The Wood of Scots Pine ( Pinus sylvestris L.) from Post-Agricultural Lands Has Suitable Properties for the Timber Industry

: Scots pine ( Pinus sylvestris L.) is a widespread species throughout Europe and at the same time is dominant in Polish forests and of key importance in the wood industry. Pine stands are subjected to numerous environmental stresses, and one of them is the di ﬀ erent physico-chemical and biological properties of post-agricultural soils compared to forest soils, which may a ﬀ ect the properties of the resulting wood and its industrial suitability. The research material taken at the height of 1.3 m from tree trunks (breast height diameter, dbh ) in the form of sections and discs was collected in an 80-year-old pine stand from four plots, representing former agricultural and ancient forest land, and two types of habitats: fresh coniferous forest and fresh mixed coniferous forest. The forest habitat trophy had a decisive impact on the dendrometric characteristics and properties of pine wood (density, modulus of elasticity, bending strength, and compressive strength along the tracheids). The history of soil use (post-agricultural or forestry) did not a ﬀ ect the analyzed pine wood properties. Regardless of the forest habitat type and soil type history, pine wood at the dbh height showed a variability of features typical of century-old cultivated stands. Individual pine trunks were characterized by signiﬁcant individual variability.


Introduction
The course of xylogenesis in trees as well as the properties of wood tissue resulting from this process are important issues in cognitive as well as practical aspects, in particular for forest management. Both the division activity of cambium underlying the entire process of secondary wood formation [1,2] as well as the subsequent stages of derivative cell differentiation are not only controlled genetically [3,4] but also epigenetically-i.e., regulated by internal and external factors acting in the environment [5,6]. Among the latter are factors basic for biogenesis, such as the temperature, light, water, soil, gravity, photoperiod, and climate factors, and occasionally such factors as wind, fires, frosts, flooding, defoliation, industrial pollution, pathogens, or breeding operations [7][8][9].
In this context, the question arises about the course of xylogenesis and the properties of wood originating from the stands growing on lands with an agricultural history that is not typical for forest production. The formation of forests on former agricultural land can arise through a natural secondary progression, but in recent times, especially in Europe, it is mainly the result of intentional afforestation [10][11][12]. This initiative was promoted by the European Economic Community's (EEC) directive 2080/92, and later the EEC 1698/2005, which established a community aid scheme for forestry measures in agriculture, with the general aim of transforming agricultural lands into forested areas. Thus, soon, part of the wood raw material will come from forested post-agricultural areas, whose soils are characterized by different physico-chemical and biological properties compared to those of forest soils [13]. Studies on the flora of secondary forests and its comparisons with primary forests gave rise to the concept and separate category of the "ancient forest" [14][15][16]. Ancient forests are not necessarily old in the sense of the age of trees-it is their permanence in a given area that is meant. This criterion is met by primary forests and those secondary forests that have been growing for a long enough time, such as for several tree generations. These ancient forests include certain plant species that, although they possess the capability to colonize secondary forests-for example, former farmlands-the process is very slow and long-lasting.
The most frequently abandoned farmlands-mainly due to economic reasons-in the Polish lowlands are poor and mesotrophic. In most cases, the potential vegetation of these areas comprises forest stands dominated by pine; at the same time, this is also the most important economic species. For this and other historical and demographic reasons, Scots pine (Pinus sylvestris L.) dominated in the post-war afforestation on former farmlands [17]. Scots pine is a typical Euro-Siberian species that is characterized by a fairly wide range of occurrence, the largest of all species of the genus Pinus [18].
The issue of the appearance, development, and characteristics of groundcover in forests on former farmlands has been the subject of numerous studies in Europe and America-i.e., [19]-but studies on the characteristics of wood originating from such forests are still scarce, although from the timber production point of view they are very much needed, especially in the era of global warming, when the role of soil and forests in the global carbon budget has been incorporated in international treaties [20].
Determining the technical quality of wood raw material is mainly based on the study of its macrostructure-i.e., the width and share of sapwood and heartwood, annual wood increments, and the share of juvenile and mature wood. However, these factors do not fully reflect the quality of the produced wood tissue. Knowledge in this area is needed in the field of the breeding and use of wood. The discussion should include, first of all, the qualitative and quantitative features of wood formed in these conditions [21]. It seems justified to research the quality of wood from stands growing in those conditions that are atypical for forest production. Based on previous research, it can be assumed that forest habitat and its previous agricultural use can be a factor modifying wood tissue conditions related to the production of wood raw material on former agricultural lands [21]. It is known that wood density is correlated with some wood mechanical traits, such as the dynamic bending strength, modulus of elasticity, and compression strength [22]. The variation in wood density depends also on a range of intrinsic (it varies significantly from pith to bark) and environmental factors, and is often used in forest biomass and carbon dynamics [23]. Therefore, wood density is thought to guide the potential uses of an individual wood species, and it is important for guiding forest management activities.
The present work is part of the research on the trunk wood structure and properties of Scots pines from stands on former farmlands or ancient forests within two habitats: fresh coniferous forest and fresh mixed coniferous forest. The research presented here concerns the characteristics of differences in Scots pine wood properties, such as the density, modulus of elasticity, bending strength, and compressive strength along the tracheids. This study aimed to determine the influence of forest habitat conditions on the investigated properties of Scots pine wood. We expected less dense wood and an increased density from pith to bark in more fertile stands.

Materials and Methods
Wood samples from a total of 40 Scots pines (Pinus sylvestris L.) aged 66 to 99 years growing in the Chojnów Forest District were taken for tests. Only trees with no visible damage or disease symptoms that were present in the main layer of the stand were selected. The trees were from four plots, representing former farmland and ancient forest and two habitat types: fresh coniferous forest and fresh mixed coniferous forest. The plots and stands were selected in the Chojnów Forest District using site inspection preceded by interviews with local forest management staff and an analysis of the history of stands taken from the forest management plans. Basic data on the plots are given in Table 1. Four plots near each other gave the opportunity to reduce the variability in environmental factors. The analysis of the research plots in terms of flora and vegetation was the subject of separate studies [24,25]. In short, the study confirms the difference between plots on agricultural land and old forests, as well as differences in trophy between fresh and fresh mixed coniferous forest habitats, which is in line with the expectations and methodological assumptions of the study-i.e., that the studied plots were diversified in terms of both land-use history and fertility. Chojnów Forest District is located in the central part of Poland in the Masovian Voivodeship on the left bank of the Vistula River, in the vicinity of Warsaw. According to the physico-geographical regionalization, the Chojnów Forest District is located in the Central Polish Lowland within the North European Plain. The Central Polish Lowland is dominated by glacial uplands with no lakes, though it also includes depressions, basins, valleys, and water accumulation plains and is partially covered with dunes [26,27]. According to the geobotanical division of Poland [28], the Chojnów Forest District is located in the Lowland-Upland Central European Province within the Baltic Sector in the Masovian Region, and encompasses two districts: the Rawa district and the Warsaw district.
For each tree, its diameter at a height of 1.3 m from the ground (breast height diameter, dbh), the total height of the tree, and the length of the trunk from the ground to the location of the living crown were measured. From each tree, a roller about 50 cm long and a cross-section disk about 10 cm thick from the part below the dbh were taken for the tests. The places of collecting the research material along the trunk are shown in Figure 1. That part was chosen according to other research findings [29,30] that the wood density and other parameters determined from the wood at dbh represent average values for all the log. The samples collected in this way were used to determine the wood density (according to [31]) and mechanical properties-i.e., bending strength (according to [32]) and static modulus of elasticity (according to [33]). After the bending strength test, the ends of the samples were cut and the compressive strength along the tracheids tests were performed (according to [34]). The parallel control of wood moisture was conducted (according to [35]). Standard laboratory equipment with the required measurement accuracy and an INSTRON testing machine (Instron ® -ITW company, Norwood, MA, USA) equipped with specialized IX-SERIES computer software for recording and analyzing the measurement results were used for the tests. The test procedures listed above should be considered as compliant with the current requirements of the international ISO standards [36][37][38][39][40][41].
The annual rings were analyzed on the transverse wood discs from pine stems. The analysis of the annual growth rings was performed using WINDENDRO TM (Ver. 2002a, Regent Instruments Inc., Quebec, Canada)-semi-automatic image-analysis software-and a high-definition optical scanner was used to measure the tree ring width and density variables (based on [42]). WINDENDRO TM is an image analysis system for tree-ring analysis. For the ring-width measurements, sanded discs were scanned (600 dpi resolution) and the images were saved as tagged information file format (TIFF) files. The WINDENDRO TM software measures the selected tree-ring parameters using a line of sensors traced along with the imported image profile. The measurement paths were selected to detect each ring boundary at right angles (four measurement paths were appointed for each wooden disc).
The results were analyzed using the program STATISTICA 10.0 PL (TIBCO Software Inc., Palo Alto, CA, USA), descriptive statistics, and the two-factor variance analysis ANOVA. All the tests were carried out for a significance level of p < 0.05.

Dimensional Features
Scots pine trees from the fresh forest were characterized by smaller diameters at dbh compared to the trees from the fresh mixed forest. Within the same habitat type, the pines grown on former agricultural lands showed smaller diameters compared to the pines grown in ancient forest land. Additionally, pine trees from mixed forests, regardless of the type of soil (post-agricultural or forestry), were characterized by a significantly greater width of the sapwood zone ( Table 2). The ANOVA confirmed the impact of the habitat and soil use history (forestry and post-agricultural) on the dimensional features (Table 3). The samples collected in this way were used to determine the wood density (according to [31]) and mechanical properties-i.e., bending strength (according to [32]) and static modulus of elasticity (according to [33]). After the bending strength test, the ends of the samples were cut and the compressive strength along the tracheids tests were performed (according to [34]). The parallel control of wood moisture was conducted (according to [35]). Standard laboratory equipment with the required measurement accuracy and an INSTRON testing machine (Instron ® -ITW company, Norwood, MA, USA) equipped with specialized IX-SERIES computer software for recording and analyzing the measurement results were used for the tests. The test procedures listed above should be considered as compliant with the current requirements of the international ISO standards [36][37][38][39][40][41].
The annual rings were analyzed on the transverse wood discs from pine stems. The analysis of the annual growth rings was performed using WINDENDRO TM (Ver. 2002a, Regent Instruments Inc., Quebec, Canada)-semi-automatic image-analysis software-and a high-definition optical scanner was used to measure the tree ring width and density variables (based on [42]). WINDENDRO TM is an image analysis system for tree-ring analysis. For the ring-width measurements, sanded discs were scanned (600 dpi resolution) and the images were saved as tagged information file format (TIFF) files. The WINDENDRO TM software measures the selected tree-ring parameters using a line of sensors traced along with the imported image profile. The measurement paths were selected to detect each ring boundary at right angles (four measurement paths were appointed for each wooden disc).
The results were analyzed using the program STATISTICA 10.0 PL (TIBCO Software Inc., Palo Alto, CA, USA), descriptive statistics, and the two-factor variance analysis ANOVA. All the tests were carried out for a significance level of p < 0.05.

Dimensional Features
Scots pine trees from the fresh forest were characterized by smaller diameters at dbh compared to the trees from the fresh mixed forest. Within the same habitat type, the pines grown on former agricultural lands showed smaller diameters compared to the pines grown in ancient forest land. Additionally, pine trees from mixed forests, regardless of the type of soil (post-agricultural or forestry), were characterized by a significantly greater width of the sapwood zone ( Table 2). The ANOVA confirmed the impact of the habitat and soil use history (forestry and post-agricultural) on the dimensional features (Table 3).  A characteristic feature recurring in all the analyzed trunks was wide annual increments next to the pith (first 8-10 increments), gradually transforming with the age of the tree into narrow ones. This arrangement is typical due to the widespread phenomenon of the formation of juvenile wood, but it is particularly strongly pronounced in trees grown in pine stands from artificial plantings [43].
Irrespective of the type of habitat, the average ring width of the heartwood was greater than the average width of the annual growth increments in the sapwood zone. In the case of Scots pine trees grown in mixed forests, the average width of the annual heartwood growth was on average over 2 mm, while in the case of trees growing in fresh coniferous forests it was on average below 2 mm (Table 4). Notwithstanding the above, the average width of annual increments for the cross-section of the whole trunk at breast height was similar everywhere and amounted to approx. 1.5 mm. In individual trees, the described features indicated significant variability, which was indicated by the high coefficients of variation within each group, even in the order of 30%. The ANOVA confirmed the impact of habitat on the width of the annual rings (Table 5). Based on previous studies [44][45][46], the width of the annual rings affects in a significant way the physical and mechanical properties of coniferous wood species, and those properties determine the suitability of the wood.

Density and Selected Mechanical Properties
The average values of the wood density and selected mechanical properties of wood from trees grown in four different stands are given in Table 6. Regardless of the habitat type of the stand and the type of previous land use (ancient forest, post-agricultural forest), the juvenile wood of Scots pine trees (including several of the oldest in-season annual rings) was characterized by a lower density (from 500 to 600 kg/m 3 ) compared to the density of mature wood (from 570 to 630 kg/m 3 ). The density values were affected randomly by the locally and irregularly occurring higher content of resin wood in the selected trunks. Within such an area, the density was higher. These results are in line with the literature data, as the density of the wood varies significantly from pith to bark and is affected by other intrinsic factors [23]. The analyzed strength characteristics also showed significant variability depending on the tested trunk. Sometimes the test results were significantly affected by defects present, such as an eccentrically arranged core (pith), flattening, scleroderma, and curvatures, as well as the related non-rectilinear course of the tracheids. The presence of resin was another disturbing factor. Wood zones oversaturated with resin were denser, but this did not cause an increase in the compressive and bending strength or the modulus of elasticity. On average, the wood obtained from former agricultural lands slightly differed in terms of its density and mechanical properties from the wood of pine trees from ancient forest land. Wood from fresh coniferous forests, while maintaining practically the same average density and compressive and bending strength, was characterized by the slightly reduced modulus of elasticity for forest soils (with this feature showing a high variability). For Scots pines wood from mixed forests, slightly higher average density and technical parameters are obtained in the case of wood from former agricultural land compared to the wood from forest land.
The density and mechanical properties of wood were more influenced by the forest habitat type than the history of soil use (post-agricultural or ancient forestry). Scots pine wood from fresh coniferous forest was characterized by higher mechanical parameters, which was confirmed by the ANOVA (Table 7). Therefore, from a practical point of view, the history of the soil (post-agricultural or forestry) does not matter in the case of the above-analyzed wood parameters. Slightly similar results were obtained during the study of Scots pine originating from the north part of Poland [21]. According to those results, the wood of pines growing on post-agricultural land in the fresh mixed coniferous forest had a higher basic density but lower compression strength along the tracheids. The performed analyses pointed to differences in the properties of wood tissues between pines growing on post-agricultural land and pines growing on typical forest soils [21]. Therefore, more verification studies need to be carried out on wood from different habitats.

Conclusions
The present work is a part of wider research on the trunk wood structure and properties of Scots pines from stands on former farmlands or ancient forests within two habitats: fresh coniferous forest and fresh mixed coniferous forest. Scots pine trunks from fresh coniferous forest were characterized by smaller diameters at the breast height and a smaller proportion of sapwood compared to the trunks of pine trees from the moist mixed forest. Based on the presented research, it was stated that the forest habitat type had a decisive impact on the dendrometric characteristics and properties of the tested Scots pine wood (density, modulus of elasticity, bending strength, and compressive strength along the tracheids). The history of soil use (post-agricultural or forestry) did not affect the properties of the analyzed pine wood. Pine wood from stands grown on forest land was characterized by a higher technical quality in terms of the parameters analyzed in this work.
The obtained research results allow for a statement that the wood of Scots pine from post-agricultural lands has suitable properties for the timber industry, which is extremely important from a practical point of view. The acquired knowledge can be used in planning the location of tree plantations for the needs of the wood industry.