Botanical features of the Algerian Sahara

by William Austin Cannon

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.[33]

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.