Unlocking the potential of fast-growing species from Indonesia: durability and sorption aspects

spec-troscopie


Introduction
Wood as a construction material offers many benefits.Despite its environmental lead as a sustainable material for construction, lignocellulosic materials such as wood are characterised by their hygroscopic and anisotropic behaviour, as well as their varying natural durability.Wood can be particularly affected by biological deterioration agents such as wood-destroying fungi, which are negatively affecting the service life of wooden components in outdoor use.In Indonesia, the core mitigation plan in the forestry sector under the Ministry of Environment and Forestry is the development of social forestry with an emission reduction target of up to 100,930 GgCO 2eq (MoEF 2017).The implementation of the social forestry programme as a climate mitigation action plan aimed to reduce deforestation rate, forest degradation, and enhance carbon sink (REDD+) with co-benefits on poverty alleviation and improvement of governance for empowering the local economy.Per July 2022, more than 1,000 units of forest management permits through forestry partnerships have been established, which cover a total of 571,053.42 ha of forested land for more than 150 thousand households (MoEF 2022).
Agroforestry practices under the social forestry programme are often adapted by communities around forested land to maintain the soil nutrient cycle and improve the microclimate.Among different types of tree species, deciduous trees are largely planted in Indonesia.In 2001, Indonesia ranked third among all Asian countries, with a forest plantation area of 9.9 million ha.The majority of the Indonesian plantations are dedicated to rubber, teak, pines, and Acacia mangium (Enters and Durst 2004).The deciduous tree species planted by community farmers in one of Sumatera's regions are Sengon/Albasia.(Paraserianthes falcataria Nielsen), Jabon (Anthocephalus cadamba Roxb.), Teak (Tectona grandis L.), Ficus microcarpa and non-woody species such as Calamus manan and Bambusoideae (Pratiwi et al. 2021).These species are planted along with coffee and cocoa plants, which have been the major export commodities from various provinces in Sumatra.Soerianegara et al. (1993) reported that the mean annual increment (MAI) for the 8-12-year-old P. falcataria rotation is around 10-25 to 30-40 m 3 /ha.Another major species is Acacia, as about 67% of the total reported area of A. mangium plantations in the world are located in Indonesia (Krisnawati et al. 2011).Further deciduous lightwood species utilised in community forests and their distribution are presented in table I.
Fast-growing species resourced by regional social forestry schemes are capable of taking a prominent position in the advancing climate agenda.However, such wood species often have relatively inferior material properties, including low biological durability.When six-year-old A. cadamba or P. falcataria trees are harvested, 100% of their formed wood is, of course, juvenile (Rahayu et al. 2014).However, there are notable differences in durability between juvenile and mature wood, which makes juvenile wood questionable for use as constructive timber.As far as durability is concerned, mature wood has higher concentrations of extractives compared to juvenile wood (Forest Products Laboratory 2010).Consequently, juvenile wood usually has lower natural durability compared to mature wood.Thus, the inherent resistance against biodeterioration is critical for any future use (Brischke et al. 2012).In addition, fungal degradation rates depend on wood moisture content, on the prevalent temperature, and on the inherent durability of a given wood piece (Zabel and Morrell 2012).

Table I.
Distribution of notable low-density deciduous wood species in fi ve major islands in Indonesia adapted from Martawijaya (2004).

Wood species
Natural For determining the expected service life of wooden components in hot-humid environments, knowledge about the inherent natural durability is required.Therefore, preventive actions such as preservation or modification measures can be applied, to assure sufficient service life.Work has been done on the durability of fast-grown wood against termite and insect attacks, but records regarding durability against decay fungi are scarcely offered (Hadi et al. 2020;Jusoh et al. 2014).Generally, cities and regencies in Java have a high to very high decay hazard index, according to Scheffer (1971), with 47% of the cities and regencies in Java having a high decay Scheffer index and 40% of the cities in the rest of Java being classified as a moderate decay hazard region (Priadi 2011).The overall purpose of this research is to assess the suitability of three fast-grown Indonesian wood species to be potentially used for engineered wood products.In particular, this paper focuses on the vapor sorption characteristics and the inherent durability of solid wood samples.

Wood origin
Various wood species (1-3, table II) were attained from different plantations in Java and Borneo, Indonesia.The tree's age ranged between five and seven years.Logs were debarked and sawn into boards.For each board, defect-free samples were cut to a size of 25 × 25 × 300 (w × h × l) mm 3 , and shipped to Austria in oven-dried condition.While most of the samples consisted of sapwood only, a larger proportion of heartwood was found for the A. mangium samples, thus some adjustments were made for decay specimens further specified in the methodology.Additional samples from Austrian-grown Paulownia tomentosa (Thunb.)Steud.and beech (Fagus sylvatica L.) were obtained from the University of Natural Resources and Life Sciences Vienna, Austria.Information about the studied wood species is displayed in table II.

Wood moisture dynamics
Oven-dried wood specimens with a dimension of 25 × 25 × 50 (w × h × l) mm 3 were sanded using multiple sandpaper grit starting from 120 grit to 600.Dust residues were removed using pressurised air.Thin slices of sapwood were then sawn using a circular saw (Format-4 Formatkreissäge Kappa 550) from the cross-section planes, having a size of 25 × 25 × 1 (w × h × l) mm 3 .The two final specimens were then cut to size 10 × 10 × 1 (w × h × l) mm 3 using razor blades and stored inside a desiccator prior to measurement.Sorption measurements were carried out in a DVS Advantage 1 (Surface Measurement Systems).The initial dry weight was determined at 0% RH (relative humi-

Coniophora puteana inoculated after 2 weeks
Trametes versicolor inoculated after 2 weeks 65 mm 110 mm dity).RH levels were applied at 10% increment, from 0-90% were set, with a total of 19 RH measurement points.As soon as the dm/dt criterion of 0.002 was reached, which is the weight change over time, the next humidity level was set.Measurements were done at three different temperatures (15, 25, 35 °C) to observe potential temperature effects on the sorption properties of the materials, and the total air gas flow was set to 200 cm 3 /s.Changes in vessel area were determined through image analysis, by using the public domain software ImageJ (Schneider et al. 2012).

Decay resistance against basidiomycetes
The assessment of the biological durability of wood specimens of the selected fast-growing species was performed according to EN 113-2 (2016), using the white rot fungus Trametes versicolor (CTB 836 A) and the brown rot fungus Coniophora puteana (CBS 230.87), respectively.Both fungal strains were obtained from the culture collection of the Institute of Forest Entomology, Forest Pathology, and Forest Protection at the University of Natural Resources and Life Sciences Vienna.Before incubation, the wood specimens were sterilised using a Cobalt 60 Gammatron 1500 following EN ISO 1348(2016) and ISO 11137 (2018).Wood test specimens (n = 13) and moisture content test specimens (n = 5) were incubated in an alternative type 1 test vessel, as shown in figure 1, for 16 weeks at 22 ± 2 °C and 70 ± 5% RH, all stored inside a Phytotron (Heraeus-Vötsch, type VB 0714).Fluctuations in temperature and RH were monitored in real-time using the Simpati ® software.
Beech (F.sylvatica L.) wood was used for virulence control of the two test fungi, which have to lead to a minimum mass loss of 20% after 16 weeks of incubation.All test specimens were produced from sapwood, except for A. mangium, which also contained larger portions of heartwood.To comply with the standard dimension, specimens were made with as little heartwood as possible.After incubation, the specimens were cleaned from adhering mycelium, oven-dried at 103 ± 2 °C, until a constant weight was reached.Mass loss was determined by weighing the oven-dry specimens before and after incubation.The inherent durability was classified according to EN 350 (2016) (table III).

Brinell hardness test
Brinell hardness tests were performed according to EN 1534 (2020).After mass loss evaluation, the specimens were stored at 20 °C and 65% RH for approximately one month, until weight constancy was reached.The test was performed with a universal testing machine Zwick Roell Z020.With an element length of less than 200 mm based on the testing norm, one testing point on the axial plane or face element was performed on each of the exposed as well as the control specimens.The hold and release of the hardness indenter were set to 30 seconds.The Brinell hardness (HB) was calculated using Equation 1: (Equation 1) where F is the maximum load (N), D is the diameter of the indentation ball (mm), and d the diameter of residual indentation (mm).A list of predetermined forces used for the measurements is shown in table IV.

Characterization of fungal decay
The level of fungal degradation after incubation on decayed specimens was observed using the digital microscope Olympus DSX 1000 and a scanning electron microscope (SEM Hitachi HT3030).Samples were prepared using a sliding microtome, by cutting tangential and longitudinal sections.Traces of fungal decay were preliminarily    observed using an energy-dispersive X-ray spectrophotometer (EDX), which was implemented in the SEM.The EDX was used to perform elemental analysis on the specimens.

Results and Discussion
The exploration of unutilized fast-growing wood species from secondary forests in Kalimantan has been studied to understand their fibre characteristics and potential utilisation in various wood products, including construction materials (Adi et al. 2014).Fast-growing wood species generally have lower density and shorter fibre length compared to slow-growing wood species (Kojima et al. 2009).They have a higher proportion of early wood, which is more susceptible to moisture movement, leading to splitting and cracking (Gril et al. 2017).Nevertheless, as an oriented strand board, they meet minimum requirement for OSB/2 properties (Dumitrascu et al. 2020).Before utilising wood species as local building materials, information regarding their properties such as strength, dimensional stability, and inherent durability in relation to sorption isotherms is vital for efficient practical use (Wegner et al. 2010).The variability of the sorption isotherm, which helps us understand how wood will absorb and release moisture, can have an impact on the durability of fast-growing wood species (Lee et al. 2021;Zhang and Richman 2021).
In this study, sorption isotherms show total vapour adsorbed by the samples as a function of relative vapour pressure (Nopens et al. 2019).All four examined fastgrowing wood species showed similar characteristics, by following the IUPAC classification type II isotherm.In figure 2, smaller hysteresis were noted at the higher temperature levels.Thus, results indicated that temperature and wood species had a significant influence on wood hysteresis and sorption isotherm.This is in accordance with Krupińska et al. (2007), who also found that wood became less hygroscopic when measurement temperature increased, as indicated by decreased moisture content.This phenomenon resulted from enhanced reaction rates due to higher water activity (Panchariya et al. 2001).Moreover, the width of the hysteresis loop depends on internal bonding between and among the cell wall polymers (Irbe et al. 2006).Higher measurement temperatures also reduced the sorption hysteresis of all wood samples, especially in the high humidity range, with a maximum hysteresis seen at 60-70% RH.The highest to lowest moisture contents at 90% RH were found in the following order: A. cadamba (highest), P. falcataria, A. mangium, and P. tomentosa (lowest).Total time to achieve equilibrium required the longest period at the RH 90%, for all measured temperatures and across all wood species.The total running times of the four-wood species were different at each set temperature, with the longest sorption period found for A. mangium, followed by A. cadamba, P. falcataria, and by P. tomentosa.
In this study, vessel area changes have been observed during swelling and shrinking at the different RH levels (figure 3).The changes in vessel area were following wood activity, depending on the adsorption and desorption stages.The graph shows that the mean vessel area was decreasing during the adsorption cycle, while on the desorption cycle, it tended to increase.Different moisture contents were recorded, and a relationship between initial mass and moisture contents was made at the end of each RH step.A previous study on cell wall structure and sorption properties using different drying methods, i.e. air drying and oven drying, showed that the higher shrinkage corresponded to a smaller surface area.In this study, the surface area of the cell wall was correlated to the total pore volumes (Broda et al. 2021).
With respect to the species resistance against decay, the specimens were classified as slightly durable to not durable (DC 4-5).The average mass losses of wood specimens exposed, from highest to lowest, irrespective of fungal species, were as follows: A. cadamba, P. falcataria, P. tomentosa, and A. mangium (table V).This is expected for fast-growing wood species, which have a high proportion of juvenile wood within the sapwood (Darmawan et al., 2013).Fast-growing wood species with short rotation periods contain high portions of juvenile wood, which means lower resistance against decay, due to fewer extractives and lower technological properties.The high proportion of juvenile wood negatively impacts wood quality and durability against biodeterioration agents (Hadi et al. 2015;Lasserre et al. 2009).
Acacia mangium behaved differently since the samples also included smaller portions of heartwood.Heartwood-free samples were not available when the specimens were procured with small-diameter acacia.To comply with the requirement of a minimum sample dimension based on EN 113-2 (2021), specimens had to be made with sapwood close to heartwood.In the literature, mass losses of Acacia hybrid, A. mangium, and A. auriculiformis due to decay by T. versicolor were 10.2%, 11.2%, and 7.3%, respectively (Jusoh et al. 2014).Those results were higher compared to the data found in this study, which had a mass loss of only 2.7% after exposure to the same white rot species.An even greater mass loss value of 14.3% was presented by another study on A. mangium, as exposed to T. versicolor (Wahab et al. 2017).Acacia mangium used in this study had a slightly higher wood density (0.70 g/cm 3 ), compared to A. mangium from Malaysia forest plantations (0.29-0.58 g/cm 3 ).Similar results as in the present study were found by Krisnawati et al. (2011), who found that A. mangium from natural forest in Indonesia had a density ranging between 0.45-0.69g/cm 3 , at 15% moisture content.
Figure 4 shows mass losses of the wood specimens due to decay caused by the white rot fungus T. versicolor and the brown rot fungus C. puteana after 16 weeks of exposure, including the mass losses of wood specimens of F. sylvatica, which were included in order to prove the virulence of the used test fungi.There was no notable difference in durability classes between brown-rot and white-rot decay; however, brown-rot exposure resulted in higher mass loss variation within the species.High data variability was recorded on P. falcataria and P. tomentosa exposed to C. puteana, with standard deviations between 12.3% and 18.8%.A former study indicated that the variation in mass loss data could be due to internal factors such as natural resistance across the wood specimens and the selectivity of the decaying fungi (Sharapov et al. 2018).
The results of the Brinell hardness tests on the wood samples are presented in figure 5. Axial hardness values before and after exposure were significantly different (p < 0.05).Species exposed to T. versicolor had considerably lower axial hardness compared to C. puteana.Species with the highest axial hardness values after incubation with basidiomycetes were from high to low: A. mangium, A. cadamba, P. falcataria, and P. tomentosa, regardless of decay type.When compared to control specimens, the axial hardness of specimens decayed by T. versicolor was reduced by 0.07%, 69.04%, 45.61%, and 55.34% for A. mangium, A. cadamba, P. falcataria, and P. tomentosa, respectively.Specimens exposed to C. puteana had slightly lower percentages of axial hardness reduction of 0.04%, 57.74%, 17.30%, and 22.24%.Brinell axial hardness is strongly influenced by the density of the surface layer, while variation in density within the sample did not influence these properties (Rautkari et al. 2013).
Test specimens exposed to brown rot revealed noticeable degradation compared to white rot in the wood substrate.Even though brown rot is causing more visible damage, the average mass loss of specimens decayed by T. versicolor was higher compared to C. puteana.Lignin plays a major role in wood degradation caused by fungi.It provides biological protection by generating enzymes to avoid fungal penetration (Arantes et al., 2011).Lignin in gymnosperms consists mostly of guaiacyl units, while angiosperms are composed of syringyl and guaiacyl units in varying ratios (Whetten and Sederoff 1995;Wool 2005).Thus, hardwood possesses lower resistance to white rot decay, which mainly attacks lignin.Additionally, fungal selectivity in degrading wood substrates is different depending on species.T. versicolor initially attacks hemicellulose and lignin, which causes strength reduction, whereas brown rot fungi only decompose hemicellulose or cellulose selectively (Qi et al. 2022).
Decay and active hyphae from fresh incubated specimens are shown in figure 6. White-rot decay activity has produced more porous zones in wood tissue, which led to the disintegration of wood samples (figure 6a).Fungal access to adjacent cells most likely took place via pit apertures or through direct penetration via cell walls (Schwarze 2007).In this study, hyphae spread through adjacent vessels (figure 6b), with the vessels filled with fungal mycelia.Cracks often occur between the radial cell walls close to the vessels.In figure 6b it is seen that hyphae grew inside the cell lumina with close contact to the cell walls.Here, active hyphae excrete degrading enzymes, causing a reduction in wall thickness over time (Schmidt 2006).
In figures 6d-f, characteristics of wood decay by C. puteana are displayed.The interface of C. puteana in P. falcataria's lumen can be seen in figure 6d.Changes in   Brown-rot decay is causing a significant decrease in the weight and mechanical properties of a given wood piece (Bouslimi et al. 2014).According to Pandey and Pitman (2004), decay by C. puteana causes the lignin content to increase with exposure time, in parallel with a mass loss as the carbohydrates are selectively removed.Using the EDX method, major portions of K (potassium) cations were found on decayed wood specimens degraded by C. puteana or T. versicolor (figure 7).Higher concentrations of Ca (calcium), Mn (manganese), and Fe (iron) in decayed wood were also found in the study of Jellison et al. (1992).Ostrofsky et al. (1997) reported that Ca and K tend to increase with fungal exposure, as seen in red spruce.A higher K cation concentration is also associated with a decrease in the electrical resistance of decaying wood (Gao et al. 2019).Increased cation concentrations are correlated with mass loss and the progression of wood decay (Soge et al. 2021).Fungal activity on wood includes the transport of ions towards the infected regions.Recent findings suggested an increase in Fe concentration in brown rot decayed specimens, with significant movement of Ca from sound wood cell walls into the area colonised by fungal mycelium (Kirker et al. 2017).
Engineered wood products, such as glued laminated timber (glulam) or oriented strand boards (OSB), can be derived from tropical fast-growing wood species.For example, Hadi et al. (2021) studied polystyrene-impregnated glulam made from tropical wood species and found mechanical properties such as MOE and MOR to be inferior, which means they did not meet the required standards.Baskara et al. (2022) tested OSB made from the tropical fast species Paraserianthes falcataria, Maesopsis eminii, and A. mangium.It was found that with sufficient isocyanate resin as the binder for OSB, most of the physical and mechanical properties did meet the standard.
The use of tropical fast-growing wood species in engineered wood products is an important area of research, given the potential for these species to serve as a valuable resource.In their solid form, wood durability, axial hardness, and sorption properties are important factors to consider when selecting fast-growing wood species for practical applications commensurate to its service life (Barbu et al. 2022;Missio et al. 2016).Wood durability is affected by its natural resistance to decay, which varies among species.Meanwhile, sorption properties are an important analytical tool for the characterization of wood or any construction materials where the impact of humidity is important to material performance.The moisture content of wood is an important parameter influencing almost all mechanical properties of wood, and strength properties of wood increase as its moisture content decreases (Cao et al., 2021).The information regarding inherent durability and sorption properties leads to a better understanding of the provision of engineered wood products made from fast-growing species, which will be the avenue for the next research activities.

Conclusions
In this study, fast-growing wood species showed low resistance to decay by basidiomycetes.Tested low-density wood had a high affinity for water.Moreover, higher temperatures during vapour sorption have led to faster running times and less expressed sorption hysteresis.The longest time period to reach moisture equilibrium was found at a relative humidity of 90%.Among the observed wood species, Anthocephalus cadamba had the highest moisture content.Changes in vessel area at different relative humidity (RH) levels were in accordance with the adsorption and desorption stages.Concerning decay resistance, specimens exposed to brown rot (Coniophora puteana) showed a higher mass loss variation within the tested species, compared to white rot (Trametes versicolor).Based on the obtained results, durability classes of wood were classified into durability classes 4-5 (slightly durable-nondurable), with the exception of Acacia mangium, which was rated as very durable (class 1).The unexpected result for A. mangium was due to the small proportions of heartwood present in the specimens.Specimens exposed to T. versicolor exhibited significantly lower axial hardness, whereas those exposed to C. puteana showed visibly extensive degradation compared to T. versicolor.High K ion concentrations were detected in decayed regions of the wood specimens.These results are crucial in determining further steps employing strategies to improve observed wood species properties for structural components in various conditions.

Figure 3 .
Figure 3. Mean vessel area of wood specimens at different relative humidity (RH) steps.

Figure 4 .
Figure 4. Mass loss results of white rot and brown rot decay after 16 weeks exposure.

Figure 5 .Figure 6 .
Figure 5. Changes in axial hardness of decayed test specimens after 16 weeks exposure.

Figure 7 .
Figure 7. EDX analysis on a sample degraded by Trametes versicolor.

Table II .
Wood species used in the experiment, mean wood density (standard deviation in parentheses), and origin.
Figure 1.Illustration of the conducted decay tests.

Table III .
Durability classes (DC) of wood according to EN 350 (2016).

Table IV .
Testing forces for Brinell axial hardness test.

Table V .
Mean density (ρ), mean moisture content before (μMC be er incubation (μMC ae ), mean mass loss (μML) and durability classes (DC) based on median ML of wood specimens (standard deviation in brackets).waterlogged test specimens; "v" indicates the species exhibits high level of variability. *