|Casuarinas: Nitrogen-Fixing Trees for Adverse Sites (BOSTID, 1984, 114 p.)|
We are, says a recent U.S. Government report, "in transition from global forest wealth to global forest poverty." Europe and North America began a century ago to balance cutting and regrowth carefully, but in the great equatorial greenbelt - at once among the richest and most fragile of resources - poverty and lack of effective controls have combined to unleash a voracious appetite for trees. Two-thirds of Latin America's original forest are gone or seriously depleted. Half of Africa's woodlands have disappeared. Thailand has lost one- quarter of its forest in the last 10 years, and the Philippines, one-seventh in the last 5.
Spreading patches of the Amazon, Central Africa, and the Himalayan foothills have taken on the barren look of North Africa and the Middle East, both of which once boasted verdant stands of trees.
The issue goes far beyond millions of acres of lost trees. Land haphazardly cleared for crops or grazing - the single leading cause of deforestation - typically replaces rich jungle with virtually useless shrubs or sharp-edged grasses.
For one-half of the world's population, the principal fuel is firewood, and the hills are stripped for it. More than 90 percent of the wood cut in Africa (nearly 2 million hectares a year) is burned as fuel. In East Africa, as trees thin out, women spend up to six days a week collecting wood simply to keep their families in cooking fuel; without it, food supplies are useless. In West African cities, where it often costs more to heat a pot than fill it, organized syndicates use trucks, donkeys, and porters to spread the destruction far into the bush.
Ultimately, the only sure way to stop deforestation is to minimize its causes - lack of fuel, fodder, and farmland for the third of humanity that lives on the edge of starvation. For most sites, this means growing more trees. And to make any impact, it will take hardy, adaptable, and vigorous trees that can colonize the harsh, eroded land and begin its return to productivity. This is where casuarinas may find a role.
Casuarinas comprise a group of about 80 species of shrubs and trees that are primarily native to the southern hemisphere, mostly to Australia, where they occur in tropical, subtropical, and temperate coastal regions as well as in the arid inland. A few species are native to Indo-Pacific areas from Peninsular Malaysia to Polynesia.
One species, Casuarina equisetifolia, has been used widely as a seaside ornamental tree and to stabilize coastal sand dunes because it thrives in sand and saline conditions. Two others, Casuarina cunninghamiana and Casuarina glauca, have also been planted outside their native habitats. But there are at least 15 more casuarinas with apparent promise, particularly as firewood crops. They deserve wider recognition.
Some casuarinas are small diffuse or spreading shrubs only a few centimeters high, while others are tall, erect, and graceful evergreens with open, feathery crowns. Some attain heights of 50 m and diameters of 1 m. Many, however, are only 15-25 m tall. Viewed from a distance, their small cones and long, drooping branchlets, which look like pine needles, make them look like conifers.
The casuarinas produce high-quality fuelwood that burns with great heat and has been called the best firewood in the world. It splits easily, burns readily even when green, has low ash content, and makes excellent charcoal. It is useful for both domestic and industrial fuel and was a prime fuel for Australian bakeries until they began to be electrified, only 50 years ago.
Casuarina wood was one of Australia's first exports: British ships that took convicts to the colony of New South Wales often loaded with casuarina and red cedar for the return voyage.
It is a very dense wood, but because it splits and warps on drying it is difficult to use for lumber or furniture. However, in the form of poles and beams it is used for construction because it is very strong. Small items such as roofing shingles and turnery products are also made from it. The wood can be pulped for paper, but so far the cost of grinding up this hard, dense wood has kept it from becoming an industrial pulpwood. Nevertheless, in Egypt, several particle board factories rely on casuarina, almost the only trees to grow in Cairo's arid environs.
Casuarina trees, taken collectively, have many other uses. Among the various species there are casuarinas capable of stabilizing shifting sand dunes, stabilizing eroding hillslopes, and reclaiming marshy soils that are periodically inundated by fresh, or even brackish, water.
Many make useful shade trees, windbreaks, and shelterbelts; the shape and form of their foilage makes them excellent for these purposes, and most species are fairly wind firm and often retain branches to ground level. They also withstand clipping and, in Hawaii, for example, they are frequently trimmed as hedges and yard trees. Casuarinas are poor forage sources, but in times of extreme drought Australian farmers use them as emergency animal feed. In Madagascar the bark, reported to contain 6-18 percent tannin, has been used extensively for tanning leather.
On suitable sites casuarinas may grow as rapidly as the fastest growing tropical trees. In parts of the Philippines Casuarina equisetifolia has been known to outgrow Leucaena leucocephala and Gmelina arborea. On granitic uplands in Puerto Rico Casuarina equisetifolia grows faster than Pinus caribaea. In India Casuarina equisetifolia saplings have been measured as growing 3 m a year.
Most casuarinas are easy to propagate. They usually set seeds in abundance, and for the majority of species, the seeds store well and germinate readily. Probably all casuarinas can be propagated vegetatively. A sterile hybrid of Casuarina equisetifolia and C. junghuhniana is propaged almost entirely by cuttings in Thailand and India, and it seems likely that other species can also be propagated this way, but most have not been tried. Cuttings have the advantage of being a rapid, sure method of perpetuating the traits of superior trees and also of allowing trees of a single sex to be planted so as to avoid weedy regeneration through the spread of natural seedlings.
With their hardy nature, casuarinas need little care. They are fairly resistant to pests and suffer few major diseases. Most species also tolerate extreme heat. Casuarina decaisneana, for example, grows in parts of central Australia where summer temperatures may reach 47°C.
In most cases, young Casuarina seedlings compete aggressively with weeds. In the Philippines Casuarina equisetifolia is recognized as one of the best trees for planting in sites covered by Imperata grass, a tenacious weed that renders large areas of the tropics useless for agriculture. In the Highlands of Papua New Guinea Casuarina oligodon is also planted in areas infested with Imperata. This ability of casuarina trees to withstand competition from grasses in their early years could give them an advantage over eucalypts, particularly in Africa and Asia, where grass competition has caused some extensive failures of eucalypt plantings.
Casuarinas seem good candidates for planting in barren or polluted areas. They have the basic physiology for survival in diverse situations, and they often serve to colonize and revegetate poor sites. Many species will grow on soils of low fertility, some thriving on light soils, others on heavy soils. Along India's west coast as well as in Madagascar, for instance, Casuarina is found on laterite - the infertile, red, iron-rich, leached soil widely found in the tropics. In Uruguay, Brazil, Senegal, and elsewhere, Casuarina equisetifolia thrives in very deep sandy soils. In Argentina Casuarina cunninghamiana has been observed colonizing bare limestone. At a lime-grinding factory southwest of Cairo, Egypt, casuarinas grow right in the lime, where no other tree survives. In Kenya Casuarina equisetifolia grows well around a cement works, and in Malaysia it colonizes sterile tin tailings. In Hawaii Casuarina equisetifolia is growing well in sterile pumice.
Some casuarinas are salt tolerant. Near Bangkok, Thailand, there are commercial plantings of the hybrid between Casuarina equisetifolia and Casuarina junghuhniana in salt-marsh areas sometimes inundated with saline water. Casuarina obesa occupies saline sites in the wheat belt of Western Australia. Casuarina glauca grows naturally on estuarine lowlands of New South Wales and Queensland that are flooded with brackish tidal water. And both Casuarina equisetifolia and Casuarina glauca thrive in dunes at or near the seaside - often directly in the path of ocean spray. Casuarina equisetifolia even grows on the shores of Hilo Bay, Hawaii, on tidal rocks often submerged in salt water.
The outstanding ability of various casuarinas to grow vigorously on poor soils is due partly to their unusual symbiosis with an actinomycete, Frankia, that enables them to use nitrogen directly from the atmosphere. Casuarinas belong to a group of more than 170 species of actinomycete-nodulated woody plants that fix atmospheric nitrogen. In moist soils at certain sites in Australia and elsewhere, the roots of one-third or more of the trees of a given Casuarina species are well nodulated by this nitrogen-fixing, bacteria-like microorganism.
Frankia infects root hairs of fine casuarina roots, forming nodules (swellings). These nodules are woody and perennial and can form large masses in the root system. The nitrogen that the actinomycete in the nodules fixes enables the casuarinas to grow well in soils that otherwise would be too deficient in nitrogen to sustain plant growth. The amounts of nitrogen they fix is roughly comparable to the amounts fixed by legumes with their Rhizobium symbionts. In sand dunes in the Cap Vert peninsula of Senegal, nitrogen in the soil around casuarinas has increased annually at rates of about 60 kg per hectare. This fixed nitrogen is added to the soil through decay of dead rootless and leafy litter.
Casuarina roots also have symbiotic relationships with three other types of microorganisms. Two forms of mycorrhizal fungi enter the root and facilitate the uptake of minerals, notably phosphorus, and some trace elements. Furthermore, a range of unidentified microorganisms interact with the plant roots to produce dense mats of "proteoid roots," the large surface area of which probably also helps to absorb phosphorus and other vital minerals. Experiments have shown that proteoid roots can double the growth of plants in soils that are very low in phosphorus.
With the combination of all four symbionts, it seems hard to imagine plants better suited than casuarinas to the reforestation of low-fertility lands.
Care must be taken when introducing a casuarina to a new area. Some species are so innately vigorous that they can get out of hand and could develop into major pests by "escaping" into natural areas. This is already happening in southern Florida (USA), where
Casuarina equisetifolia and Casuarina glauca in many instances are becoming undesirable weeds. Casuarina glauca is also a pest in Hawaiian pastures, where it spreads by root suckering. Because of this, casuarinas are not recommended for planting in areas where their wood is not needed.
Along the edges of roads and farm fields, as well as in lawns, root suckers are a nuisance and have to be cut back continually. On the other hand, in many situations root suckering is an advantage - in fuelwood plantations, for example, the cut trees rapidly regenerate from root sprouts and do not have to be replanted. This is particularly true in eroded "blowouts" where the suckering species can quickly occupy the whole area.
Casuarinas have still other limitations. Because the branchlets are often rich in selenium, silica, and salt, the copious leaf litter under casuarinas may be toxic to nearby plants. On some sites the thick litter under the trees may contribute to acidification or salinization of the soil, and casuarinas, therefore, may not be good companion crops. In such cases, the effects are probably similar to those of pine trees.
Although most casuarinas are well adapted to survive fires (some by producing shoots from thick woody rootstocks, others by coppice shoots from buds in the stem), a few are fire sensitive. Casuarina equisetifolia, for example, is readily killed even by light fires.
Without their Frankia symbiont, casuarinas are often disappointing when planted as exotics.
Casuarinas occur in a wide range of habitats, from rainforest to desert, from seashore to high mountaintop, and from cool temperate regions to the hot humid tropics. They have modest site requirements and most are rapid-growing, trouble-free trees. Many may be salt tolerant, others are especially wind firm, and all seem adaptable to poor soils. Casuarina trees form root nodules in symbiotic association with the soil actinomycete Frankia that fixes atmospheric nitrogen. They also form symbioses with at least two different groups of fungi, which endow various species with the ability to extract phosphorus and other minerals from even the poorest soils.
Casuarinas can be used for many purposes, from amenity planting, land reclamation, shelterbelts, and dune stabilization to the production of shingles, particle board, tannin, timber and roundwood, and perhaps paper pulp. The wood makes outstanding fuel.
During the past century, most casuarinas have been largely neglected in comparison with many Australian Eucalyptus species that have become established forest resources in many of the world's warm regions. Now casuarinas warrant increased attention. One day they may complement eucalypts as a global wood resource, especially as they have the advantage of fixing nitrogen and seem to tolerate competition from weedy grasses better than eucalypts.