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.[34] 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.