Comparative studies of the macro- and microstructures of stump-root wood and stemwood

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 (Picea abies Karst.), larch and pine (Pinus sylvestris L.), there occur medullary rays without horizontal tracheids. In fir (Abies alba), the medullary rays have marginal parenchymal cells, strongly elongated along the ray, with similar outer walls. The resin ducts in the wood of the roots are entwined with a large number of cells of the leading parenchyma, forming continuous bands or one-sided accumulation. The properties of fibres in SRW are somewhat different from the properties of fibres in SW. The shortest tracheids are located in the root collar – from 1.87 to 2.43 mm – as described by Geles (1978). As the distance from the centre of branching of the root system increases, the tracheids' length increases. On a straight section of the root, the length of tracheids does not differ from their length in SW, which is 3.3–4.1 mm. On average, the tracheids of the SRW are somewhat shorter than those in the SW. Scientists have not investigated the transverse dimensions of the tracheids.

Some comparative studies of different parts of birch (Betula pendula Roth.) and aspen (Populus tremula L.) trees were conducted by Mutovina et al. (1983), which noted differences in fibre length and cell wall thickness. However, the diameters of vessels and libriform fibres, in particular, in the roots of these species, were not investigated. In the studies of Vintoniv et al. (2007) and Yatsenko-Khmelevskyy (1954), the internal structure of SW of some species is specified, but the features of macro- and microstructures of SRW are not presented.

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 (Pinus sylvestris L.), spruce (Picea abies Karst.) and fir (Abies alba) – and libriform fibres in broadleaved species – aspen (Populus tremula L.) and birch (Betula pendula Roth.).

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 (Pinus sylvestris L.), spruce (Picea abies Karst.) and fir (Abies alba) – and the diameters of the vessels and fibres of the libriform in the SRW of hardwoods – aspen (Populus tremula L.) and birch (Betula pendula Roth.). 3). To compare the internal structure of stump-root systems with the SW of the studied species.

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 (Picea abies Karst.), fir (Abies alba), pine (Pinus sylvestris L.), aspen (Populus tremula L.) and birch (Betula pendula Roth.) were selected according to GOST 16438.6:1980 and ISO 3129:2019, and for each species, wood samples were taken from three trees. Three stump-root system samples were cleared from the ground and prepared for research. The SRW samples were taken from the area of the root collar and at a distance of 50–60 cm from the transverse root discs: for pine (Pinus sylvestris L.) and fir (Abies alba), from the roots of vertical orientation and for spruce (Picea abies Karst.), from the roots of horizontal orientation. The SW samples in the form of discs were obtained from sections at a height of 1.3 m from the root collar. Two blocks from each stem disc and two to seven blocks from each root disc were examined. A microtome was used to cut thin layers. After pre-cooking in water with the addition of glycerol, the sections were cut perpendicular to the fibres. Thin sections of spruce (Picea abies Karst.), fir (Abies alba), pine (Pinus sylvestris L.), birch (Betula pendula Roth.) and aspen (Populus tremula L.) were stained with a solution of safranin, dehydrated in alcohol, purified from xylene and made into preparations by placing them between glass plates. To measure the size of block of cells, we used wood from growth rings 1 and 2 closest to the pith, rings 9 and 10, and the last two rings closest to the bark. An MBI-1 biological microscope with a total magnification of 56–1350 times was used for the experiments. Photographing was carried out using the MFK-20 photo attachment with a Zenith-12 D camera. Statistical processing of research results was carried out according to the recommendations of GOST 16483.0:1989 using statistical program.

The results of comparative studies of the macrostructure of SRW and SW are given in Table 1.

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 (Pinus sylvestris L.) – by 84.62%; in spruce (Picea abies Karst.) – by 73.68%; in fir (Abies alba) – by 93.75%; in aspen (Populus tremula L.) – by 35.71% and in birch (Betula pendula Roth.) – by 105.00%. It should also be noted that the content of late wood in the SRW of conifers is lower than in the SW, in particular, in pine (Pinus sylvestris L.) – by 19.60%; in spruce (Picea abies Karst.) – by 27.30% and in fir (Abies alba) – by 21.69%, which is primarily due to the leading function of the stump-root systems of trees.

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 (Pinus sylvestris L.) – by 46.05%; in spruce (Picea abies Karst.) – by 42.31%; in fir (Abies alba) – by 47.54%; in aspen (Populus tremula L.) – by 27.78% and in birch (Betula pendula Roth.) – by 52.00%. The eccentricity of the annual layers of perennial roots with different deviations can also be observed in microsections (Fig. 1). Individual annual layers show quite different deviations.

Figure 1

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.

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 (Pinus sylvestris L.) – by 32.78% and in spruce (Picea abies Karst.) – by 18.67%; in fir (Abies alba) – by 22.54%; in the tangential direction near pine (Pinus sylvestris L.) – by 8.03%; in spruce (Picea abies Karst.) – by 1.21% and in fir (Abies alba) – by 14.92%. Regarding the late tracheids, we obtained the following results: in the radial direction, there was an increase in pine (Pinus sylvestris L.) – by 14.45% and in fir (Abies alba) – by 7.98%; in spruce (Picea abies Karst.), there was a decrease by 17.82% and in the tangential direction, there was a decrease in all species: in pine (Pinus sylvestris L.) – by 15.30%; in spruce (Picea abies Karst.) – by 4.01% and in fir (Abies alba) – by 5.32%. The interval of the sizes of early and late tracheids both in root wood and in SW in the radial direction was approximately 1.5–2.5.

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 (Pinus sylvestris L.) – by 23.81% for early tracheids and by 13.64% for late ones; in spruce (Picea abies Karst.) – by 19.35% and 24.00%, respectively, and in fir (Abies alba) – by 27.78% and 12.20%, respectively (Tab. 2).

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 (Populus tremula L.) – by 20.41% for vessels and by 12.95% for libriform fibres and in birch (Betula pendula Roth.) – by 20.69% for vessels and by 18.41% for libriform fibres.

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 (Pinus sylvestris L.) root. But most part of his work was engaged in studying only the middle part of roots of coniferous species.

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 (Pinus sylvestris L.), spruce (Picea abies Karst.) and fir (Abies alba), the same alternation of annual layers of early and late wood can be seen, as well as in the trunks.

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 (Pinus sylvestris L.), spruce (Picea abies Karst.), fir (Abies alba)]. Bordered pores are often found on the tangential walls of the late and early tracheids (except juniper). This feature of the structure reflects the great adaptation of the root wood to perform the water supply function.

The alternation of annual layers is also observed in birch (Betula pendula Roth.) and aspen (Populus tremula L.), the wood of their roots being dominated by vessels and libriform fibres. The annual layers of the roots differ from the annual layers of the trunk in their uneven development and indistinct boundaries. The annual layers are harder to count in the roots than in the trunk since they are poorly distinguished there.

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 (Pinus sylvestris L.), even in a relatively small area, can develop eccentrically in different directions (Fig. 2).

Figure 2

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 (Abies alba), they are polygonal, and in spruce (Picea abies Karst.) and pine (Pinus sylvestris L.), they are of a rectangular shape with a much larger side in the radial direction. The late tracheids of this wood have almost the same shape. They are somewhat compressed in the radial direction and elongated in the tangential direction (Fig. 3). Early SRW tracheids, as evidenced by the data in Table 2, have thinner walls (by 19–28%) and a larger internal cavity (by 15–25%) compared to similar SW elements (Fig. 4). However, there is no significant difference between the late tracheids of these types of wood: the walls are very thick and the internal cavities are small; within one growth layer, the transition of early tracheids to late ones is gradual.

Figure 3

Figure 4

The thickness of the tracheid shell of pine (Pinus sylvestris L.) roots during the transition to the late zone first increases to a maximum and then decreases again near the border of the annual ring. Manwiler (1972), during his study of the southern pine (Pinus pinaster Ait.), found that the tracheids of the roots were one third longer and one third larger in diameter, their walls were 18% thinner and the gaps were almost two thirds larger than the tree trunk tracheids measured at stump height. The tracheids of horizontal roots were longer and had thicker walls than those of the roots of other orientations; length, cell diameter and gap diameter increased along the root. In non-horizontal roots, the diameter of cells and gaps decreased, while the length and thickness of the wall increased. Along the taproot, all dimensions increased to a maximum and then they decreased. His research was concerned with only one species of pine (Pinus sylvestris L.). In spruce (Picea abies Karst.) and fir (Abies alba), large, developed roots have fairly wide tracheids with thicker walls than in pine (Pinus sylvestris L.) roots but are smaller in size than in SW.

The study of the microstructure of hardwoods emphasises that the wood of aspen (Populus tremula L.) roots (Fig. 5) and that of birch (Betula pendula Roth.) (Fig. 6) differs little in structure from SW. Being representatives of diffuse-porous species, all vessels in their wood are small, relatively evenly scattered on the annual ring and, in some places, several pieces are located together, forming radial or tangential groupings.

Figure 5

Figure 6

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.