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Introduction

World-wide there are more than 60000 known wood species. In the Amazonian forests approximately 40 % of the world’s wood species can be found (Cazzolla Gatti et al. 2022), a large part of which can be found in Perú. There are approximately 237 wood species which could be commercially exploited (SERFOR 2016). Currently only a small part of this huge potential is processed into wooden products for specific application while a greater part is processed into low value products, e.g. pallets, packaging, formwork, scaffolding, among others. (PCM, USAID, US Forest Service 2021) or not used at all. In Perú, the Andes mountains separate the Amazonian Forest from the densely populated coastal regions where the major part of the wood processing and furniture manufacturing companies are located. In the Amazonian part of Perú the sawmills are located close to the rivers. Log supply is almost to 100 % based on upstream rafting and ship transport. All sawn timber products for export or for being used in the densely populated coastal regions need to be transported by trucks over the Andes (Mejía 2015, CNF 2015, DIREPRO 2012).

Most of the Peruvian timber production is consumed domestically. According to recent estimates no more than 10 % of national production is exported (USAID 2019). According to FAO statistics (FAO 2020) 735000 m³ non-coniferous sawn timber were produced in Perú in 2020, of which 72000 m³ were exported, representing an export value of US$ 45 million. Perú imported 122000 m³ coniferous sawn timber in 2020, representing an import value of US$ 35 million. This coniferous timber is mainly used by the furniture industry, even though there is an abundance of broadleaved forest resources which could be used for the same purposes (USDA 2017).

For increasing the commercial utilization of the Peruvian wood resources, there is an urgent need to improve the knowledge about the properties and technical features of the lesser-used species. Information about wood/water relations, such as shrinkage properties, sorption characteristics, and electrical properties are crucial when searching for fields of application and establishing appropriate methods of harvesting, sawing, drying, processing, storage prior to further processing, and long-distance transport of wood products in case of export.

Materials and methods

Selection of species

Due to the huge number of lesser-used tree species in the Amazonian part of the Peruvian forests and due to the limited resources in terms of time and manpower, a selection of species had to be carried out to conduct the research tasks. In this study, a multi-criteria decision analysis approach (Saaty 1990) was used for the selection process of 15 species out of several hundreds. First, a long list of selection criteria was set up. The five most important criteria and the associated weighing factors were defined by five experts attending a consensus meeting. The selection criteria and the corresponding weighing factors were: frequency of occurrence (30), lack of physical and mechanical information (30), possible implications for local markets (15), possible importance for export (15), and market price (10). The numbers in round brackets are the weighting factors (sum = 100). The experts rated the five criteria for each of the species on the list by giving up to 10 points (0 = not relevant, 10 of utmost importance) to each criterion. Multiplication of points with weighing factor yielded the total score (max = 1000). For this study, the 15 species with the highest scores were selected. To make sure that the natural variability between different trees and in individual stems is considered in the determination of the wood properties, the test material for production of the specimens had to comprise: sections from at least three different trees, sections from butt and middle locations in the stem, specimens taken from both, sapwood and heartwood, no previous treatment with preservatives, no prior drying at elevated temperatures. The list of selected species is presented in Table 1.

Physical properties

Investigations focused on the wood/water relations with the following properties and behaviors examined: sorption characteristics, shrinkage behavior, electrical properties. Standardized methods for determination of moisture content (ISO 13061-1 AMD 1:2017-06 2017), density (ASTM D2395:2017 2017, ISO 13061-2:2014-10 2014) and shrinkage properties (ISO 13061-13:2016-11 2016, DIN 52184:1979-05 1979) were used.

Sorption characteristics

According to standard procedures, scanning sorption isotherms at 23 °C+/-2 °C were determined by exposing small specimens (20 mm x 20 mm and 6 mm in longitudinal direction) to a series of constant climate conditions starting with an average moisture content (MC) of approximately 12 %. For each species 10 specimens were tested (n=10). Absorption was carried out in several absorption steps ending at 98 % relative humidity (RH), followed by various desorption steps ending with 33 % RH. Finally, after determination of the oven dry weight of the samples, the equilibrium moisture content of the samples at the end of each climate step was calculated and the sorption isotherms were visualized.

Parallel to this standard procedure, a dynamic water vapor sorption device for determining sorption isotherms (ProUmid, Germany) was used by a project partner. Here, small samples of wooden particles (millings) comprising a mix of various trees and locations in the stems were placed in small containers. 19 samples from different tree species were placed onto a rotating rack (round table) and exposed to a series of pre-programed climate conditions. By continuous sequential weighing of the loaded containers moisture equilibrium of their content was determined.

Shrinkage behavior

To describe the shrinkage behavior quite a number of different measures can be chosen (Noack et al. 1973). Sargent (2019) gives an overview about procedures and characteristic values which are described in various standards and textbooks.

Climate chambers are indispensable to determine wood/water relations. However, climate chambers are expensive due to high cost for investment, maintenance and energy. In order to conduct the investigation in a reasonable time span, several climate chambers were required to provide different climate conditions at the same time. As neither several climate chambers nor sufficient funds for purchasing additional chambers were available, two insulated chambers with internal air circulation and temperature control were allocated in a laboratory room with well controlled temperature. In each chamber six small, and hermetically sealed, removable containers were positioned. Each container was equipped with a small fan for internal circulation of air. The relative humidity of the circulating air was conditioned by means of a basin filled with a saturated salt solution placed at the bottom of the container. The wooden samples were positioned on a rack approximately 4 cm - 5 cm above the surface of the saturated salt solution. By keeping the temperature of the chamber constant and keeping the salt solutions in saturated condition, various constant relative humidity (RH) conditions could be provided at the same time. Figure 1 shows the overall setup, Figure 2 the design of the small climate containers. In Table 2 the salt solutions used and their corresponding relative humidity are listed.

For determination of shrinkage behavior, specimens (n=10 for each species) with dimension 20 mm x 20 mm x 10 mm were submitted to the conditioning steps illustrated in Figure 3. To identify the point of reaching equilibrium condition in the specimens, consecutive weighing of the test samples was carried out. Dimensional changes were measured using calipers when equilibrium was reached.

Saturation with water to guarantee maximum swelling was achieved by submerging the specimens under water in a vacuum-tight container and extracting air by several vacuum cycles as shown in Figure 4.

Oven drying according to ASTM D4442-20 (2020) at the end of all climate steps was applied to calculate moisture content (MC) at various climate conditions.

The following features were determined:

Maximum tangential shrinkage (Ct_tan) in percent (between MC_sat and MC₀)

Maximum radial shrinkage (Ct_rad) in percent (between MC_sat and MC₀)

Maximum volumetric shrinkage (Ct_vol) in percent (between MC_sat and MC₀) is achieved by calculating the sum of tangential, radial, and longitudinal shrinkage. But, because of the very small values of longitudinal shrinkage, the sum of tangential and radial shrinkage will deliver almost the same result.

Coefficient of shrinkage h in percent/percent RH

Ratio of shrinkage q in percent/percent MC

Shrinkage anisotropy (ratio of tangential to radial shrinkage)

Electrical properties

For measuring moisture content (MC) with hand-held MC-meters the functional relation between electrical resistance and moisture content must be known. Electrical resistance was determined by placing stainless steel screws spaced 30 mm in small wooden blocks. Electrical resistance was measured using a commercial wood moisture meter (Brookhuis FMD6) equipped with the feature that indicates absolute values of electrical resistance instead of transformed moisture content readings. The instrument’s accuracy was checked by comparing the instrument readings with those of a high precision tera-ohm-meter (Knick H12) while measuring the electrical resistance of six fixed resistors ranging from 1,87 MOhm to 5000 MOhm.

To establish the electrical resistance / moisture content relation, wooden blocks (n=10) of each species (15) with the dimensions 30 mm x 45 mm x 25 mm were exposed to five different climates, which cover the whole range of indoor climates experienced in furniture or building applications. The absolute electrical resistance values obtained were transformed into ln (resistance). For establishing the functional relation, the statistical functions of MS-EXCEL were used. Selecting the logarithmic function yielded the best results.

Results and discussion

Sorption characteristics

Figure 5 shows the sorption isotherms of the 15 Peruvian wood species included in the investigation comparing the results achieved by the standard method. All sorption isotherms show the typical shape of isotherms of hygroscopic materials. Between approximately 35 % and 85 % a hysteresis between absorption and desorption is visible. The hysteresis determined in our experiments ranged between 1,6 % and 2,6 % at 65 % relative humidity. These average hysteresis values are lower than those reported by other researchers (Fredriksson and Thybring 2018, Simo-Tagne et al. 2016) who determined sorption isotherms of other wood species. This probably is due to the fact that in our experiments some fluctuations in temperature and relative humidity could not be avoided. As a consequence, our desorption and absorption lines did shift towards the mean line between de- and absorption. In the lower range of relative humidity (until approx. 55 %) all species examined showed almost the same behavior. The mean values between desorption and absorption at 65 % relative humidity (EMC_65) and 98 % humidity (EMC_98) are listed in Table 3. Pronounced differences between the 15 species are only present at higher relative humidity (98 %). Here the equilibrium moisture contents (EMC) vary between 16,6 % (Andira macrothyrsa) and 23,0 % (Otoba parvifolia).

Shrinkage behavior

Table 4 lists the results of the various properties describing the shrinkage characteristics of 15 Peruvian wood species. In many cases documentation for lesser-used species only provides information about maximum shrinkage values in tangential and radial direction. Sometimes, only information on volumetric shrinkage without differentiating the anatomical direction is provided. For practitioners the use of h- and q-values is more important as its use is to predict the dimensional changes of wooden parts during usage. The ratio of tangential and radial characteristic values is a good measure for anisotropy and dimensional stability. Low absolute shrinkage values in combination with low tang/rad-ratios characterize wood species with excellent dimensional stability, while high tan/rad-ratios indicate a high propensity for distortion and dimensional change under use conditions in regions with pronounced fluctuations of relative humidity (e.g. low indoor RH in winter due to central heating or RH variations due to alternating dry and humid seasons). Low equilibrium moisture contents at high relative humidity combined with good dimensional stability qualify species for high quality indoor and outdoor applications.

Electrical properties

In Table 5 the regression parameters of the functional relation between electrical resistance and wood moisture content for 15 Peruvian lesser-used species are presented. In Figure 6a and Figure 6b examples of the graphical presentation of measured values and the corresponding curve fitting is shown for the two species Clarisia racemosa (Mashonaste) and Apuleia leiocarpa (Ana caspi). What can easily be seen from the graphical presentation is that there is some scattering of values around the regression curve, even though the correlation coefficient R² is very high. This scattering reflects the natural variation of electrical properties, both within a species and between individual specimens, due to the varying location in the stem and sapwood/hardwood fraction.

In industrial practice, measuring uncertainty results from the fact that boards and wooden products often exhibit moisture gradients. As a consequence, when measuring moisture content with resistance type moisture meters, obtained results are strongly influenced by the presence of moisture gradients, type of measuring electrodes used (insulated or non-insulated) and penetration depth of the electrodes. In order to avoid such uncertainties in the present investigation a lot of attention was given to avoid moisture gradients in the test specimen. Only when equilibrium conditions determined by securing constant weight between consecutive weight measurement were achieved, the electrical resistance measurement was carried out. Therefore, scattering of values in the graphical presentations is only attributable to natural variation of electrical properties within the species and not to moisture gradients.

For practitioners, knowledge about the functional relation between electrical resistance and wood moisture content is less relevant. The practitioner wants to know how to adjust the setting of his moisture meter to obtain reliable results. To secure this, the functional relations shown in Table 5 must be compared with the functional relations of the setting parameters of the instrument. The manufacturers of the commercial wood moisture meters provide a list of species and recommend corresponding setting parameters for proper use of the instrument. Many of the instruments provide the option of choosing Group 1, 2, 3, or 4. Others offer more choices either by a two-parameter X/Y-setting or by numerical codes each of which may represent a specific resistance/moisture content relation.

Figure 7 shows two examples of species for which (a) an almost perfect fit between instrument setting and functional resistance /moisture content relation could be found, while for (b) the best fitting instrument setting or code only results in good moisture content values around 12 % moisture content. Using a non-appropriate instrument setting will result in false moisture readings.

Conclusions

Determination of wood/water relations is important for describing the behavior of wood during drying, and also during further processing and usage. Proposing and introducing lesser-used species to the market will only be successful if potential users can judge the suitability of a species for a specific application. Data on physical properties and wood/water relations (e.g. density, shrinkage behavior and sorption characteristics) as presented in this research study can help to fill the gap on information for the 15 lesser used Peruvian species. Such data can be incorporated in data bases like macroHOLZdata (GFF-Holzwissenschaft 2022), Holzdatenbank (TU-Dresden 2018), or Wood Properties Techsheets (USDA 2022), which already provide information on wood properties for a large number of species. However, for proposing applicability of a lesser-used species for specific end-uses, wood/water relations are only part of the information needed. Without data on mechanical behavior, chemical composition and durability against fungi and wood destroying insects, the picture is not complete, thus further research is needed. Detailed wood property tables are necessary to support the search for alternative species for specific uses and to replace endangered species on the timber markets.

Acknowledgements

This research has been financed by CONCYTEC-World Bank Project 8682-PE “Mejoramiento y Ampliación de los Servicios del Sistema Nacional de Ciencia Tecnología e Innovación Tecnológica”, through its executing unit PROCIENCIA (Contract Nº 046-2019-FONDECYT-BM-INC.INV). The study was carried out at the Centro de Innovación Productiva y Transferencia Tecnológica de la Madera (CITEmadera Lima) of the Instituto Tecnológico de la Producción (ITP).

The authors would like to thank the members of the entire working group at CITEmadera Lima for their support, without which the successful implementation of the project in times of the COVID pandemic would not have been possible.

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Author notes

^♠Corresponding author: welling@holz-expertise.de

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