by JEAN MESSINES
Lately FAO Forestry Adviser in Libya
The problems involved in afforestation and sand-dune fixation in Libya apply to most of the Near East. Thus, while this article is limited to a discussion of these problems in Tripolitania, M. Dressiness conclusions apply to a far greater area than that with which lie is specifically concerned.
The first impression of a traveler visiting Tripolitania is that all is desolation and waste. This is due to the immensity of the steppes and the lack of forests but, above gill to the invasion by sand. Were it not for an occasional coastal oasis, the cultivated Italian concessions, a few half-hearted attempts at afforestation, and the rather beautiful scattered plantations along the coast, one could well believe that the desert had conquered the whole country.
One does not have to go far into the interior to find desert. The suburbs of Tripoli are surrounded by vast stretches of almost bare sand, sometimes flat but more often forming unending small dunes. These phenomena of wind erosion are much more noticeable in Tripolitania than in other parts of North Africa. Their causes -ire manifold and mainly originate in the unfavorable topography and climate.
The lay of the land in Tripolitania constitutes only a very slight barrier against the wind from the south, the GNU There is no high screen of mountain range separating the true desert from the sea, but only an irregular plateau at an elevation of 300-600 m., rising in some places to a maximum of 800 m., which overlaps a large coastal plain, the Jefara. This is undoubtedly enough of a barrier to allow Tripoli and the coast sector to enjoy a comparatively favorable climate, but as an obstacle it is neither high enough nor sufficiently continuous to prevent the Ghibli from blowing over the plain, occasionally with incredible violence, picking up, transporting, and depositing thousands of tons of sand.
The climate also favors wind erosion. Aridity causes the vegetation to remain stunted, providing inadequate soil cover. Rainfall is low, always less than 400 mm. Mean maximum precipitation figures collected at the Agricultural Experiment Station at Sidi-Mesri, 5 km. from Tripoli where the dunes begin, average only 379 mm. Throughout all the Jefara, rainfall varies from 100 to 350 mm. Furthermore, it occurs irregularly, and is badly distributed. It only rains in the autumn and the winter, the three summer months being entirely dry.
Summer temperatures are sometimes very high; a maximum of 58°C. (136.4°F.) has been reached at El Azizia. On the coastal plain, at Misda for instance, the heat may become as intense as in the desert. Despite the mild winter temperatures, the extreme summer heat is responsible for a great range of temperature between the seasons.
There are three climatic zones in Tripolitania:
1. The entire central and eastern mountain (Jebel) area, the coastal plain of Dafnia and lesser Jerald, the littoral from Sabratha to Misurata, via Tripoli, all have a semi-arid Mediterranean climate;
2. The Jebel-Nefusa, from Yefren and the eastern portion of the great Jefara around El Azizia, are in the and Mediterranean sector.
3. The entire western greater Jefara and the plateaux south of the Jebel, including Misda and Beni-Ulid, are in the Saharan area.
Naturally, there are some variations in each of these zones. For instance, Tripoli and the northern portion of the lesser Jefara enjoy the moderating effect of sea temperatures and moisture, and are the most favored regions from the standpoint of rainfall.
Taking these factors into account, what parts of Tripolitania could be successfully afforested? Generally speaking, afforestation is possible (even without any special precautions, such as the regular watering of plants during their first years) in the entire area of regular rainfall of over 200 mm. This area has a semi-arid Mediterranean climate, where the growing of olive trees is practicable. The chief problem in afforestation of this area is sand, for it is this region that sand invades the most. This threat can be met, however. There are means of combating wind erosion, at least in areas where the sand is still shifting.
There are sand dunes throughout Tripolitania; the desert is indefatigable in its attack. The tiny particles of sand, blown gradually or by a storm over the inland hills, are deposited over the entire plain. When the Ghibli blows, the sky darkens and assumes a characteristic reddish color. The mobility of the sand presents a constant menace, imparting to the lesser Jefara, and particularly the areas south and east of Tripoli, the appearance of a desert. Thus, in this Mediterranean zone, the sand forms true ergs of the Saharan type. From the interior of the African continent come the red (Heix) sands, composed mainly of very fine quartz granules. But there are also the sea sands, white, with rounded, rough textured and more calcareous grains, less able to retain water than the continental sands. Sea winds, blowing mainly from the northeast, form vast seashore dunes along the entire coastline from Zuara to Misurata, particularly at Side-Benur, Zliten, etc. These red and white sands meet in the coastal area, and the dunes which they form are frequently very close to one another. Ordinarily, there is bare steppe between these dune formations, which are still mobile and consist of pure sand. Sometimes the steppe itself overlies old deposits of sand which had once invaded the area and covered all other formations. This sandy, undulating terrain, sometimes levelled for the shifting cultivation of cereals, is still grazed by herds of small livestock. However, the equilibrium seems precarious, and it would probably take very little to upset it.
Overgrazing of steppe pastures and damage to vegetation by the goat - "the razor of the globe," as F. Cavara so aptly describes it1 - has resulted in extreme deterioration of the soil. When drought occurs and lasts for several years, the soil, deprived of its vegetative cover, is blown away by the Ghibli and is gone for ever.
1 CAVARA, F., Vegetation in Tripolitania, and economic use and agricultural development of the new colony. Naples, 1914.
The vegetation of the Tripolitanian steppe is poor and sparse with little variety of species. Those species which do grow are more or less drought-resistant. Being xerophytes, they generally have a highly developed root system and their foliage is reduced to a minimum; their leaves are generally filiform, dry and shiny, the epidermis thickened and cutinized. Many species are spherical in shape. Most of the bushes are spiny and prickly.
In the Jefara, the steppe looks in spots like the Mediterranean coast; elsewhere, like the interior pre-desert continental areas and, in the extreme western section, just like the Sahara.
In the past, forest certainly covered large expanses of Tripolitania. What type of forest it was is not precisely known, but there are still remnants of ancient stands. The most remarkable is the forest park of Acacia, tortilis (Arabic: Talha), in the greater Jefara, north and northeast of Gasr-el-Hag in an area where precipitation is less than 150 mm. Trees are spaced from 50 to 100 m. apart, but at least they are trees. The best-known stand of this species in North Africa covers about 30,000 ha. in southern Tunisia. Much smaller stands of acacias are found in several parts of Algeria and Morocco and in the desert steppes of Cyrenaica as well. Under present conditions, this tree has no great economic value; it is too slow-growing, and in this arid climate may only produce a stand with a density of 20, to 25 trees par ha. at best. While this acacia does not seem suitable for general afforestation purposes, the stand at Gasr-el-Hag might be set aside as a national forest reserve where livestock-grazing is prohibited. If this step were taken, regeneration would occur there naturally without much difficulty.
In the same region, along the road from El Uotia to Giosc at a place called El Ghezuania, an old stand of Tamarix articulata (Arabic: Etal) existed until a few years ago, when nomads destroyed it completely. Despite the age of the trees, shoots are still growing from the old stumps. This remnant of ancient forest ought also to be conserved and set aside as a national reserve.
The most common woody species of Tripolitania is Zizyphys lotus (Arabic: Sedr). This may grow into a tree, but it generally becomes bushy since it is browsed by goats. Its spiny and tufted branches have a special propensity to hold moving sands which accumulate in curious hillocks around networks of branches which grow taller as the sand becomes fixed. This type of vegetation, with dispersed clumps of trees, covers large areas. In less abundance are Pistacia atlantica (Arabic: Batum), found at the foot of the Jebel, and Rhus oxycantha2 (Arabic: Gedari). Other natural woody flora include Nicotiana glauca, Ricinus communis (Arabic: Gherua), Calotropis procera (Arabic: Osciur-Cronca) and several species of Tamarix (gallica, africana). These are all bushes, but, apart from the castor-oil plant, the usefulness of which in fixing sands will be discussed below, these species are not suitable for afforestation in Tripolitania because of their slow growth and insignificant, economic value.
2 In Tripolitania, one does not find the very special community of Rhus and Pistacia ientiscus, in the form of scrub, which is so common in Cyrenaica on the coastal plain between Tocra and Tolmeita.
Plants of the Steppe
The steppes themselves vary greatly in appearance and features. During the rainy season, in areas of stable sands, a fine grass cover, rich in legumes and including many perennials, blankets the soil. The types of perennials which grow on the steppes serve to classify the land structure and vegetation community, as follows:
1. Asphodelus microcarpus (Arabic: Belus), and less frequently Squill (Urginea maritima - Arabic: Ansil) are types of vegetation found in moist, deep, flat sand. Occasionally Squill constitutes the only steppe vegetation, and sometimes the two plants form a community.
2. Artemisia campestris (Arabic: Sciaha) requires the same conditions as Asphodelus and grows with it. It prefers undulating, sandy soils.
3. Imperata cylindrica (Arabic: Dis)3 grows on the poorest soils, still deep and moist, but sometimes slightly salty. This plant takes root in sand and, as will be shown below, may be useful for dune fixation. Another bushy, graminaceous plant, Psamma arenaria, equally useful in sand-dune fixation, grows along the littoral in the seashore sands.
4. The presence of Retama retam (Arabic: Rtem) indicates deep, rather dry soil with undulating surfaces. The xerophylous nature of this plant is very marked. It frequently constitutes a community with Aristida pungens (Arabic: Drinn) and Ricinus communis, the bush mentioned above. Overcutting of Rtem for fuel has frequently led to soil deterioration and the formation of shifting sand dunes.
Drinn, which grows in conjunction with Rtem in shifting sands, has long, creeping roots which spread out over the sand and try to penetrate it. These two plants are invaluable along with Dis in sand-dune fixation work for afforestation purposes. Sometimes Drinn alone is found to constitute the entire steppe vegetation. It is characteristic of a dune which has not yet become fixed.
5. Calycotome intermedia (Arabic: Gandul) is a small spiny bush which prefers deep, stony soil. It betrays the presence of a hard-pan below the surface, a characteristic not only of this area, but also of all arid countries. These hard-pan or crust soils of the arid Mediterranean climate are found widely, particularly in the Jefara, where they are often overlain by sand deposits of more recent origin.
6. Where the soil is somewhat thicker and the sand cover less heavy, Deverra tortuosa (Arabic: Guzzah) and Thymelaea microphylla (Arabic: Metnam) occur. Large areas in the Dafnia plain toward the east between the Homs (Lebda) and Misurata are covered with this type of vegetation. On the more superficial terrains, where the hard-pan crops out on the surface, the characteristic plant of the steppe is Gymnocarpus fruticosus (Arabic: Gescrud) or Rantherium suaveolens (Arabic: Arfège).
7. Mountain-types of vegetation and, more frequently, the pre-desert types which are more common to the Jebel and the southern plateaux also occur in parts of the plain, especially in the western greater Jefara and in the area south of Taourga. Artemisia herba alba is an outstanding example in this category.3 Arabic: "Dis" stands for Imperata cylindrica and Psamma arenaria, in Tripolitania, and Ampedolesma mauritanicus in Algeria.
These various types of steppe vegetation are of interest to the agronomist as well as the silviculturist. Of all the types of terrain discussed, only the first two, the steppes with Asphodel and Artemisia campestris, can still be used for agricultural purposes. Assuming a minimum of 200 mm. rainfall annually, these steppes, as well as those characterized by Dis, Rtem and Drinn, can be afforested.
Map of Tripolitania.
This map should be compared with the account in page 51 of the three climatic zones of Tripolitania. The area suitable for afforestation is roughly that within the limits of the olive culture line.
Sand-dune Stabilization and Afforestation
The climate limits feasible afforestation in the plain to the lesser Jefara and the coastal area. The prime objectives of forestry in this region should be to fix and afforest: (1) the more or less mobile continental sands; (2) the steppes which are more or less covered by vegetation and occasionally those which are almost entirely devoid of vegetation, and (3) in areas where such protection measures are urgently needed, to undertake the difficult operation of afforestation of seashore sands.
Italian foresters worked out successful techniques for stabilizing the sands, and as a result of their efforts several thousand hectares of dunes have already been fixed and afforested. After their initial research excellent practical results were obtained, particularly from the standpoint of protection of highways. A drive along the 30 km. road from Tripoli to Castel Benito suffices to show how successful the work has been. Before the Italian Forest Service started its work, the roads from Tripoli to Homs (Lebda) and into the interior were unsafe and necessitated many detours. Today, the main roads are well-established and safe.
While the technical methods for dealing with seashore dunes and with Continental dunes are almost identical, there is an essential distinction to be drawn between the two.
Continental sands comprise very fine grains which can be fixed by a network of low, thickset hedges, and dead vegetation litter (dune grass, rushes, etc.). Dis (Imperata cylindrica) is the plant most commonly used for this purpose. Small, flexible and penetrable obstacles considerably hamper the shifting of sand but do not completely prevent it. They enable trees to gain a foothold in the square or oval patches of dis, averaging 4 x 5 m., and can fix sands for young forests. The topography is altered as little as possible. Rigid obstacles and high barriers are proscribed; palisades, high rigid and continuous fences cause heavy accumulations of sand which pile up and consolidate, just the opposite of the desired result.
For seashore sands, on the other hand, high continuous barriers are preferable. They help build up artificial littoral dunes, in the shelter of which the sands invading the inland palm groves can be fixed and covered with small, low thickets. Thus, a combination of both methods is called for.
The maritime sand dunes are as great a threat as those of the desert, as this picture from near Zuara shows. The oasis itself is threatened by the dunes' invasion, and the fig trees in this photograph are already half buried.
A technique which several specialists consider to be an indispensable adjunct to sand-dune fixation, namely the covering of an entire stretch of sand with branches or sticks, has been abandoned in Tripolitania. Another idea that has been given up is the artificial sowing of grasses, such as Rtem, or the planting of woody species like the castor-oil plant, as a help in stabilizing the sand and allowing other plants to gain a foothold. Experience has shown that seeding by appropriate plants will occur quite naturally.
A long process of elimination has finally resulted in the selection of the following woody species suited to sand fixation:
1. Acacia spp., above all, Acacia cyanophylla. This is a variety of A. saligna, Bentham, from which it differs only in the color of its leaves. Elsewhere in Tripolitania all transitional forms of this species are found. This acacia flourishes on the southern coast of Western Australia,4 in the areas of dry summers and rare winter rains. It is interesting to note that it grows there on sandy moors over deep calcareous sands near stands of Eucalyptus gomphocephala, which has been almost as successful as Acacia cyanophylla in Tripolitania. The climate of Libya is admirably suited to both species.44 Located between 32° and 33° latitude North, Northern Tripolitania from the standpoint of climate is very similar to Australia, which is located in approximately the same part of the world but at the antipodes. This leads us to expect that varieties of Acacia and Eucalyptus from southwestern Australia can be used successfully for afforestation in Tripolitania.
A. cyanophylla, because of its abundant flowering and excellent capacity for survival, is the tree par excellence for Tripolitania. It fixes the soil well by its extensive root network, both close to the surface and to great depths and affords even better shelter to grass than eucalyptus. It also encourages humus and top soil. It throws out new shoots and suckers prolifically when young to ensure the future of the stand, and it produces an abundance of low branches which spread out and form a canopy, so that there is no heavy undergrowth and little fire hazard.
The wood is only useful for fuel and charcoal, but experiments have already been made, and are worth continuing, on ways of utilizing the tannin content of the bark on an industrial scale; thus far, the results have been inconclusive. The tannin content of the bark of A. cyanophylla is lower than that of other Australian acacias (mollissima, picnantha), but unfortunately the latter species have proved less successful for sand-dune fixation.
For afforestation work, 1-year-old plants raised in pots in nurseries are used.
2. Tamarix. The second species selected is Tamarix articulata, which has also been highly successful, especially in maritime sands. It can grown on salty lands and is resistant to sea winds, fog, spray and storms. At Zuara, on the coast, Tamarix seems to be the species best adapted to the environmental conditions. Seven-to eight-month rooted slips from nursery beds are used.
3. Castor-oil plants. Finally the ordinary castor-oil plant (Ricinus communis), highly prized for its beans and lubricating oil (in great demand as an export commodity) has many uses. It is good for sand-dune fixation and as a wind-break, and does not need irrigation. The Libyans are familiar with this plant and have been using it successfully around Tripoli. Its use is therefore particularly to be advocated on private holdings.
Robinia pseudo-acacia was at one time used on a large scale by the Italians, but with few exceptions the results were not apparently very good. The cost of planting this species is less than for A. cyanophylla because seedlings without special root protection can be grown in the open and the young trees constitute a windbreak very quickly and therefore very economically. However, the resultant stands generally look sickly, are of irregular growth and yield very little wood. Black and Carolina poplar, which have also been tried, proved disappointing and were quickly discarded.
Eucalypts can be very valuable provided they are not used as the initial species for dune fixation. They may be used on stable, sandy steppes, to prevent the shifting of sands, and as a substitute species after the first acacia crops have been cut on dunes already fixed and afforested. The two species most widely found in Tripolitania, and which have already proved their worth, are E. rostrata and E. gomphocephala. Both have a highly satisfactory rate of growth but the latter seems to have a longer life and grow to greater dimensions, giving more and better quality sawn timber. Furthermore, it is adaptable to calcareous soils and should definitely be given preference.
Pinus halepensis and P. pinea can be used on a small scale for the afforestation of sandy steppes between shifting sands. Their rate of growth is slow and, for sand-dune fixation, they are less effective than acacias. On the other hand, the pines yield timber, both saw wood for construction and small dimensional stock and box wood, and are therefore of commercial value. They can be planted in shelter-belt rows like Eucalyptus, Cupressus sempervirens, Casuarina equisetifolia, and Acacia horrida, and in borders around cultivated fields and along roads and pathways in agricultural areas, both on private lands and state-owned areas.
The question of the density of stocking of plantations on sands raises the delicate problem of water resources.
The water table of the Tripolitanian littoral and coastal plains as well as throughout the Jefara lies at varying depths. Near the sea, water is found at a depth of 4 to 5 m., even under the sand deposits. Innumerable wells (over 8,000) around Tripoli account for the prosperous littoral oases. In the Jefara, wells are much fewer and have to be dug to greater depths to reach water, up to 40 m. at El Azizia and over 80 m. further inland, near the hills. These wells are used mainly to water livestock on the grazing lands. The water is generally cool and drinkable but the supply is not inexhaustible and the flow diminishes seriously in summer. At present the water table seems to have reached a disturbingly low level, which indicates a lessening of the ground water reserves and a need for prudence in the development of agriculture and forestry.
In arid and semi-arid areas, rainwater alone does not suffice to maintain the vegetation which must depend to some extent on water translocation within the earth's strata. In Tripolitania, runoff water very seldom reaches the sea; rainwater either infiltrates into the soil or evaporates. There are two kinds of evaporation processes, namely:
1. Physical evaporation, which depends on whether or not the soil is covered with vegetation. It has been measured experimentally, and in the temperate zone from a quarter to a half of the total rainwater is found to be lost through evaporation. It has been calculated that in desert or subdesert areas potential evaporation could be 30 to 40 times the total rainfall. On forested land, the rate of evaporation is estimated at one-third of that on bare areas.
2. Physiological evaporation, which results from transpiration and the assimilation requirements of the vegetation. This varies from species to species, depending on the part of the world where they grow and on the season of the year. Investigations in this field are as yet too incomplete and fragmentary to draw any satisfactory conclusions.
COMPARISON OF CLIMATIC CONDITIONS OF SEVERAL SITES IN LIBYA AND AUSTRALIA
Take the case of a eucalyptus plantation with spacings of 6 x 6, which means about 270 stems per ha. A single, well-grown tree, under the Tripolitanian climate in the middle of the summer, will through transpiration give off at least 200 litres of water per day. Even allowing for a considerable decrease in the rate of transpiration during the winter or when the vegetative growth is slowed down (which is only for a short period under the climatic conditions of Tripoli), the physiological evaporation of a eucalyptus stand of this type would be from 8,000 to 10,000 m3 by volume per ha. per year. This corresponds to a water level of from 800 to 1,000 mm.
There are obviously many unknown factors to such a calculation and the figures are only given with the greatest reserve. However, they do tend to suggest that, in Tripolitania, a spacing of eucalyptus trees 6 m. apart is too close. One need only inspect the stands in the public domain at the Hascian Preserve in the sandy region, to be certainly convinced that trees spaced 5 x 5 are overcrowded. Growth would have been much better if they were spaced 7 x 7 or even 8 x 8. With only 150 trees per ha., the aggregate volume of timber would have been much greater and the quality of the product better.
Before World War II, much good work was done by the Italians in the way of dune fixation and afforestation, and this desolate area was once covered with coppice of Acacia cyanophylla, one of the most successful species, which has now been clear-felled.
To revert to the question of evaporation, we have already estimated that a eucalyptus plantation can transpire or absorb from 800 to 1,000 mm. of water per ha. annually.5
5 For comparison purposes, the following figures are cited by Furon (L'érosion du sol [soil erosion], Raymond Furon, Paris 1947) from Keilhack and other authors, for evaporation under the temperate climate of Europe:Coniferous forest: 0.5-1 mm. per day up to 300 min. annually.
Cereal crops: 2.2-5 mm. per day, or for 4 months of active growth: 270-600 mm. annually.
Meadows with grasses and clover: 2.1-7.3 mm. per day, or for 8 months of active growth: 500-1,000 mm. annually.
Under the arid climate of Tripoli, physiological evaporation from eucalyptus or acacia plantations on sandy ground (not to mention physical evaporation, which is obviously proportional to the water content of the soil) must normally require water supplies other than rainwater, or the vegetation would suffer and die. Fortunately dew helps. The condensation of water vapor at night has not been studied thoroughly, at least quantitatively; as a matter of fact, condensed dew is difficult to measure. Some observations indicate that certain types of vegetation, particularly forest crops, draw from the atmosphere as much dew water as rainfall.
In any case, there must be some water equilibrium in Tripolitania, since dry cultivation is possible in areas where the rainfall is only 200 mm. Also, many plants, particularly desert plants, can draw water from the sand through their roots from depths of 5 and 6 m. or more; others spread their runners over 20 sq. m. or more. In Tripoli, some eucalypts growing alongside roads have roots 65 m. long. This suggests that trees should be planted very far apart, as in the forest reserve of Acacia tortilis at Gasr-el-Hag. Acacia cyanophylla should be planted at a maximum rate of two plants per Dis cellule or 1800 to 900 plants per ha. With eucalyptus on stable, sandy soils, only 150 plants per ha, should be used, spaced 8x8. No attempt should be made to cover the site uniformly. In Tunisia, on the dunes of Garmath, where the climate is more humid than in Tripoli, young A. cyanophylla plantations were formed with twice or three times the number of plants per hectare here suggested, that is 2,000-2,500 plants. The plantations now look like dense, barely penetrable thickets and are entirely different in appearance from the stands in Tripolitania, where the trees are well spaced and compact, having adapted themselves to the aridity of the area. The struggle for existence plays a very important role in these regions.
Eucalypts may provide one of the solutions to Tripoli's afforestation problems. This grove in the state forest at Hascian shows how well they do. The white stem on the rights is that of E. rostrata, behind it being several E. gomphocephala. The trees are 18 years old, spaced 5 X 5 m.
A study of Tripolitania's problems of afforestation and sand-dune fixation leads to conclusions which are valid for all arid countries. In a country like Tripolitania it would clearly be folly to try to create a complete forest canopy, or to plant up whole areas uniformly. Furthermore, the species chosen for afforestation and sand fixation must be both frugal with water and drought-resistant, and it would appear that the use of many of the Australian acacias and eucalypts will provide the best solution of the problem.
Among the former, experiments might well be undertaken with A. cyclops, A. picnantha and A. mollissima, with emphasis on tannin production. The two last might do well in the coastal areas; another Australian species worth trying is A. aneura.
Of the eucalypts, E. gomphocephala has already proved useful in this area. Other species suggested for trail include E. cladocalyx and E. sideroxylon for all arid soils; E. occidentalis for humid, salty soils, and E. astringens because of the quality and high tannin content of its bark. E. salmonophloia might be tried on clayey soils, E. brockwayi on muddy soils, E. oleosa on sands, and E. saligna. on deep, moist soil. There are, possibly, other species which might also be used and about which information can be found in existing literature.
Our present state of knowledge is insufficient to enable us to calculate scientifically the extent to which afforestation is possible. But it is certainly safe to say that nature will quickly efface any imprudent, over-ambitious schemes.
Readers of the above article will be interested in an article on "Afforestation in Arid Zones", by M. Metro, which will appear in the next issue of UNASYLVA. Reference should also be made to M. Metro's paper, "Ecology of Eucalypts", which appeared in No. XLIX of the proceedings of the Society of Natural Sciences of Morocco, Oct. 1949. A study tour to enable foresters of and zone countries to observe eucalypts in their natural environment has been organized by FAO, to take place in Australia, Sept. 1952; finally, a study of eucalypts is also being prepared by M. Metro for FAO.