Biological resistance and chemical properties of wood from an agroforestry system

1 Introduction

Agroforestry systems (AFSs) represent a sustainable approach that integrates trees, agricultural crops and/or animals in the same space (Nair et al. 2021). This combination not only increases productivity, but also improves ecological resilience and soil quality. Species diversification and the efficient use of natural resources are fundamental principles of AFSs, promoting long-term preservation. In addition, they contribute to food security and, above all, income generation for farmers, making them a viable solution for family farming (Sgarbossa et al. 2020; Thomas et al. 2021).

The selection of species to be used in agroforestry systems is a crucial factor in their development, and can vary depending on the region and the specific objectives of the AFSs (Ruticumugambi et al. 2024). It is possible to include native tree species, such as Parapiptadenia rigida (Benth.) Brenan (Angico), Peltophorum dubium (Spreng.) Taub. (Canafístula), Schizolobium parahyba (Vell.) Blake (Guapuruvu) and exotic species, such as Eucalyptus urophylla × Eucalyptus grandis, because the diversity of species maximizes the efficient use of resources, such as light, water and nutrients.

Therefore, studying the characteristics and technological properties of wood, whether it comes from AFSs or not, is essential in order to determine its viability and potential for use and to add value to it. Characteristics such as density, moisture content, mechanical strength and natural durability are essential for using wood in different sectors, such as construction, furniture and other industries (Andrade and Paes 2023; Eloy et al. 2024), as in the manufacturing of wood-based panels, for example.

In addition to physical and mechanical properties, the biological and chemical properties of wood are equally crucial (Medeiros Neto et al. 2024). Given its nature, wood is subject to the action of biodegrading agents, including termites, borers and fungi, which use wood as a source of energy and shelter (Medeiros Neto et al. 2024). Among these agents, fungi are responsible for the largest proportion of damage caused to wood (Moreschi 2013), especially in terms of rot, which causes the greatest damage; the degrading organisms can be classified as soft, brown, white and dry rot, as well as staining, varying according to the deterioration of the different compounds in the wood.

As such, the attack of these organisms is influenced by the biological resistance of the wood, which is mainly determined by the presence, type, quantity, and location of extractives in the cell. Only some extractives – such as starches and other simple sugars – and the main cell wall components (cellulose, hemicellulose, and lignin) serve as a food source for xylophagous organisms (Medeiros Neto et al. 2024; Paes et al. 2013). Thus, detailed studies into the chemistry of wood make it possible to improve its durability and functionality (Batista et al. 2020). These characteristics are essential for determining the best way to process and use wood, as well as for developing treatments that increase its useful life. With this in mind, this study aims to determine the biological resistance and chemical properties of the wood of four species from an agroforestry system. This work is novel in evaluating lesser-studied native species cultivated in integrated systems, providing insights that can support sustainable use and valorization of these woods. The hypothesize that the wood from these species, due to their growth in diversified ecological conditions, will exhibit variable resistance and chemical composition profiles, potentially influencing their durability and industrial applicability.

2 Materials and methods

2.1 Sampling

Four nine-year-old forest species were used to carry out the work, three of which were native: P. rigida (Benth.) Brenan (Angico), P. dubium (Spreng.) Taub. (Canafístula), S. parahyba (Vell.) Blake (Guapuruvu) and a hybrid of E. urophylla × E. grandis. Table 1 shows the average values for total height, diameter at breast height (DBH) at 1.30 m above the ground, basal area and estimated volume. The cultivated area was occupied by different crops throughout the years/harvests, such as: Glycine max, Zea mays, Phaseolus vulgaris, Triticum aestivum, and Avena strigosa Schreb. The species were collected from an Agroforestry System with planting spacing of 12 m between rows and 1.5 m between plants in the row. This experiment was conducted in an experimental area belonging to the Federal University of Santa Maria Campus in Frederico Westphalen, Rio Grande do Sul, Brazil, located at an altitude of 480 m with the geographical coordinates 27°22′S; 53°25′W.

Table 1:

Dendrometric conditions of the evaluated trees.

Species	DBH (m)	H (m)	BA (m2)	V (m3)
Parapiptadenia rigida	0.17	18.01	0.024	0.205
Peltophorum dubium	0.16	19.33	0.020	0.152
Eucalyptus grandis × Eucalyptus urophylla	0.34	26.99	0.095	0.839
Schizolobium parahyba	0.25	21.04	0.049	0.467

1. DBH, diameter at breast height (1.30 m above the ground); H, height; BA, basal area; V, volume.

The sampling process involved felling five representative trees of each species, selected on the basis of their average diameter. A 2-m-long log was then removed in the diameter at breast height (DBH) region, located 1.30 m from the ground, as well as a disc in the same position. Central planks were made from these logs and used to prepare the samples for biological and chemical analysis (Figure 1).

Figure 1:

Sampling scheme for the biological and chemical analysis of wood from the agroforestry system. DBH, diameter at breast height; BOD, biochemical oxygen demand.

3 Results and discussion

The analysis showed that there were statistical differences in all the biological and chemical variables. Table 2 shows the average mass loss for the four species evaluated for each type of fungus.

Based on the results, when observing the biological resistance of the species studied, the fungi that most degraded the wood of P. rigida were GA (5.9 %) and PS (6.9 %), both of which cause white rot. For P. dubium, the fungus with the least influence was GT, a brown rot fungus, which accounted for 2.3 % of the mass loss, followed by PS and GA with 3.9 % and 4.7 %, which did not differ statistically from each other, while the fungus with the greatest degradation was TV with 6.0 %.

The fact that less mass loss occurred in GT is in line with expectations, since among the wood-degrading fungi evaluated in this study, it is considered to be the least aggressive, as it mainly degrades polysaccharides, while white rot degrades lignocellulosic materials (Leão et al. 2018).

In E. grandis × E. urophylla, the fungi with the least influence were PS (4.8 %) and TV (6.0 %), both classified as white rot, while when the species was subjected to the brown rot fungus (GT), there was a significantly greater loss of mass.

Schizolobium parahyba wood was the most affected among the species, with a 17.6 % loss of mass when subjected to the GA fungus, followed by TV (10.2 %), which did not differ from GT (7.5 %), and the latter did not differ from PS (6.3 %). According to the classification of the American Society for Testing and Materials (2005), the only species evaluated in this study that cannot be classified as highly resistant to deterioration is S. parahyba, as the loss of mass was greater than 10 % during the period it was subjected to the GA fungus.

This behavior may be directly related to the basic density and porosity of the wood and can be used as a comparison factor between forest species in terms of natural durability, since in most cases, denser wood offers greater resistance to deterioration by xylophagous organisms (Panshin and De Zeeuw 1981). This explains why the wood of S. parahyba was the most affected, since it has the lowest density among the species studied, corresponding to 277 kg m⁻³ (Table 3). Table 3 shows the results obtained for the chemical properties of the species studied.

The test of means for the chemical variables showed that the lignin content was highest for E. urophylla × E. grandis (21.5 %), differing only from P. rigida (15.0 %). As for the extractive content, the opposite was true: the highest percentage of this variable was found in P. rigida (19.8 %) and the lowest in E. urophylla × E. grandis (9.9 %) and S. parahyba (11.6 %).

As for the results obtained for the extractive content, Medeiros et al. (2016) and Hsing et al. (2016) found values of 6.45 % and 6.51 % for this variable, when studying the E urophylla × E. grandis species at 2.25 and 4 years of age, respectively. These values are lower than those of the present study, justified by the age of the material, as the extractive content increases as the tree ages (Hillis 2012; Santos et al. 2024). Furthermore, it can vary depending on the planting environment, cultural practices and plant spacing.

The holocellulose content was higher for S. parahyba (68.7 %) and E. urophylla × E. grandis (68.0 %), the latter not differing from the other species. Allesi et al. (2021) and Silva (2018) found values of 70.89 % and 70.24 %, respectively, for the S. parahyba species, while Trugilho et al. (2014), when studying 15 clones of E. urophylla × E. grandis at four years of age, found values ranging from 70.89 % to 82.04 %. Medeiros et al. (2016), on the other hand, found data closer to that of the present study, 66.12 %.

The ash content of the P. dubium species was statistically different and higher than the others, with an average value of 1.6 %, followed by P. rigida, with 1.3 %, S. parahyba, with 1.0 % and E. urophylla × E. grandis with 0.6 %. Similar results for the ash content of S. parahyba were identified by Allesi et al. (2021) and Silva (2018), who found averages of 1.09 % and 1.19 % respectively. For E. urophylla × E. grandis, Medeiros et al. (2016) and Hsing et al. (2016) reported ash contents of 0.67 % and 0.68 %, respectively, values similar to those found in the present study. It is important to consider that eucalyptus clones can be developed through genetic improvement for different industrial purposes, which may influence the chemical composition of the wood, including ash content. Therefore, variations among clones may reflect selected genetic traits according to specific end-use objectives.

Table 4 shows the Pearson correlation between mass loss and the chemical properties of the species studied. This relationship shows biological resistance, since according to the data presented, the two are directly linked.

Table 4 shows that there was no significant correlation between mass loss and lignin content, indicating that the amount of lignin did not directly influence the material’s mass loss. However, lignin is an important structural component that can provide resistance to degradation by fungi, indicating that species with a higher lignin content can provide the wood with greater protection against decay organisms (Daniel and Nilsson 1997; Lombard et al. 2014). This is related to the type of fungus (brown or white rot) and the type of lignin (syringyl and guaiacil), with syringyl being more easily degradable, as it forms less compacted structures.

The variables extractive content (−0.59), ash (−0.60) and basic density (−0.84) showed results that were inversely proportional to mass loss, suggesting that higher quantities of these components result in greater durability. Therefore, extractives influence the biological resistance of wood (Coldebella et al. 2018), and higher values, as in the case of P. rigida (19.8 %), can prevent fungal attack. This is directly linked to the higher concentration of toxic extractive compounds, such as alkaloids, phenols and terpenes, meaning that the higher the proportion of this variable, the lower the susceptibility to decay (Keržič et al. 2024; Nault 1988). Furthermore, the place where such extractives are concentrated in the cell, those linked to the cell wall are more effective than those existing in the cell lumen.

However, it is important to note that not all extractives confer resistance. Some extractives, particularly those soluble in cold water, such as simple carbohydrates and starches, can serve as a nutrient source and attract rot fungi, especially soft rot fungi. Thus, the chemical nature and solubility of extractives play a critical role in determining their contribution – whether protective or susceptible – to biological degradation.

Ash content indicates the presence of inorganic compounds in the wood, which can favour resistance to fungal attack, as high levels of minerals can inhibit fungal activity. Therefore, wood with a low ash content, such as E. urophylla × E. grandis, may have a feature that is more easily degraded by fungi, although this also depends on other factors such as extractive content.

Holocellulose was the only variable with a positive correlation with mass loss, i.e. the greater the amount of holocellulose, the greater the degradation of the material, which can be an indicator of the amount of polysaccharides (cellulose and hemicellulose) (Mujtaba et al. 2023). This explains the greater loss of mass in S. parahyba, since it had a holocellulose content of 68.7 %, higher than the other species evaluated, making it more susceptible to degradation by rotting fungi.

The species studied were more affected by white rot fungi, as these have the ability to degrade all the components of the cell wall (polysaccharides and lignin) by means of an arsenal of hydrolases, oxidative enzymes and low molecular weight mediators (Leonowicz et al. 1999). On the other hand, brown rot fungi, basidiomycetes, are capable of metabolizing cell wall polysaccharides, but not lignin, although the arrangement of the latter polymer can be modified during an attack (Arantes et al. 2012). This behavior explains why the wood of P. dubium was less affected by the GT fungus than the others, since it had the highest lignin content of all the species studied.

Basic density may be an important factor in determining the physical resistance of wood to fungal attack. P. rigida had the highest density (652 kg m⁻³), suggesting that the wood is denser and possibly more resistant to fungal attack. On the other hand, S. parahyba had the lowest density (277 kg m⁻³), which may indicate a less dense wood that is more susceptible to mass loss by fungi. This behavior is in line with the literature, such as the study carried out by Costa et al. (2011), who subjected the wood of Simarouba amara, with a density of 335 kg m⁻³ and Carapa guianenis with a density of 527 kg m⁻³ to the GT and TV fungi, in both of which the wood with the lowest density was more affected.

The correlation shows that lignin is inversely proportional to the extractive content, and these results are in line with those presented by Medeiros et al. (2016). It can also be seen that the higher the extractive content, the higher the ash and density content and the lower the holocellulose content.

Principal component analysis (PCA) showed that the first and second principal components accounted for 68.51 % and 18.14 % of the accumulated variance, respectively. In this way, the PCA technique was able to explain more than 86 % of the accumulated variance for the variables studied for the different species (Figure 2). The results of the PCA showed that S. parahyba is more susceptible to fungi than the other species, as it had more contribution, i.e. mass loss. As can be seen, each forest species was arranged in a different quadrant, indicating contrasting differences between them.

Figure 2:

Principal component analysis (PCA) of the chemical relationship and degradation of the wood of four forest species (Eucalyptus grandis × Eucalyptus urophylla, Peltophorum dubium, Parapiptadenia rigida and Schizolobium parahyba) from an agroforestry system. ML, mass loss; LC, lignin content; EC, extractives content; HC, holocellulose content; AC, ash content; BD, basic density.

In the analysis of the individual contribution of the variables to explaining the variance of each component, PC1 had the highest contributions from EC (20.9 %), HC (22.6 %), AC (14.5 %) and BD (15.1 %). PC2 had the highest contributions from ML (25.5 %) and LC (51.8 %). The vectors in different directions of the BD and AC variables indicate an inverse relationship with ML and HC. The same inverse relationship occurred for EC and LC.

In general, the results showed that holocellulose content and density directly influence susceptibility to fungal attack of species such as S. parahyba, with a high holocellulose content and low density, were more degraded, while P. dubium, with higher lignin, showed greater resistance.

4 Conclusions

Species with a higher density, extractive and lignin content, such as P. rigida and P. dubium, are more resistant to fungal attack, while species with a lower density and higher holocellulose content, such as S. parahyba, followed by E. urophylla × E. grandis, are more susceptible to decay. The greatest loss of mass occurred with the inoculation of the white rot fungus, G. applanatum, for all species. The principal component analysis corroborated the relationships presented, showing that the variance between the species can be explained mainly by the chemical and physical characteristics of the wood.

The results obtained in this study help to highlight the relevance of understanding wood properties in the context of agroforestry system management. It was observed that, when appropriate treatments are applied to mitigate the action of biodegrading agents, there is potential for the use of wood from agroforestry systems in specific industrial applications. Although further studies are needed, this information may assist in the selection of species aimed at improving forest management efficiency and the utilization of timber products.