Part 16

The most obvious features of desert plants are associated, in whatever way, with the subaerial portions. Leaves are usually greatly reduced or wanting, during the dry seasons at least. Spines are frequently present and the exposed parts are often well covered with hairs. The stomata are sometimes deeply sunken, the cuticle often very heavy, and a waxy substance may cover leaves or stems. The chlorophyll-bearing cells are arranged with the long axis at right angles to the leaf or stem surface. All or most of these characters are associated with the low humidity of the air. In certain deserts plants are also to be found with greatly enlarged stems and branches which serve as water-storage organs. It should be noted, however, that plants which do not have a constant surface undergo many marked changes with a betterment of the water relations, particularly if this comes when the temperatures are favorable. For example, many cacti organize leaves which are unsuited in structure for periods of extreme drought, and which consequently fall away soon after the close of the rainy season. These leaves enormously increase the rate of transpiration at a time when this is not injurious.

It is not in the subaerial parts alone, however, that the plants of one desert are different from those of another, that plants are unlike in the same desert, or that plants of a desert are different from those of the more humid regions. The root-habits also exhibit not a little diversity and show marked reactions to the pressure of their environment. For instance, the desert shrubs of the region surrounding the Desert Laboratory at Tucson have well-marked root-systems, apparently constant under natural conditions, which may roughly be designated as the tap-root type, the superficial type, and the generalized type. Other conditions being equal, species with characteristic root-types have also characteristic distribution, or exhibit in other regards consistent reactions. Thus, the widely extending and superficial type of roots is confined, among independent plants, to such as have water-storage capacity. The relation of this root-type to the distribution will be mentioned later. Plants with a dominating tap-root are confined to areas where the soil is relatively or actually deep, while species having generalized roots have a local distribution which may be considered the maximum.

The relation of the superficial type of root-system to the distribution of the species is not so apparent in this relation in plants as with the other types of roots, but is undoubtedly close and possibly definitive. The absorption roots of plants with water-balance mostly lie less than 10 cm. beneath the surface and are thus subject to extreme desiccation for the maximum time, or in other words they are exposed to favorable moisture conditions the minimum time. How long the optimum water-absorption time may be for such species is not known, but that it is longer than might be supposed (from the period certain species can survive without water) is highly probable from the facts concerning their distribution. In brief, the best development of the fleshy species in question occurs where the rainfall is periodic, occurring twice each year, and they are wanting or sparse where the rainfall is uncertain or occurs but once annually. Had these plants a deeply penetrating type of root-system the local as well as the general distribution would be very different from what it now is. Owing to the unfavorable character of the rainfall in southern Algeria, plants with a water-balance are wanting there, just as they are wanting in portions of southwestern United States where the amount or the character of the precipitation is likewise unfavorable.

Besides the characteristics of the root-systems of the desert perennials whose significance has been sketched, there are other features of importance. For example, the roots lying near the surface bear tufts of delicate roots, which are formed during periods of active growth and perish when such seasons cease. By the organization of deciduous rootlets the absorption area of the species is enormously increased, and quickly, without at the same time increasing the distance of water transport.

There need be mentioned here only one additional feature of the desert plants. It is now known, in brief, that the non-fleshy perennials of the desert, not halophytes, may possess a very dense cell-sap. This fact has been demonstrated in the subaerial parts of several species, and is assumed to hold for the roots also of the same species. As suggested, not all of the desert plants, however, are capable of developing dense juices. Thus, certain fleshy species, and such mesophytes of the desert as have been studied in this connection, do not have more highly concentrated cell-sap than the ordinary plants of the humid regions. Further, it appears that desert species which, under natural conditions and during the dry seasons, form extremely dense juices, lose this capacity when grown under humid conditions. So far as is now known, species capable of developing a cell-sap with high osmotic power have generalized roots, although this may be of no especial significance; but the relation of this capacity to survival in an arid substratum is apparent and vital.

ENVIRONMENTAL FEATURES OF THE FLORA OF ALGERIA.

The physical environment of the plants of southern Algeria is, in a few broad features, similar to that of the southwestern portion of the United States. These regions have about the same latitude, both are separated from a large sea by mountains, and the range in altitude is similar. There are other features, however, particularly as regards the amount and the distribution of the precipitation, in which the two widely separated regions are very unlike, and a correlated difference in the habit and composition of the floras of the two regions is apparent.

The Algerian climate as a whole is a mild, temperate one, but very diverse. The latitude and topography taken in connection with the presence of large seas to the north and west, and a large continent leading away to the south are its chief determinants. The climate, therefore, of the northern portion is coastal; that of the southern portion continental.

Probably the most important of the secondary factors which modify the climate of Algeria is its highly varied topography. An important mountain system, the Atlas, made up of many more or less detached groups and secondary systems, a plateau or steppe lying 3,000 feet more or less above the sea, and finally the northern edge of the Sahara, which has a very diverse topography of its own—such is the surface of Algeria.

Algeria is divided into three climatic provinces corresponding to the leading topographical differences: the Tell, including the littoral, or portion between the maritime Atlas and the Mediterranean; the High Plateau, or steppe, which lies between the Tellian Atlas and the Saharan Atlas; and the desert. These provinces have marked individual differences in rainfall, temperature, and other climatic features.

In the Tell and the High Plateau the winds from the sea deposit most of their moisture. Along the coast as much as 700 mm. of rain is recorded, while in other parts of the Tell it is about 570 mm. On the High Plateau the yearly precipitation sinks to 310 mm. In the desert south of the Saharan Atlas, however, where the altitude is lower and the temperature greater than in either of the other provinces, the yearly rainfall is 200 mm. and less. In some years, in fact, no precipitation whatever is reported in the desert.

The seasonal distribution of the rains in any arid or semi-arid region is of great importance as a factor in shaping the character of the vegetation. For example, in the semi-arid region of the southwestern part of the United States, in the Tucson region, there are two distinct rainy seasons—the rains of winter and those of summer—and here the plants with a water-balance are an important feature; but farther to the west, where there are no summer rains, there are no succulents. In Algeria, also, there is but one rainy season and it has already been noted that the absence of plants with water-storage facilities is one of the leading characteristics of its vegetation. The seasonal distribution of rains is as follows: In the Tell, in winter it is 42 per cent, in spring 27 per cent, in autumn 27 per cent, and in summer only 4 per cent. On the High Plateau the percentages are 30, 20, and 34 for winter, spring, and autumn, and 16 for summer. In the desert the percentages for winter, spring, and autumn are 37, 39, and 20 respectively, while in summer 4 per cent of the entire rainfall occurs. On the High Plateau, however, and in the Saharan Atlas the distribution of rain is much more equable, since 16 per cent falls on the High Plateau in summer and 13 per cent in the Saharan Atlas the same season. We have, therefore, the interesting result that both in the Tell and on the desert there is a long, dry summer season, but in the intervening country more or less rain falls at this time of year. It seems very probable that a careful study of the plants of these regions would show reflected in the vegetation this peculiar character of climate.

The mean relative humidity changes in a marked manner as one passes from the Tell, across the High Plateau, and enters the desert. For example, at Fort National the mean relative humidity is 85 per cent. On the desert it varies from 54.6 per cent at Ouargla to 42.6 per cent at In Salah. At times in midsummer the humidity in the desert is too low to measure with instruments; it is often 7 or 9 per cent. On the other hand, the humidity in autumn is surprisingly high, owing in part to the lower temperature and in part to the northerly winds. However, no dew is reported and probably its occurrence is rare.

With so great difference between the Tell and the desert in relative humidity is associated marked variation in the rate of evaporation. For example, at Algiers the total annual evaporation is 1,654 mm., while at Ghardaia it is 5,309 mm., which is possibly the greatest amount of evaporation thus far reported. Thus, the difference in evaporation between the Tell and the desert is nearly as the ratio 4 to 1.

The evaporation-rainfall ratios for the Tell, High Plateau, and the desert are of great interest. The seasonal evaporation-rainfall ratio for the littoral is 2.5 to 1; that of the Tell is 3.5 to 1; that of the High Plateau is 9.4 to 1; and that of the desert is 46.5 to 1. If we represent the evaporation-rainfall ratio as unity, the ratio for the Tell becomes 1.4 and the ratio of the High Plateau becomes 9 to 3.7, while the desert ratio is 18.6.

The annual and daily variations of temperatures in the desert are naturally relatively great. At Algiers the annual variation is approximately 40.7° C., while at Ghardaia it is 47.9° C. As great an annual variation as 57° C. has been observed at Ghardaia. The daily variation of temperature is especially marked on the High Plateau and the desert, ranging 17° C. more or less on the High Plateau, and 20° C. more or less on the desert. These figures are occasionally overstepped: for example, at Batna in July, 1904, when the maximum daily range was 21.8° C.; and an observation made by the writer in the open desert between Ouargla and Touggourt, in November, 1910, showed a variation of temperature of 24.7° C. between 3 o’clock in the afternoon and 6 o’clock the following morning.

The absolute maximum temperatures in southern Algeria are fairly high. At El Golea, for example, they are 47°, 46.5°, 48°, and 49.2° C., while at In Salah, about 700 miles from Algiers, the maximum temperatures for four years have been found to be 50°, 49.2°, 50°, and 48° C. It is interesting to note that at Ouargla, which is much nearer the coast than In Salah, even higher temperatures have been recorded. The maxima for as many years are as follows: 50.2°, 51.0°, 52°, 49°, and 48.4° C. Usually in winter freezing temperatures are experienced at all stations in southern Algeria.

Very little has been done on the soil temperatures in the desert region, but at Ghardaia, in July, 1911, the temperature of the soil 15 cm. beneath the surface of the soil ranged, maximum from 36° to 37° C., minimum from 31° to 33° C., giving an absolute range of 6° C.

In addition to the rainfall, the evaporation, and the temperature, there is another important climatic factor which should be taken into account, but which can not be stated in accurate terms, i.e., air-currents. It is a matter of common experience that one rarely observes a calm day on the desert, but that usually the wind, which is often of considerable force, is found to be blowing. This is unquestionably an important factor in raising the total of evaporation and therefore in increasing the arid conditions of this region. The winds which are most effective in the direction mentioned are those which come from the desert and are known as the “sirocco;” these are most likely to blow in spring and summer, although they occur in autumn also, and to a very limited degree in winter. When the winds blow from the north cooler conditions occur, the relative humidity is lower, and therefore the evaporation rate is less. The sirocco, or desert wind, crosses the Mediterranean and is sometimes felt in southern Europe. It does not generally last more than three days at one time, but at Batna, in July, 1902, it was reported for eight consecutive days. When the sirocco blows the humidity is likely to be markedly affected; for example, at Batna during the eight-day sirocco alluded to, the relative humidity fell from 25.6 to 16 per cent on the first day.

SOME EFFECTS OF TEMPERATURE AND RAINFALL IN SOUTHERN ALGERIA.

While it is recognized that, generally speaking, climate shapes the character of the vegetation, its immediate effects can not well be measured, or, at least, have not been accurately measured, so that it seems necessary to confine one’s observations to supposed or probable effects, however unsatisfactory this may be. So far as suits the present purpose, climatic effects may conveniently be separated into those which are direct and those which are not direct, remembering at the same time that the division is purely arbitrary, since the climate is a complex of various factors and its effects on vegetation are also complex. Among the climatic factors whose effects are most striking are the air temperature and the rainfall, and only certain effects resulting from a variation of these will be commented on here.

In parts of the Sahara visited where the most rain is reported, especially Laghouat and Biskra, plants were observed to exhibit exposure preference. Here the south or southerly facing slopes may have a floral composition different from the opposite exposure. In each instance the soil conditions, and apparently the moisture conditions also, were alike. Exposure preference was not noticed in the southern portions of the colony. Another temperature relation was observed, namely, the renewal of growth in the autumn. This is probably direct effect, although the point is not certain. In Ghardaia it was seen that many of the perennials were taking on new growth and coming into flower, although no rain had fallen for 12 months. Analogous conditions, with a significant difference, occur each year in the Tucson region. Here with the change from a cooler to a warmer temperature, as from winter to spring, or from spring to summer, fairly independent of the rainfall, many perennials organize flowers or shoots. But, so far as is known, no species renews its vegetative activities with the coming of winter, or with a decreased temperature and also independent of the rains, although there are characteristic winter and summer plants. Judging from analogy, therefore, it would appear that the stimulus to development on the part of the M’Zabite plants may be from the relatively better water relations made possible by a lower temperature without rain. In November at Ghardaia the evaporation rate is much below that of summer, that during the night being very small. Further, it was told me by good authority that the same species seen growing in autumn renew growth whenever rain chances to come, whatever might be the season. But it should be remembered that rain most commonly occurs in this region in winter, so that the plants may have a rhythm to which they usually conform, but from which they may depart, and that both stimuli (better water relations and lower temperature) are the annually recurring factors by which it may have been induced. Reference, of course, is made to perennials only, as no annuals were seen until the rains of spring made conditions favorable for their appearance.

The effects of a varying amount of precipitation are naturally the most marked of any climatic factor. It is especially striking as one goes south from the Mediterranean, crossing the Tell and the High Plateau and entering the desert proper. Whether the effects would be increasingly striking with deeper penetration of the desert is doubtful. As is well known, a leading characteristic of the vegetation of the littoral and of the Atlas Mountains is the presence of forests of whatever species. As the littoral is left behind the forests disappear until on the High Plateau there are only straggling trees along the dried water-courses. This steppe bears mainly shrubs, many of which are halophytes, with the perennial grass, Stipa tenacissima, and Artemisia herba-alba away from salt spots. Such low forms are present in sufficient numbers as to give character to the landscape and to conceal the surface of the ground fairly well.

South of the Saharan Atlas a marked change occurs. Here, with a rainfall of 200 mm. and less, the trees are confined to the dayas, a narrow belt, the vicinity of oueds, and the oases, exclusively. The shrubs of the hamada also decrease in numbers as one goes south, and where the annual precipitation is least, as on the Gantara between Ghardaia and Ouargla, large barren areas extend. At no place on the hamada of the M’Zabite region are the shrubs present in sufficient numbers or size to conceal the surface of the ground or to give character to the landscape.

Aside from the effects following a lessened annual precipitation there is also to be taken into account the increasing uncertainty of rain, or its irregularity, which is also a marked characteristic of the Saharan climate. In the desert, also, storms are likely to be of the torrential type. I did not observe vegetation characteristics which appeared directly traceable to the irregularity in rainfall, but Hayward reports an interesting condition observed by him in the southern Sahara, near Kidal, where large tracts of Mimosa had died from an unusually long period (five years) without rain. It is not at all improbable that to the cause named much is directly traceable which is generally attributed to insufficient rainfall taken in the usual sense. It is probable that the vegetation of the desert—the amount as well as kind—is due to the capacity of desert forms to meet successfully the occasional, even rare, conditions, of whatever sort.

THE SOIL RELATION IN SOUTHERN ALGERIA.

A very important environmental factor, although one which can not at this time be adequately presented, is the soil relation. Nowhere is the edaphic factor more important than in the desert, where quantity and quality are always important and occasionally even determining factors. In this connection I do not refer particularly to dunes or to chotts, but to country soils, that is, the sort most commonly to be found, which in southern Algeria is a clay with sand present in greater or less amount. So far as the relation of plant to soil refers to the presence or the absence of the plant, the problem can be briefly stated thus: Given similar kinds of soil and an equal precipitation, areas where, within limits, there is greatest depth of soil will have the largest number of plants, and areas with light soil covering will have few or no plants. Also, having given sufficient soil, the kind of plants present, together with certain root-types, will depend on the soil depth. It should be understood that these generalizations are supposed to apply to southern Algeria and not the deserts in general, or at least not to semi-deserts where in certain regards a very different condition obtains. The plant distribution thus dependent on soil depth, and the root character also having relation thereto, will be presented below; the soil conditions, so far as they are known to me, can be given briefly in this place.

From a few excavations on the hamada near Laghouat, on an analogous area by Tilrempt, on the hamada at Ghardaia, between Ghardaia and Ouargla (Gantara), and by Biskra, in each instance with analogous topographical conditions, it was learned, in short, that on formations of this sort the soil is usually less than 50 cm. in depth, although where there are rocks or in drainage depressions the depth may be greater. In the oueds, where a different type of soil occurs, greater soil depth is naturally found; also on the reg, or alluvial plain, frequently at least the flood-plain of oueds, the soil conditions are peculiar and the soil is deeper than on the higher hamada. The special significance lies not so much in the differences in the soil per se, but in the differences in the water relation occasioned by variation in depth. Owing to want of data in regard to penetration of water and its retention on the Algerian soils, the soil-moisture relation can only be gathered from inference. As regards rains, it is probable that light showers, those so slight as not to penetrate over 1.0 cm., have little or no direct influence on the perennial vegetation, but greater penetration directly benefits such plants. The first conclusion is drawn from the observation that filamentous rootlets (seemingly like the deciduous rootlets of the perennials of the Tucson region) on Haloxylon at Biskra were not found nearer the surface than 8 cm. Should the more superficial soil layer be moist for any considerable time, there would apparently be no reason why such temporary rootlets should not be formed nearer the surface. However, should there be sufficient moisture in the soil to permit absorption by roots, provided a slow rate was adequate to replace the transpiration loss, which rate was made lower by a more moist air, a slight rain would be of great significance, even if it did not penetrate to any appreciable depth. On the intensely arid desert such slight modifications of the water relations as the lowering of the temperature as winter approaches, causing decreased evaporation or rains, although actually small in amount, may be of large moment to plants. Such a condition was noted at Ghardaia, where there had been a drought for over twelve months, but on the return of the cool season, with a lower evaporation rate, growth was resumed and several plants came into flower. This appeared to be not wholly the stimulus of lower temperature, since I was informed by good authority that the plants renewed their various activities whenever rains chanced to come, whatever might be the season.