The structure of wood is a fascinating subject with some amazing practical applications. By analyzing the cellular structure and annual growth rings of wood it is possible to determine the ages of trees and shrubs and even gather data concerning past climates. Ancient creosote bushes (Larrea tridentata) in the Mojave Desert have been found to be well over 10,000 years old. In fact, the cell structure of these slow-growing shrubs is so dense that the dry wood actually sinks in water. Comparative studies of the wood of living and recently deceased trees with logs found in abandoned Indian villages have been used to date extinct cultures. One of the most damaging pieces of evidence that led to the conviction of Bruno Hauptmann, the man accused of kidnapping Charles A. Lindbergh's son in 1932, was the ladder used in the crime. Microscopic examinations of the ladder revealed that it was homemade and from precisely which kinds of wood. One rail on the ladder was found to match a floorboard from Hauptmann's attic, and it was proven that the rail and board were originally from the same piece of wood.

There are many native hardwood trees and shrubs in San Diego County, California with wood that is very dense but still floats in water because its specific gravity is less than 1.0. At least two species, hollyleaf cherry (Prunus ilicifolia) and Engelmann oak (Quercus engelmannii), have wood that will barely float. The following table compares some of the native woods in San Diego County, California with woods from other areas, including insular and northern California, the eastern United States and tropical regions:

Many of the native hardwood trees and shrubs in the southwestern United States were used by Indians and early settlers in a variety of ways. Logs of the hardest woods, including desert ironwood (Olneya tesota) and mountain mahogany (Cercocarpus betuloides), were used as fuel, burning for hours and giving off intense heat before they turned into fine white ashes. The wood of native oaks, particularly the canyon live oak (Quercus chrysolepis), were used by early pioneers for wedges (with iron-rimmed heads) for splitting redwood ties, and for the heads of mauls (heavy wood mallets). During the days of wooden carriages and wagon building, canyon live oak was a prized wood for singletrees, axles and wheels. Bigleaf maple (Acer macrophyllum) was also popular for singletrees, a wooden bar hooked at either end to traces of the harness. Desert willow (Chilopsis linearis), a member of the Bignonia Family (Bignoniaceae) was highly prized by local Indians for making bows. Unlike the weak-limbed true willows (Salix), the wood of desert willow is very stiff and strong. Of course, one of the finest bow woods in North America was the osage orange (Maclura pomifera) of the eastern United States, a member of the Mulberry Family (Moraceae).

The genus Eucalyptus comprises about 95 percent of the forest trees of Australia, with over 500 described species--including some of the tallest trees on earth. Some of the species are called "ironbarks" because their bark superficially resembles the color and texture of black or reddish (rusted) iron. Although the dry, seasoned wood of most eucalypts will float in water, there is at least one species called "wandoo" (E. redunca) that is definitely an ironwood. California has literally hundreds of species of trees, but there were few forested regions in the vicinity of the booming coastal cities where railroads were being constructed. During the late 1800s and early 1900s several species of Australian Eucalyptus gums (including E. camaldulensis and E. globulus) were extensively planted in California for lumber, firewood, windbreaks and railroad ties. Although the species selected for extensive plantings grew into forests very rapidly, the wood proved very undesirable for lumber and railroad ties because of extensive splitting during the drying process. Today, these forests have forever changed the character of coastal southern and central California.

The name "gum" can be traced back to the voyage of Captain James Cook to the South Pacific in 1770. Captain Cook discovered the east coast of Australia, called New Holland at that time. In one harbor, the ship's naturalists found so many unusual and beautiful plants that they named it Botany Bay. Eight years later, a fleet of eleven English ships reached Botany Bay with 1,530 people, 736 of them convicts. This marked the establishment of England's most important prison camp of the nineteenth century, and the European settlement of a vast land called Australia. The actual discovery of the genus Eucalyptus is credited to the ship's botanist, Joseph Banks (later Sir Joseph Banks). One of the newly discovered species "red bloodwood" (E. gummifera) had a reddish gum exuding from its trunk, and the naturalists called it a "gum tree."

Other species of eucalyptus with persistent bark fall into five additional groups, called ironbarks (bark hard and deeply fissured), peppermint barks (bark finely fibrous), stringy barks (bark long and fibrous), boxes (bark rough and fibrous), and bloodwoods (bark rough, cracked and scaly on trunk and large limbs). Another group of large trees, called ashes, have rough bark on the trunk but smoother bark on the branches. In fact, the mountain ash (Eucalyptus regnans) rivals the California redwoods as the world's tallest trees. With about 500 described species dominating more than 80 percent of Australia's forests, it is convenient to categorize them within different groups based upon their bark type. In fact, one of the most striking species with thick, deeply furrowed, persistent black bark is the red ironbark (E. sideroxylon), commonly planted at Palomar College. In addition to tree forms, there are numerous drought resistant, shrubby eucalyptus called mallees. Some of these resprout from subterranean lignotubers like many of our chaparral shrubs. One of these (Eucalyptus macrocarpa) produces spectacular red blossoms and the largest seed capsules of any eucalyptus. Some mallees of parched desert regions store water in their roots, a fact well-known to Australian aborigines.

One of the most interesting uses for Eucalyptus wood is the sacred Aboriginal wind instrument called a "didgeridoo." Developed by indigenous people of northern Australia, this is one of the oldest known wind instruments, dating back at least 1,500 years. It is traditionally made from Eucalyptus trees which have their interiors hollowed out by termites. Finding the right tree requires diligent searching. If the hollow is too big or to small, it will make a poor quality instrument. Generally, the longer the instrument, the lower the pitch or key.

Chemically the eucalyptus "gums" are rich in tannins (kinotannic acid) and are similar to another phenolic compound called catechu. They are known in the trade as kinos or gum kinos and are used as tannins to convert animal hide into leather. One of the main Australian sources of kino is the common red gum (Eucalyptus camaldulensis), naturalized throughout San Diego County. Kino gums are also used medicinally as astringents to relieve throat irritation, dysentery and diarrhea. True polysaccharide gums, such as locust bean gum from the carob tree (Ceratonia siliqua), and chicle, a terpene gum from the latex sap of the sapodilla tree (Achras zapota), are chemically quite different. They all probably serve to seal off wounds and prevent bacterial and fungal infections.

Oil of eucalyptus (eucalyptol) is a volatile terpene compound (called an essential oil) which is distilled from the leaves of several species. It is used for flavorings, dentifrices, cough drops, and for the synthesis of menthol. The lemony fragrance from the leaves of lemon-scented gum (E. citriodora) is due to another volatile terpene called citronellal. One of the reasons that few plants will grow well beneath naturalized gum forests in southern California is that volatile terpenes from fallen leaves are leached into the soil, thereby inhibiting seed germination and growth of competing species.

The wood of different species of eucalyptus varies considerably, from wood as soft as pines to very hard, close-grained wood as dense as oak and hickory. Eucalypts constitute most of the forest vegetation of Australia and are one of the most important hardwood timber resources in the world. There are a number of species that provide excellent lumber for furniture, wood-carving and construction, including karri (E. diversicolor), spotted gum (E. maculata), blackbutt (E. pilularis), and jarrah (E. marginata). In fact, jarrah is stronger and more durable than oak and resistant to termites and marine borers.

One of the most common naturalized species of eucalyptus throughout central and southern California is the red gum (Eucalyptus camaldulensis), also known as E. rostrata. This species is a prolific seeder and has literally taken over canyons and river valleys where sufficient moisture can support groves of these large forest trees. Red gums seemed immune to any insect predator--that is until 1998 when the leaves of a tree in Los Angeles County were found to be infested with the red gum lerp psyllid (Glycaspis brimblecombei), an Australian insect that severely defoliates this species of eucalyptus. Psyllids are also called jumping plant lice and belong to the Order Homoptera, along with scale insects, aphids and cicadas. In fact, they superficially resemble a miniature cicada except they are the size of aphids (less than 4 mm long). Both immature and adult psyllids feed by sucking plant juices out of the leaves, causing serious leaf drop which weakens the trees and makes them more susceptible to other pests, such as wood-boring beetles. The wingless, yellowish immature form (nymph) secretes a waxy protective cover called a lerp. "Lerp" is a term derived from an aboriginal Australian language describing this cover. The nymphs also secrete large amounts of sticky honeydew on the leaves which results in blackened foliage due to the growth of sooty mold. This insect infestation has rapidly spread throughout Los Angeles, Orange and San Diego Counties, and at the time of this writing (March, 2000) is causing major defoliation of red gums. A tree that once thrived in this region without any major insect predators is now facing a serious threat to its survival.

The tropical American rain tree or monkey pod (Samanea saman) has been introduced throughout tropical regions of the world. A member of the legume family Fabaceae, it is also listed in some references as Pithecellobium saman. Introduced to the Hawaiian Islands in the 1800s, this rain forest tree reaches heights of 80 feet (24 m) with a symmetrical, spreading crown. The beautiful, porous, dark-grained wood has a specific gravity of 0.5 to 0.6, similar to Honduran mahogany.

Athough the sunflower family (Asteraceae) is the largest plant family, with approximately 24,000 species, very few of these are classified as trees. But there is one African species with a dense, brownish-yellow wood and a specific gravity of nearly 1.0. Some references give a value of 1.1 which would place it in the category of an ironwood in this report; however, my samples appear to be less than 1.0 because they float in water. The tree is called muhuhu (Brachylaena huillensis) and it is indigenous to Angola, Kenya, Mozambique, Tanzania and Uganda. It occurs in dry lowland forests and semi-deciduous dry upland forests. In Kenya 50% of the trees harvested for wood carving are muhuhu. In addition to carving, this beautiful wood is used for flooring, furniture and bowls. A fragrant essential oil is also extracted (distilled) from muhuhu wood. Because muhuhu trees have been overexploited throughout their range, carvers have been encouraged to use other imported trees, such as neem (Azadirachta indica), jacaranda (Jacaranda mimosifolia) and silk oak (Grevillea robusta).

Another heavily expoited and endangered African hardwood is known in Rhodesia and Zimbabwe as muninga or mukwa wood (Pterocarpus angolensis). This fine-grained, beautiful hardwood is commonly used for furniture and wood carvings. It is comparable in hardness to walnut, ash and pecan in this article, with a specific gravity of 0.6 to 0.7. Like other bloodwoods of the genus Pterocarpus, the sap is deep red. In fact, native Rhodesians use the sap as a permanent red dye for their clothing.

But of all the so-called "hardwoods," the most spectacular species are the "ironwoods" that actually sinks in water. Fifteen of the world's heaviest ironwoods with specific gravities between 1.11 and 1.37 are listed in the following Table 2. All of these seasoned, dry hardwoods will sink in water, with a specific gravity greater than one:

Some readers might wonder why I placed the South African black ironwood (Olea laurifolia) at the bottom of my list, while each year the Guinness Book of World Records lists this tree as the world's heaviest wood with a specific gravity of 1.49. In addition to the fact that most wood references do not concur with the Guinness Book, I studied a sample of this hardwood and the highest specific gravity I could obtain was 1.11, far below many other ironwoods. In fact, Acacia nigrescens (Acacia pallens), a common tree of the Kruger National Park region of South Africa, has a much heavier wood with a specific gravity of 1.19. Called knob-thorn acacia, the trunk is covered with woody, conical knobs each tipped with a thorn. Like other African acacias, the knob-thorn acacia is a painful tree to climb with bare hands. Some remarkable swollen-thorn acacias of Africa and Central America have hollowed-out thorns inhabited by symbiotic stinging ants that protect the trees from herbivores and epiphytes. According to Wood Density Phase 1--State of Knowledge, Australian Greenhouse Office, Technical Report No. 18 (Oct. 2000), there are several Acacia species in Australia that could be classified as ironwoods. Acacia xiphophylla has a specific gravity of 1.3 and another species called waddy wood (A. peuce) has a density of 1425. If this value is in kilograms per cubic meter, its specific gravity would be 1.425.

Actually, the heaviest and hardest wood I have ever seen is a block in my laboratory labeled "snakewood," an unknown hardwood species presumably from South America. [This may be Piratinera guianensis of the mulberry family, although its specific gravity is higher than published sources for this species.] The block has a specific gravity of 1.43 which is about the same density as vegetable ivory. On the 10 point scale of mineral hardness, the block of leadwood is similar to vegetable ivory with a rating of 2.5. Compare this value with 3.5 for a copper U.S. penny and 10 for diamond. There is a species of Condalia called tropical ironwood with a reported specific gravity of 1.42, which makes it one of the heaviest woods on earth. Interestingly enough, we have a native species of Condalia (Ziziphus) in San Diego County called desert jujube (C. parryi). This spiny shrub occurs on the dry slopes along the western edge of the Colorado Desert. It belongs to the Buckthorn Family (Rhamnaceae) along with Florida black ironwood or leadwood (Krugiodendron ferreum), perhaps the heaviest wood in the United States. Another rare ironwood tree in the Buckthorn Family called "kauila" (Alphitonia ponderosa) is endemic to the Hawaiian Islands. The hard, heavy wood was used for a variety of tools and objects by early Hawaiians, including clubs, beaters, anvils, digging sticks, spears, and weights for fishing lines.

Two of the hardest and heaviest native woods in southern California and adjacent Baja California are desert ironwood (Olneya tesota) and mountain mahogany (Cercocarpus ledifolius). The dried wood of both species readily sinks in water, and can quickly dull a "smoking" saw blade. In fact, chain saws have been known to break while cutting through large trunks of mountain mahogany. Mountain mahogany belongs to the Rose Family (Rosaceae) along with Santa Cruz Island ironwood, but the wood of mountain mahogany is much harder than its island relative. The dry fruits of mountain mahogany are easy to spot in the summer and fall, with their silvery, feathery styles glistening in the sunlight. The distinctive heartwood of desert ironwood is almost black and takes a high polish. It is similar in color and hardness to ebony (Diospyros ebenum), and is made into beautiful wood carvings sold in gift shops throughout the southwestern United States and Mexico. Some of these native shrubs can be seen at the Wild Animal Park in the Baja California and Native Plant Gardens.

Any discussion of ironwoods would not be complete without mentioning the Caribbean tree called lignum vitae (Guaiacum officinale), one of the world's hardest and most famous woods. A similar species (G. sanctum) is native to the Florida Keys. Both species are members of the Caltrop Family (Zygophyllaceae) along with North and South American creosote bush and the ubiquitous puncture vine (Tribulus terrestris). The name lignum vitae means "wood of life," owing to the medicinal properties of the heavy, resinous wood. During the days of masted sailing ships, the wood and sweet-smelling resin globs were sought after for treatments and cures for a variety of human ailments, including gout, syphilis and rheumatism. Today the resin is still used for expectorants and as a dye to detect the presence of occult (hidden) blood. During an annual physical examination, gum guaiac (also called guaiac) is the reagent used to check for blood in a stool sample. Peroxidase enzymes in the blood cells oxidize chemicals in the resin resulting in a characteristic blue-green color change. The raw resin, called gum guaiac, contains about 15 percent vanillin (artificial vanilla), resulting in the sweet aroma. The density and high resin content of the wood make it extremely resistant to friction and abrasion and account for its remarkable self-lubrication properties. In fact, under certain conditions the wood wears better than iron. Because of this, the wood has been highly valued for pulley sheaves, bearings, casters, food-handling machinery, and especially for end grain thrust blocks which line the propeller shafts of steamships. During World War I, attempts were made to use other ironwoods such as Tabebuia guayacan from Central America for propeller shaft bearings, but the wood lacked the oily resin of lignum vitae.

Africa has several different species of ironwood trees that belong to entirely different plant families. One of these is called leadwood or wawra (Combretum imberbe), a member of the Combretaceae. It is native from Tanzania south through Mozambique and Zimbabwe to South Africa. Wawra is a slow-growing deciduous tree, with some individuals in South Africa documented at over 1,000 years old. The dark wood is very hard with a specific gravity of 1.23. It is reportedly difficult to saw and very resistant to termites. Wawra wood is highly prized by woodcarvers. African artist Alex Chitura from Zimbabwe carves beautiful African mammals from this wood including elephants, rhinos and hippos. He also has huge scupltures of these exotic animals in wood and stone. His work is for sale at the San Diego Wild Animal Park and San Diego Zoo.

There are so many fascinating stories about ironwoods that it would be impossible to condense them all into one short article. At least one species has probably touched your life in one way or another--perhaps as a beautiful ornamental tree, a lovely carved figure, an appliance or clock with wooden gears and roller bearings, or perhaps a voyage on an old ocean liner.

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