Exceptional physical properties of texas mesquite wood

Forest Ecology and Management, 16 (1986) 149--153 Elsevier Science Publishers B.V., Amsterdam -- Printed in The Netherlands

149

Exceptional Physical Properties of Texas Mesquite Wood DEWAYNE WELDON Texas Forest Service, Lufkin, TX 75901 (U.S.A.)

(Accepted 11 March 1986)

ABSTRACT Weldon, D., 1986. Exceptional physical properties of Texas mesquite wood. For. Ecol. Manage., 16: 149-153. The mesquite tree has long been considered a pest tree in the southwestern United States. Only in recent years have the exceptional physical properties of the wood gained the widespread recognition that it deserves in this country. The natural colour, grain pattern and texture of mesquite wood can equal that of the finest hardwoods. High strength values give mesquite wood advantages over many other hardwoods. Mesquite wood has no peers when it comes to dimensional stability. Mesquite wood also excells in its ability to resist degredation by decay organisms. Although some obstacles remain, the future for mesquite wood appears bright.

T h e T e x a s F o r e s t P r o d u c t s L a b o r a t o r y is o p e r a t e d b y t h e F o r e s t P r o d u c t s D e p a r t m e n t o f the Texas F o r e s t Service. T h e T e x a s F o r e s t Service was established b y t h e Texas Legislature in 1 9 1 5 as a p a r t o f t h e Agricultural and M e c h a n i c a l College o f the state. T h e l a b o r a t o r y is still an integral part o f t h e T e x a s A & M University S y s t e m . Since t h e very beginning, T h e T e x a s F o r e s t Service has been interested in the u t i l i z a t i o n o f m e s q u i t e trees. Bulletin N o . 3 b y F o s t e r a n d Krausz ( 1 9 1 7 ) r e p o r t e d o n p o t e n t i a l uses o f the tree. While research o n the utilization o f m e s q u i t e w o o d has n o t been one o f o u r m a j o r objectives, t h e r e has been a c o n s t a n t interest f r o m t h a t early b e g i n n i n g u n t i l n o w . Marshall ( 1 9 4 5 ) rep o r t e d o n chemical studies d o n e b y this l a b o r a t o r y a n d reviewed p o t e n t i a l uses f o r m e s q u i t e . Several limited research projects have b e e n c o n d u c t e d since 1 9 6 4 , and t h e y are c o n t i n u i n g . A n e w p r o j e c t is c u r r e n t l y g e t t i n g u n d e r w a y t o l o o k m o r e closely at the e x c e p t i o n a l p r o p e r t i e s o f m e s q u i t e w o o d using s o m e 100 m e s q u i t e trees c u t near C u e r o , Texas. M a n y q u e s t i o n s have been a n s w e r e d b y t h e s e studies b u t even m o r e have been p o s e d and it has been s h o w n t h a t o n e o f t h e m o s t e x c e p t i o n a l properties o f the m e s q u i t e tree is its e x t r e m e variability.

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150 There are some 40 to 50 valid species of Prosopis, although m a n y others and numerous varieties have been named (Burkart, 1976). With the exception of a few species in Africa and Asia, all others are f o u n d in the New World, where the genus originated. Whether considered friend or foe, mesquite is a remarkably adaptable tree which can grow in almost any soil type. Mesquite tolerates wide extremes of temperatures from desert heat to areas of upwards of 100 days of frost per year. Rainfall requirement ranges from a m i n i m u m of 15 cm y-1 to upwards o f l m y -~. The best known of the northern species is the c o m m o n mesquite Prosopis glandulosa, with several varieties growing in the southwestern United States and Mexico. Most of our work has been with honey mesquite, Prosopis glandulosa var. glandulosa (Tort). Honey mesquite exhibits a wide variety of growth forms f r o m single stemmed trees upwards o f 61 cm in diameter and 15 m tall to multi stemmed shrubs less than 1 m tall, with every conceivable form in between. The physical properties of the wood from these trees seem to vary almost as much. The wood of the mesquite tree is a very remarkable resource. The natural beauty of this dense, hard, and strong w o o d is comparable to t h a t of the best furniture woods. The sapwood has a pale yellowish white colour and is restricted to a narrow band which changes very little in width, no m a t t e r what size or age the tree is. The heartwood in freshly cut trees m a y range from a yellowish brown through shades o f grey brown to deep reddish, almost purple brown. As it is exposed to ultraviolet light rays, however, it changes to a fairly uniform warm dark brown, no matter what its original colour was. Mesquite wood is medium to coarse textured, and the grain is irregular for the most part. It is easy to work, finishes s m o o t h l y and takes a high natural polish. When dried it is exceptionally stable and quite resistant to insects and decay. All in all, it is a highly prized wood for furniture, flooring, doors, decorative trim, carving and novelty products. The major deterrent to widespread use of mesquite wood is the nature of the tree itself. While a few large specimens do exist, most mesquite trees are fairly small as timber trees go. The main t r u n k is short, branching freely and producing a widely spreading crown. Marketable logs are typically no more than 2 m long and 15 to 30 cm in diameter in most parts of Texas. The incidence of natural defects is high in mesquite. Rogers (1984) studied the effect of these defects on lumber recovery. Worm holes, ring shake and areas of decay are quite c o m m o n . The value of the few good trees which do occur, however, can usually pay for clearing away the inferior trees and this has historically been the goal of most Texas land owners with stands of mesquite trees. Extensive strength studies were undertaken at the Texas Forest Products Laboratory by Westbrook and Adams (1972). Some 100 logs were harvested from the Miller Ranch near Falfurias, Texas and transported to Lufkin,

151 Texas w h e r e t h e y were s a w e d i n t o l u m b e r . Small clear test s p e c i m e n s were p r e p a r e d f r o m t h e l u m b e r a c c o r d i n g t o A S T M s t a n d a r d D 143. F o l l o w i n g this standard, the principal s t r e n g t h tests were p e r f o r m e d o n s a m p l e s c o n d i t i o n e d to 10% m o i s t u r e c o n t e n t and o n samples in the green state as c u t f r o m the tree (65% m o i s t u r e c o n t e n t ) . Results o f these tests are s h o w n in Table 1. TABLE 1 Strength properties of mesquite Test

Static bending

Compression parallel to grain

Strength property

Unit of measure

Moisture content 65%

10%

Fiber stress at proportional limit Modulus of rupture Modulus of elasticity

kPa kPa kPa

42 800 80 700 7 870 000

60400 110000 9520 000

Max. crushing strength

kPa

43 200

56 600

Compression perpendicular to grain

Fiber stress

at proportional limit

kPa

19 300

23100

Shear parallel to grain

Max. shearing strength

kPa

9 560

15 000

Tension perpendicular to grain

Max. tensile strength

kPa

Side hardness

Load to embed 1.12 cm ball to 1~ its diameter kg

End hardness

Load to embed 1.12 cm ball to 1~ its diameter kg

__a

1 010

967

4 850

1

060

1 070

aNot determined.

S o m e o f t h e strength c h a r a c t e r i s t i c s f o u n d in this s t u d y were very interesting. The hardness values d e t e r m i n e d here i n d i c a t e d t h a t m e s q u i t e w o u l d be especially desirable for f l o o r i n g and o t h e r such uses w h e r e a high degree o f resistance t o wear, s c r a t c h i n g and d e n t i n g is i m p o r t a n t . This has indeed, been the case and several f l o o r i n g m a n u f a c t u r e r s are n o w o p e r a t i n g in Texas. All was fine until we s t a r t e d getting requests for assistance to d e t e r m i n e w h y some m e s q u i t e floors w e r e s h o w i n g excessive wear and marring. During t h e past 5 years, several a d d i t i o n a l hardness studies have been carried o u t . Results have varied drastically. Values o f less t h a n 500 kg and over 1 4 0 0 kg have been o b t a i n e d f r o m selected samples. M o s t o f t h e subseq u e n t tests indicate, h o w e v e r , t h a t t h e side hardness and e n d h a r d n e s s values f r o m t h e original s t u d y m a y be a b o v e the species average. E x t e n s i v e

152 testing on a large number of samples from widely varying geographical locations will hopefully give a more accurate average hardness figure for mesquite. Such tests are currently u n d e r w a y at our laboratory. Another important and exceptional property of mesquite is its dimensional stability. Total shrinkage from green to oven dry condition for mesquite and two c o m m o n commercial species, White oak (Quercus alba) and Pecan (Carya illinoensis) is shown in Table 2. This is one p r o p e r t y t h a t does not seem to vary appreciably. Numerous studies have been c o n d u c t e d by our laboratory during recent years and mesquite invariably shows very low shrinkage values when compared to other wood. Volumetric shrinkage has consistently been in the 3 to 5% range, with the highest individual test just over 8% in these studies. Total volumetric shrinkage for most commercial domestic hardwoods ranges from a b o u t 12% up to 19% (FPL, 1974).

TABLE 2 Shrinkage comparison Species

Radial shrinkage (~)

Mesquite 2.2 White oak 5.3 Pecan 4.9

Tangential shrinkage (%)

Volumetric shrinkage (%)

2.6 9.0 8.9

4.7 15.8 13.6

Coupled closely to this dimensional stability are the exceptional drying characteristics of mesquite wood. Mesquite has historically been considered a difficult wood to dry. Wiley (1977) demonstrated, however, t h a t 2.54 cm mesquite lumber can be dried quickly and easily with m i n i m u m degrade. Practical experience by local wood workers indicates, however, t h a t 5.08 cm thick lumber does n o t fare so well. The current series of studies under way at our laboratory will evaluate the drying characteristics of 5.08 cm thick lumber. The final characteristic of mesquite wood that has been considered as exceptional is its natural durability. The high extractives c o n t e n t of mesquite (Adams, 1972) is presumably responsible for resistance to deleterious effects of insects and fungi. At any rate, mesquite w o o d has been renowned for its longevity in areas favourable to decay. Fence posts and paving blocks are two uses which have been i m p o r t a n t in the past. Those of us who know and appreciate the wood are often discouraged because it is not utilized to the full~st extent possible. As more products are available to the general public, the exceptional properties we appreciate will c o m m a n d attention and respect for mesquite.

153 REFERENCES Adams, D.G., 1972. Mesquite: extractives, density, moisture content, and swelling. Texas Forest Products Laboratory Misc. Report. Lufkin, TX,. 8 pp. Burkart, A., 1976. A monograph of the genus Prosopis (Leguminosae, subfam. Mimosoideae). J. Arnold Arbor. Harv. Univ., 5: 2 1 9 - - 2 4 9 , 4 5 0 - - 5 2 5 . Forest Products Laboratory, 1974. Wood Handbook. USDA Agric. Handbook No. 72. Madison, WI, 421 pp. Foster, J.H. and Krausz, H.B., 1917. General survey of Texas woodlands including a study, of the commercial possibilities of mesquite. Bulletin 3, Dept. of Forestry, A. and M. College of Texas, Lufkin, TX, 26 pp. Marshall, E.D., 1945. Utilization of mesquite. Texas Forest Products Notes No. 3. Texas Forest Service, Lufkin, TX, 14 pp. Rogers, K.E., 1984. Lumber and clear cutting recovery from mesquite (Prosopis spp.) Logs. Texas Forest Service Publication No. 135, Lufkin, TX 8 pp. Westbrook, R.F. and Adams, D.G., 1972. Mechanical and physical properties of mesquite. Texas Forest Products Laboratory Misc. Report. Lufkin, TX, 35 pp. Wiley, A.T., 1977. Moisture relations of mesquite wood. Texas Forest Service Publication No. 113. Lufkin, TX, 6 pp.