Faculty of Agricultural, Veterinary and Forest Sciences
University of Chile
One of the major constraints in the use of wood is its degree of resistance against the action of innumerable destructive biological agents (primarily fungii and insects).
Indeed, due to its organic origin, wood is exposed during its useful life to organisms which, using the energy stored in the various constituents of the wood for their nutrition, provoke processes known as biodegradation (decay, stains and rot). The mechanisms involved in the deploymerization of such compounds have already been studied, although many questions still remain open (6, 8).
Natural durability of the wood is thus defined as the resistance which it shows against the aforementioned action by lignivorous fungii. Durability depends on the forest species involved, and on the xylematic portion involved (i.e., sapwood and/or heartwood).
As a general standard (2), it has been established that the natural durability of wood must be referred to the effects of lignivorous fungii on acount of their vast range of occurrence, their polyphagy and voracity, and their frequency, adaptation and the characteristics of its action.
The timber with low natural durability (9) faces serious limitations in its utilization in certain environments (8). However, with adequate preservation techniques this disadvantage can be offset significantly.
At the present time, the main preservation processes are based on the impregnation of the wood with biocides of proven effectiveness (6).
The aptitude of the ligneous material for its impregnation depends basically of its permeability, defined as the degree of resistance of a material to the passage of a fluid resulting from a pressure gradient (3).
There is no data regarding the natural durability of tamarugo and algarrobo wood, nor of their permeability, for species from the Tamarugal Pampa; furthermore, these technological characteristics of the genus Prosopis are seldom mentioned in international literature (1).
To determine the natural durability and permeability of tamarugo and algarrobo wood from the Tamarugal Pampa (Region I).
The laboratory test to determine the natural durability was based on British Standard BS No. 838, for agarmalt culture medium and the interpretation of the findings was based on the guidelines described by Findlay (1938).
The test fungii used were Polystictus versicolor (Linn) Fr. and Lentinus lepideus Fr., the former corresponding to white rot and the second to brown or gray rot, both included in the reference standards.
Although originally Lenzites trabea (Pers.) Fr. had been considered, also causing brown rot, it was finally decided to use L. lepidus on account of its greater resistance to fungitoxic organic products, as additional data obtained in this research made it possible to assume the existence of such compounds in extractives typical of the wood from these tree species.
Four test specimens with sapwood and four with heartwood were used, for each test fungus, age class and species. Thus, eight sapwood test specimens and eight heartwood ones were tried for 22-year-old algarrobo and an equal amount for 50-year-old algarrobo, with the same procedure for tamarugo.
The test specimens had the following dimensions: 5 × 2.5 × 1.5 cm, the length parallel to the fibers.
Once the test specimens, healthy and free of defects, had been selected, their basic density was determined to an accuracy of ± 0.01 gr/cm3. They were subsequently stabilized to an average moisture content of 25%, and then introduced into 500-ml Kolle flasks, wherein the corresponding test fungii had previously been cultivated.
Two test specimens were put into each flask, sapwood and heartwood, respectively, establishing contact with the mycelium of the respective fungus. The flasks were incubated in darkened controlled environment chambers for a period of four months (75–80% R.H.; 22 ± 1° C).
Once the trial period was over, the test specimens were sterilized and removed; the adhering mycelium was also removed and, after bringing the wood to anhydrous weight, the weight loss was calculated by established formulas (4, 7).
To determine the permeability of the wood, the trial was based on the work of Chudnoff (5), modifying the size and shape of the test specimens.
Two sapwood and two heartwood test specimens were made from each of the trees selected in the general sampling, each approximately two cm to the side (8 cm3). Twenty sapwood test specimens and a similar amount of heartwood test specimens were obtained with this procedure for each age class (2) of both woody species (algarrobo and tamarugo).
Once their green volume had been determined, with an accuracy of ± 0.1 mm, the test specimens were brought to anhydrous state in an oven at 105° C, up to constant weight. They were subsequently weighed with an accuracy of ± 0.1 gr.
Homogeneous groups of five test specimens, separated by type of wood (sapwood-heartwood), age, and species, were floated in water and subjected to vacuum and stabilization series (4) (60 cm Hg), the latter lasting as long as the vacuum stage.
At the end of each trial series, they were removed from the water and surface-dried before being weighed. Thus, the amount of water absorbed was determined, expressed in liters/m3 of wood.
RESULTS AND DISCUSSION
1. Natural Durability
Table 1 presents the findings for the resistance of the wood of both species to the action of the test fungii used. Basic average densities are provided for each type of wood tested in this trial.
It may be observed that sapwood natural durability is very low, independently of age and species, unlike that of heartwood, which is resistant to rot in both species.
On the other hand, a slight difference in the resistance of the wood to the type of rot was also detected, brown rot more active than white rot, particularly in heartwood.
This is probably due to the fact that L. lepideus is less vulnerable to the presence of normally fungitoxic organic extractives.
These extractives, present in higher amounts in heartwood, are responsible for the different durability of sapwood and heartwood in the same species (7).
The remarkably high density of these woods has no apparent relationship with their natural durability, which coincides with what Southam and Ehrlich reported years ago (10).
Table 2 classifies the species according to the guideline suggested by Findlay (9) for natural durability.
Resistance to rot of algarrobo and tamarugo sapwood and heartwood caused by the attack of Polystictus versicolor and Lentinus lepideus (test fungii)
|Basic density||Weight loss caused by fungus (%)|
|Sapwood Heartwood||Polystictus versicolor||Lentinus lepideus|
Classification of the wood according to their natural durability
|Species||Age||Type of wood||Quality|
|Algarrobo||(22–50 years old)||Heartwood||Hardy|
|Algarrobo||(22–50 years old)||Sapwood||Very susceptible|
|Tamarugo||(22–50 years old)||Heartwood||Hardy|
|Tamarugo||(22–50 years old)||Sapwood||Very susceptible|
The above table shows sapwood of both leguminous species to be very susceptible to biodeterioration, which would bar their utilization in humid environments on in contact with sources of fungal inoculum, unless they are subjected to a preservative treatment (impregnation). This alternative is feasible due to the high permeability of this type of wood.
Table 3 shows the amount of water absorbed by type of wood, age and species, in each trial series.
The sapwood of both species absorbs a considerably higher amount of water than the heartwood, with a tendency to increased permeability in wood from older trees (50 years of age).
Vacuum-time effect of atmospheric pressure treatment
|Species||Duration of the treatment|
|(min. in vacuum-min. in atmospheric pressure)|
|22 years old|
|50 years old|
|22 years old|
|50 years old|
The 30 – 30 series (vacuum - atm. pressure) constitutes a clear inflection point of the absorption curve, which indicates that the water absorption limit of this wood is being approached in the treatment (50% M.C. sapwood), with the exception of 50-year-old algarrobo sapwood.
Table 4 was made on the basis of the findings presented in Table 3, —modifying the Chudnoff standards (op. cit.), which are referred to the absorption of oily substances—, and summarizes the permeability of the species studied.
Its interpretation indicates that heartwood is definitely impermeable to water, unlike sapwood, which is perfectly permeable.
These data suggest, therefore, that treating sapwood with water-soluble preserving agents is perfectly feasible, and that greater possibilities exist of applying simple drying programs or processes.
Last, it is interesting to notice that the capacity for water absorption —and, therefore, the permeability of the wood— is independent of their respective densities.
Wood permeability to water
|(60 – 60 min. vacuum/min. atmospheric pressure)|
|Impermeable (0 – 144 1/m3):|
|Algarrobo (22 – 50 years old) Heartwood|
|Tamarugo (22 – 50 years old) Heartwood|
|Semi-permeable (145 – 288 1/m3)|
|Permeable (289 l/m3 and up):|
|Algarrobo (22 – 50 years old) Sapwood|
|Tamarugo (22 – 50 years old) Sapwood|
1. BOLZA E., KEATING W.G. 1972. African timbers. The properties, uses and characteristics of 700 species. CSIRO, Div. Build Res. Melbourne, Australia.
2. CARTWRIGHT K.ST.G., FINDLAY W.P.K. 1958. Decay of timber and its prevention. H.M. Stationary Office, London.
3. CHEN PETER Y.S. 1970. Wood permeability and its importance in preservation and drying. Southern Lumberman. U.S.D.A.
4. CUEVAS E., ULLOA I. Durabilidad natural del coigüe (Nothofagus dombeyii (Mirb) Oerst.) y tepa (Laurelia philippiana Looser). Asociación Chilena de Ingenieros Forestales. Actas de las Terceras Jornadas Forestales, Nov. 1967, Valdivia, Chile.
5. CHUDNOFF M. 1970. Water assay for treatability of wood. For. Prod. J. 20 (9): 103–106.
6. DARREL D. NICHOLAS, ed. 1973. Wood deterioration and its prevention by preservative treatments. Syracuse University Press. Vol. II New York, USA
7. DONOSO J., VELOSO E. 1968. Durabilidad natural de algunas especies forestales de interés comercial en nuestro país. U. de Chile, Esc. Ing. Forestal, Bol. Téc. No 17.
8. DONOSO J. 1972. Agricoles lignicoles de la Forêt Valdivienne du Chili. Introduction a l'étude de la biodétérioration du Bois. Thèse Doctorat Fac. Sciences Sorbonne U. de Paris. 128 pp. France.
9. FINDLAY W.P.K. 1983: The natural resistance to decay of some Empire Timbers. Emp. For. J. 17: 249 – 259.
10. SOUTHERN C.M., EHRLICH J. 1943. Decay resistance and physical properties of wood. J. For. 41:666:673.
Faculty of Agricultural, Veterinary and Forest Sciences
University of Chile
This study on the chemical composition of wood and bark of Prosopis tamarugo and Prosopis alba from the Refresco area of the Tamarugal Pampa, in Northern Chile, was carried out through a public tender on a project of the Production Development Corporation (CORFO) in 1981, and corresponds to one of the stages of an in-depth study on renewable natural resources within the framework of the Desert Zone Research Program. It was conducted concurrently with other research on the anatomic structures and physical and mechanical properties of these two major species of the Refresco Farm.
The purpose was to determine the chemical composition of heartwood, sapwood and bark of the stem's basal section, for 22- and 50-year old specimens of both species, the predominant ages at the area. Some chemical tests were also made to determine the responses of the wood, and the heartwood calorific value was determined as a function of its moisture content.
The only background data available was similar research conducted on Prosopis juliflora, particularly in the United States of America and Mexico, although no chemical studies on the wood of these species could be found.
The material consisted of 1 m long sections taken from the base of the stem of 5 trees of each species, for the age ranges of 22 and 50 years.
The material was subsequently separated into heartwood, sapwood and bark, dried and ground to a size of 0.25 mm. Each of the different parts was stored hermetically and kept in a dark place until the analyses were carried out.
The analyses were made with a material corresponding to an equal-weight mix of the ground parts of the 5 trees from each species and age range.
ASTM American Standards were applied in the following determinations:
Standard D-1109 was additionally applied for c), since the usual standardized methods are not effective for wood with high polyphenol content, embodying errors particularly in the determination of lignin. This has been observed by these authors in former research and it is, furthermore, considered as an accepted method by most researchers (Hillis 1962).
All the remaining determinations were made with usual techniques of analytical chemistry and, as the processes have not yet been officially standardized, they are briefly described below:
Tannins were estimated by means of the Stiasny method, which consists of preparing a water extract of the wood, adding 1% of sodium sulphite and thereafter inducing precipitation in an acid medium with formaldehyde. Insolubles are expressed as “tannins”, i.e., the set of polyphenols active towards the collagen. Concurrently, tannins were estimated by descending chromatography in W1 paper, one- and bi-dimensionally, with butanol-acetic acid-water (BAW) systems and diluted acetic acid, developing with phenol reagents. Those labelled as “non tannin”, i.e., phenols and others not active towards collagen, were estimated by difference.
Similarly, descending chromatography in paper was applied to determine the monosaccharides. A hydrolizate with acid in autoclave was prepared, neutralized with Ba(OH)2 and CO2, and concentrated in a vacuum.
The chromatograms were processed over a period of 60 hours with BAW 4:1:5 and developed with ammoniacal silver nitrate. In all cases standard controls were used for comparison, consisting of pure monosaccharides, specific for this analytical process.
As to the polyphenols, for which no standards were available, it was only possible to determine the amount of phenols present, approximating the group to which they belong by colorimetric reactions.
It must be kept in mind that, generally speaking, the determinations obtained by paper chromatography are of a qualitative nature, and only through the comparison with known standards can an approximate quantitative measure be provided, e.g. with the monosaccharides.
The development of the polyphenols was made with ultraviolet light and specific reagents, particularly iron chloride-potassium ferricyanide, ammoniacal silver nitrate and diazoted benzidine, among others.
The mineral elements in the ash were determined with an absorption spectrophotometer, and the acidity of the wood was determined taking the pH of an extract obtained by diluting 4 g of the ground material (mesh 0.25 mm) in 25 ml of distilled water, at 25° C, according to a method recommended by Browning (1967).
As a complement to the chemical analysis, some simple reactions were tested on the wood samples from both species which may help to their rapid characterization, namely, the direct application of the reagents on the surface of the wood.
Last, the higher calorific value of heartwood and bark from both species was determined with a Parr adiabatic calorimetric bomb. From this datum, the respective lower calorific value was computed, assuming 6% of hydrogen for the average gross elemental composition; the lower calorific power —also by computation— was expressed as a function of the moisture content.
RESULTS AND DISCUSSION
Tables 1, 2, 3 and 4 below show the general results obtained for the chemical composition of the two species studied.
The large amount of extractives present in these two timbers is striking, particularly those that are soluble in water and water-1% NaOH. This indicates the importance of polyphenols and acid polysaccharides in these woods. It may be estimated, in fact, that approximately 40% of the dry weight of tamarugo and 30% in algarrobo correspond to extractives, an extremely high proportion giving a special interpretation to the high density of these species.
It is also interesting to observe the high amount of extractives in water in young algarrobo sapwood, which in 50-year-old algarrobo appears as soluble in 1% NaOH.
Similarly, the findings for the inorganic matter present are very high, determined as ash, and even more striking in algarrobo, since, apart from the high value observed, the content in heartwood exceeds that of sapwood significantly.
According to data from research presently under way, tamarugo has a high content of silica, a trait that was not investigated particularly in this study, but basing on the calcium, potassium and magnesium findings, that possibility seems to be ruled out. It would be very interesting to clarify that doubt in future research.
As regards the structural substances of the cell wall, holocellulose and lignin, both species fall within ranges considered normal for most of the broadleaf species. However, the lignin proportion detected may be considered relatively low in comparison with other Prosopis described but this fact could be explained by the ramified and tortuous way these trees grow, originating significant amounts of tension wood.
General findings of tamarugo chemical composition
|Constituent (%)||Age (years)|
|Extractives 1% NaOH||6.93||8.82||17.94||10.40||11.98||20.41|
|Methoxyl groups **||—||14.60||—||—||16.00||—|
* Percentages expressed on the basis of extractive-free anhydrous wood.
** Percentages expressed on the basis of anhydrous lignin.
*** Estimated by the Stiasny method. Non tannis obtained by difference from the extract.
General findings of algarrobo chemical composition
|Constituent %||Age (years)|
|Extractives NaOH 1%||8.67||12.87||15.96||14.14||10.64||14.97|
|Methoxyl Groups **||—||15.75||—||—||16.50||—|
|Tannins ***||1.31||4.00||10.24||< 1||7.65||8.90|
|Non tannins||16.69||11.45||16.12||not computed||11.98||16.11|
* Percentages expressed on the basis of anhydrous wood free of extractables.
** Percentages expressed on the basis of anhydrous lignin.
*** Estimated by the Stiasny method. Non tannins obtained by difference of the extract.
An algarrobo trunk (Argentina). (Photo D. Huss.)
Elemental composition of 50-year-old tamarugo and algarrobo heartwood ash
|Element||Heartwood 50-year-old tamarugo||Heartwood 50-year-old algarrobo|
Monosaccharides and uronic acids identified in tamarugo and algarrobo heartwood for the age ranges of 20 and 50 years
|20 years||50 years||20 years||50 years|
|Glucose||+ (60)*||+ (60)||+ (65)||+ (60)|
|Xylose||+ (10)||+ (15)||+ ( 5)||+ ( 5)|
|Manose||+ (10)||+||+||+ ( 5)|
|Arabinose||+||+ ( 5)|
|Uronic acids||+ (15)||+ (20)||+ (25)||+ (25)|
(*) Figures in parenthesis indicate an estimate of the compound's share in the total sugars. Those without a figure are present, but in such small and inaccurate amounts that the estimate was omitted.
Tamarugo has a much higher tannin content than algarrobo. Generally speaking, it must be accepted that if the standard ALCA method were used, the figures would be even higher, according to what both Browning (3) and Wise (9) indicate. Tannin contents of 15% —as in 50-year-old tamarugo heartwood—are found in the species exploited commercially for that purpose, such as E. wandoo (12%), A. catechu (15%), or S. lorenzii (16.5%) (Hillis 1962).
The bark of both species is even richer in active tannins, as they even exceed R. mangle (20–22%), A. dealbata (20%) and P. lingue (17–22%) (Hillis, op. cit.).
The analysis of the tannins showed them to be condensed tannins derived from cathequine and gallocathequine. The so-called “non-tannins”, i.e., not active towards collagen, are constituted mostly by polyphenols belonging to the flavonoid group.
The chromatographic analysis of the fractions soluble in alcohol/benzene and absolute alcohol showed the presence of a considerable variety of these products, as shown below:
|Number of phenols|
In the low-molecular-weight tannin and polyphenol fraction, at least three of them were found to be common to both species, accounting for a significant amount of the respective total. Furthermore, from their chemical reactions, UV fluorescence and Rf values, it may be stated that they are leucoantocyanines (flavan-3, 4-dioles), flavonoles and cathequines.
The genus Prosopis is one of the genera containing “kino”, an exudate generally produced by the tree to neutralize the action of some type of aggresive agent. For this reason, an analysis was made of a black sticky mass exuded by an algarrobo branch attacked by insects, found on the ground.
|This analysis showed the following results:|
|Solubility in ether||0.19%|
|Solubility in ethyl acetate||2.82%|
|Solubility in methanol||73.41%|
|Initial moisture content||16.10%|
A complex mix of phenols was detected with the chromatography of the methanolic extract; no free sugars were detected, although their presence was not determined as glucoside. It is noteworthy that, among others, two polyphenols were found in this material of the same kind as those in the wood. The high ash content is derived from the sand adhering to the sample.
As to the sapwood, the chromatographic analysis of the “non tannin” fractions and of those soluble in water and 1% NaOH had special relevance in the case of algarrobo. A product was extracted which, while it gave no positive reaction with Fehling liquor, when hydrolized with certain energy it showed positive ractions in sugars. Its chromatography was very diffuse, but it was possible to separate uronic acids, glucose and one monosaccharide which could correspond to pentose, with very similar Rf value as that for arabinose.
The above findings confirm the presence of gums and tannins, particularly in algarrobo with healthy wood, a situation which is substantially altered when, responding to a pathology, the tree exudes “kino”, modifying completely the chemical composition. In this respect, Hillis (6) cites examples of eucalyptus in Australia containing bags of up to 45 liters of “kino”, as a defense of the tree against aggresive agents of various kinds.
To summarize, it may be said that healthy algarrobo contains a small amount of “kino”, but a large amount of gums, which are acid polysaccharides. This agrees with the opinion of several authors, Hillis among them, who state that the gums, in this species of Prosopis, have a back-up function, serving at the same time as precursors for the in situ formation of the polyphenols making up the “kino”, since it is an accepted fact that the phenols are not translocated, the glicosides normally located in the sapwood and the aglucomes in heartwood.
Another characeristic determined for these species was their acidity. As stated previously, a method recommended by Browning (3) was adopted for this purpose, with the following findings:
It may be concluded that tamarugo wood is much more acid than algarrobo's although both barks are of the same type and much more acid than their respective woods.
Practically all the characterizing reactions made with the heartwood of both species showed the same results, with the exception of the action of concentrated sulphuric acid, which had a different response in tamarugo by forming a distinct clear deposit, not readily explained by its greater calcium content, particularly considering that algarrobo has a higher total organic content.
Positive reaction was observed with the Maüle test, Wiesner reaction, diazoted benzidin and ferric chloride in alcoholic solution.
Last, the calorific value of the heartwood and bark of both species was tested, for 50-year-old trees, computing the lower calorific value and its relation with the moisture content. The findings thereof are presented in Table 5 below.
Higher and lower calorific value of 50-year-old tamarugo and algarrobo heartwood and sapwood and its relation to moisture content
|Higher calorific value anhydrous wood (kcal/kg)||5,065||4,760||4,720||4,702|
|Lower calorific value anhydrous wood (kcal/kg)||4,801||4,496||4,456||4,438|
|Lower calorific value wood with 10% moisture (kcal/kg)||4,310||4,032||3,996||3,980|
|Lower calorific value wood with 20% moisture (kcal/kg)||3,901||3,647||3,613||3,598|
|Lower calorific value wood with 30% moisture (kcal/kg)||3,555||3,320||3,289||3,275|
|Lower calorific value wood with 40% moisture (kcal/kg)||3,258||3,040||3,011||2,998|
This study may be considered a first open door toward knowledge on the composition and chemical properties of tamarugo and algarrobo wood.
Sources of inaccuracy —additional to the experimental errors inherent to this type of research—, are the natural variability of the properties of any woods, and, in this case, the “abnormality” to be expected from the presence of tension wood derived from the gnarled growth of these species.
The general findings, however, provide an image of these two forest species which can be used as a basis for those planning to establish plantations and utilize their wood in the arid and semiarid zones of the North of Chile or elsewhere.
As can be easily concluded from this paper, there is much still left to clarify, verify and learn about the chemistry of tamarugo and algarrobo wood. Among the more relevant subjects, in the opinion of the authors, are the studies on gums and polyphenols, which are a noteworthy response pattern of these species against the action of pathogens and aggressive agents.
Also considered of interest in the short term is the study of these species growing under different types of climate and soil, as they are being introduced in other regions where the original plant cover has been decimated, to check erosion and provide a range of products useful to the human settlements in those areas.
1. AUSTRALIAN COUNCIL FOR SCIENTIFIC AND INDUSTRIAL RESEARCH. 1947. The chemistry of wood. Trade circular No 28. Division of Forest Products. Melbourne.
2. BLOCK R. 1958. Paper chromatography and paper electrophoresis. Academic Press Inc. New York, USA.
3. BROWNING B.L. 1967. Methods of wood chemistry. Interscience Publishers. New York, USA.
4. COVACEVIC R. 1979. Poder calorífico de pino insigne y de otras especies forestales. Deg. Thesis, Universidad de Chile, Facultad de Ciencias Forestales. (Mimeo).
5. FIESER L., FIESER M. 1948. Química Orgánica. Editorial Atlante S.A., México.
6. HILLIS W.E. 1982. Wood extractives. Academic Press. New York, USA.
7. HARBONE J.B. 1964. Biochemistry of phenolic compounds. Academic Press. London, England.
8. PRIDHAM J.B. 1959. Phenolics in plants in health and disease. Pergamon Press Ltd. London, England.
9. WISE L. 1946. Wood chemistry. Reinhold Publishing Corp. New York, USA.