.33 | 3.33 | |
percentage of late wood, % | 23 | 22.10 | 19.43 | 24.73 | 0.976 | 21.18 | 4.42 | |
Wood | Indicators for fir | N, pcs | Maver | Mmin | Mmax | ± | V, % | P, % |
SW | average width of annual rings, mm | 41 | 1.60 | 1.38 | 1.78 | 0.023 | 9.38 | 1.46 |
number of annual rings in 1 cm, pcs | 41 | 6.10 | 5.26 | 6.77 | 0.145 | 15.25 | 2.38 | |
percentage of late wood, % | 23 | 18.90 | 16.31 | 20.97 | 0.882 | 22.38 | 4.67 | |
SRW | average width of annual rings, mm | 41 | 3.10 | 2.68 | 3.44 | 0.033 | 6.77 | 1.06 |
number of annual rings in 1 cm, pcs | 41 | 3.20 | 2.76 | 3.55 | 0.117 | 23.44 | 3.66 | |
percentage of late wood, % | 23 | 14.80 | 12.77 | 16.42 | 0.667 | 21.62 | 4.51 | |
Wood | Indicators for aspen | N, pcs | Maver | Mmin | Mmax | ± | V, % | P, % |
SW | average width of annual rings, mm | 41 | 2.80 | 2.40 | 3.11 | 0.039 | 8.93 | 1.39 |
number of annual rings in 1 cm, pcs | 41 | 3.60 | 3.09 | 4.00 | 0.117 | 20.83 | 3.25 | |
SRW | average width of annual rings, mm | 41 | 3.80 | 3.26 | 4.23 | 0.137 | 23.16 | 3.62 |
number of annual rings in 1 cm, pcs | 41 | 2.60 | 2.23 | 2.89 | 0.061 | 15.00 | 2.34 | |
Wood | Indicators for birch | N, pcs | Maver | Mmin | Mmax | ± | V, % | P, % |
SW | average width of annual rings, mm | 41 | 2.00 | 1.68 | 2.23 | 0.050 | 16.00 | 2.50 |
number of annual rings in 1 cm, pcs | 41 | 5.00 | 4.20 | 5.57 | 0.186 | 23.80 | 3.72 | |
SRW | average width of annual rings, mm | 41 | 4.10 | 3.45 | 4.57 | 0.075 | 11.71 | 1.83 |
number of annual rings in 1 cm, pcs | 41 | 2.40 | 2.02 | 2.67 | 0.089 | 23.75 | 3.71 |
SRW – stump-root wood; SW – stemwood; N – sample volume; Maver – average arithmetic value; Mmin – minimum value; Mmax – maximum value; V – coefficient of variation; P – coefficient of accuracy of the experiment.
It is found that in all species, the width of the annual rings in the SRW is greater than in the SW, in particular, in pine (
It is estimated that in all species, the number of annual rings per 1 cm in SRW is less than in the SW, in particular, in pine (
An analysis of the microscopic sections of coniferous roots indicated some differences in the shape and size of the main constituent elements – tracheids. Statistics and comparison of characteristics of tracheids of coniferous wood are shown in Table 2.
Statistics of the size of the tracheid wood roots and trunk of conifers
Wood | Indicators for pine | Layer | N, pcs | Maver | Mmin | Mmax | ± | V, % | P, % |
Stem | radial direction, μm | early | 20 | 36.00 | 29.80 | 40.23 | 0.682 | 8.47 | 1.89 |
late | 20 | 17.10 | 14.16 | 19.11 | 0.823 | 21.52 | 4.81 | ||
tangential direction, μm | early | 20 | 27.40 | 22.68 | 30.62 | 1.199 | 19.56 | 4.37 | |
late | 20 | 28.10 | 23.26 | 31.40 | 1.319 | 21.00 | 4.69 | ||
wall thickness, μm | early | 20 | 2.10 | 1.74 | 2.35 | 0.054 | 11.43 | 2.56 | |
late | 20 | 4.40 | 3.64 | 4.92 | 0.101 | 10.23 | 2.29 | ||
Root | radial direction, μm | early | 20 | 47.80 | 39.57 | 53.42 | 1.811 | 16.95 | 3.79 |
late | 20 | 19.40 | 16.06 | 21.68 | 0.962 | 22.16 | 4.96 | ||
tangential direction, μm | early | 20 | 29.60 | 24.51 | 33.08 | 0.917 | 13.85 | 3.10 | |
late | 20 | 32.40 | 26.82 | 36.21 | 1.610 | 22.22 | 4.97 | ||
wall thickness, μm | early | 20 | 1.60 | 1.32 | 1.79 | 0.034 | 9.38 | 2.10 | |
late | 20 | 3.80 | 3.15 | 4.25 | 0.087 | 10.26 | 2.29 | ||
Wood | Indicators for spruce | Layer | N, pcs | Maver | Mmin | Mmax | ± | V, % | P, % |
Stem | radial direction, μm | early | 20 | 33.20 | 27.74 | 37.04 | 0.521 | 7.02 | 1.57 |
late | 20 | 20.20 | 16.88 | 22.54 | 0.557 | 12.33 | 2.76 | ||
tangential direction, μm | early | 20 | 24.80 | 20.72 | 27.67 | 0.881 | 15.89 | 3.55 | |
late | 20 | 29.90 | 24.99 | 33.36 | 0.973 | 14.55 | 3.25 | ||
wall thickness, μm | early | 20 | 3.10 | 2.59 | 3.46 | 0.080 | 11.61 | 2.60 | |
late | 20 | 5.00 | 4.18 | 5.58 | 0.206 | 18.40 | 4.11 | ||
Root | radial direction, μm | early | 20 | 39.40 | 32.92 | 43.96 | 1.851 | 21.02 | 4.70 |
late | 20 | 16.60 | 13.87 | 18.52 | 0.796 | 21.45 | 4.80 | ||
tangential direction, μm | early | 20 | 25.10 | 20.97 | 28.01 | 1.219 | 21.71 | 4.86 | |
late | 20 | 28.70 | 23.98 | 32.02 | 1.292 | 20.14 | 4.50 | ||
wall thickness, μm | early | 20 | 2.50 | 2.09 | 2.79 | 0.038 | 6.80 | 1.52 | |
late | 20 | 3.80 | 3.18 | 4.24 | 0.186 | 21.84 | 4.88 | ||
Wood | Indicators for fir | Layer | N, pcs | Maver | Mmin | Mmax | ± | V, % | P, % |
Stem | radial direction, μm | early | 20 | 35.50 | 30.25 | 39.75 | 1.737 | 21.89 | 4.89 |
late | 20 | 16.30 | 13.89 | 18.25 | 0.749 | 20.55 | 4.60 | ||
tangential direction, μm | early | 20 | 31.50 | 26.84 | 35.28 | 1.677 | 23.81 | 5.32 | |
late | 20 | 26.30 | 22.41 | 29.45 | 1.006 | 17.11 | 3.83 | ||
wall thickness, μm | early | 20 | 1.80 | 1.53 | 2.02 | 0.040 | 10.00 | 2.24 | |
late | 20 | 4.10 | 3.49 | 4.59 | 0.116 | 12.68 | 2.84 | ||
Root | radial direction, μm | early | 20 | 43.50 | 37.07 | 48.71 | 2.102 | 21.61 | 4.83 |
late | 20 | 17.60 | 15.00 | 19.71 | 0.868 | 22.05 | 4.93 | ||
tangential direction, μm | early | 20 | 36.20 | 30.85 | 40.54 | 1.793 | 22.15 | 4.95 | |
late | 20 | 24.90 | 21.22 | 27.88 | 1.219 | 21.89 | 4.89 | ||
wall thickness, μm | early | 20 | 1.30 | 1.11 | 1.46 | 0.060 | 20.77 | 4.64 | |
late | 20 | 3.60 | 3.07 | 4.03 | 0.078 | 9.72 | 2.17 |
It was found that in all coniferous species, the transverse dimensions of early tracheids in the SRW were greater than in SW, in particular, in the radial direction in pine (
It should also be noted that the thickness of the walls of early and late tracheids in the SRW of coniferous species was less than that of SW in all species, in particular, in pine (
There were differences in the dimensions of the vessels and fibres of the libriform, the dimensions of which were larger in the root wood. The vessels of this wood had slightly wider cavities. Statistics and comparison of diameters of vessels and fibres of libriform wood of broadleaved species are given in Table 3. It was found that in broadleaved species, the diameters of vessels and libriform fibres were larger in the SRW than in SW, in particular, in aspen (
Statistics of the diameters of vessels and libriform fibres in the stump-root wood and stem wood of deciduous species
Wood | Indicators for aspen | N, pcs | Maver | Mmin | Mmax | ± | V, % | P, % |
Stem | vessels, μm | 20 | 98.00 | 84.48 | 108.83 | 0.461 | 2.10 | 0.47 |
fibre libriform, μm | 20 | 22.40 | 19.31 | 24.88 | 0.241 | 4.82 | 1.08 | |
Root | vessels, μm | 20 | 118.00 | 101.72 | 131.04 | 0.528 | 2.00 | 0.45 |
fibre libriform, μm | 20 | 25.30 | 21.81 | 28.10 | 0.192 | 3.40 | 0.76 | |
Wood | Indicators for birch | N, pcs | Maver | Mmin | Mmax | ± | V, % | P, % |
Stem | vessels, μm | 20 | 87.00 | 72.14 | 96.79 | 0.691 | 3.55 | 0.79 |
fibre libriform, μm | 20 | 20.10 | 16.67 | 22.36 | 0.224 | 4.98 | 1.11 | |
Root | vessels, μm | 20 | 105.00 | 87.07 | 116.81 | 0.532 | 2.27 | 0.51 |
fibre libriform, μm | 20 | 23.80 | 19.73 | 26.48 | 0.125 | 2.35 | 0.53 |
The issues of the structure and properties of wood of some tree species and their roots were first dealt with by Chernyaev (1864), who described the structure of the middle part of pine (
The results of our studies on the macroscopic structure of SRW and SW show that the transition from SW to root wood is rather smooth and there is much in common in their structure. In the cross section of perennial roots of pine (
The sharply marked boundary between the trunk and the root is not easy to find, as the tissues of the trunk gradually pass into the tissues of the root. The structure of the wood of large perennial roots has much in common with the structure of the wood of the trunk, as Ugolev (1986) noted in his work.
Since the main function of large roots is to conduct water, their wood is dominated by wide-cavity thin-walled elements, which give them a porous structure, as noted in the works of Guz (1996), Kalinin (1983) and Kalinin et al. (1998). It is also described that the boundary between the annual layers is not very noticeable, and the transition from early to late wood is smoother. This is due to the lack of sharp seasonal fluctuations in temperature and humidity of the environment and moisture content in the soil. But nothing is said in their works about comparison of the internal structure of the SRW and SW. Vikhrov (1953) in his comparative studies noted that the annual layering becomes less pronounced with distance from the root collar. Tracheids in the secondary wood of the root, as well as in the wood of the trunk, are arranged in regular radial rows. They contain bordered pores from one to three rows [in pine (
The alternation of annual layers is also observed in birch (
The study of Tumanyan (1953) showed that in some hardwoods, vessels in the wood of the roots are strongly developed, the annual layers are narrow and poorly visible, and there is almost no difference between early and late wood. In particular, the wood of the roots of hardwooded broadleaved species contains a large amount of woody parenchyma, and it is present in higher amounts there than in the trunk.
The roots of coniferous and broadleaved species under study do not have a permanent type of eccentricity. One and the same root of pine (
The peculiarities of the microstructure lie in the fact that, in terms of the cross-sectional shape, the early tracheids in the SRW of coniferous species can take very different shapes – from square, rectangular to polygonal – while in the SW of fir (
The thickness of the tracheid shell of pine (
The study of the microstructure of hardwoods emphasises that the wood of aspen (
The bulk of the cells of all hardwoods consists of libriform fibres. On the cross section of the root wood, the libriform has the appearance of predominantly rounded cells with narrow cavities and thick walls, and on the microsections of the SW, the libriform fibres are characterised by mostly angular cells with gaps and thick walls.
Thus, the results of macro- and microscopic studies show that there are differences in internal structure between the SRW and the SW. The wood of the roots is dominated by large, wide-cavity, thin-walled elements that give it a porous and brittle structure, which is less pronounced in the wood of the trunk.
The results of comparative studies of SRW and SW allowed us to draw the following conclusions:
It is established that in the structure of wood with perennial roots, mostly the same structural elements are present as in the SW. But, along with this, there are many differences, although it is impossible to indicate a sharp border between them, as their tissues gradually merge into one another. It was found that the main features of the macrostructure of the roots were the uneven development of annual layers and fuzziness of their boundaries, absence of a constant type of eccentricity and a sharp transition from early to late wood. It was found that in all species, the width of the annual rings in the SRW was greater than in the SW, in particular, in pine ( It is estimated that in all species, the number of annual rings per 1 cm in the SRW is less than in the SW, in particular, in pine ( Differences in the microscopic structure between the SRW and SW are revealed, which consist in the difference in sizes of the main constituent elements: tracheids in conifers and vessels and libriform fibres in hardwoods. It was found that in all coniferous species, the transverse dimensions of early tracheids in the SRW were greater than in SW, in particular, in the radial direction in pine ( It is established that, in terms of the cross-sectional shape, the early tracheids in the SRW of coniferous species can take very different shapes – from square, rectangular to polygonal, while in the SW of fir ( It is established that the thickness of the walls of early and late tracheids in the SRW of coniferous species is less than that of SW in all species, in particular, in pine ( It was found that in broadleaved species, the diameters of vessels and libriform fibres in the SRW were larger than in SW, in particular, in aspen (