It will be well to summarize some of the main facts regarding another
and important environmental feature of the Saharan plants, namely,
the relation to herbivorous animals. In the western Sahara, wherever
there is any forage, animals which subsist on it are to be found. Of
the wild animals the gazelle is probably the most numerous and the
most destructive. All travelers across the desert have noted the
presence of this animal. In the northern Sahara Tristram remarked its
abundance fifty or more years ago, and it may be frequently seen by
the traveler at the present time. In addition to the native animals,
the domestic animals, especially the sheep, goats, and camels, are
very numerous, very destructive of plants, and range great distances
for food. As a result, an area around every well or oasis, extending
as far as 40 kilometers or even much more than this,[38] is repeatedly
grazed over and has been utilized in this manner for centuries. As a
result only the poisonous or the distasteful species, or the plants
especially well armed, are left undisturbed to grow and reproduce,
while the balance are more or less consumed, frequently so much so as
to be quite unrecognizable. From the large number of camels, sheep,
and goats which range the desert pastures it might be concluded
that the leading types of plants to be found would be such as are
not eaten by them, but this is not the case. On the other hand,
possibly the most generally consumed form is _Haloxylon_, which
grows on the oued flood-plain, the reg, and the hamada. Thus,
so far as this type is concerned, there is probably little or no
diminution in numbers because of the attacks of animals.
A similar conclusion would doubtless be drawn after study of other
forms, but there lies at least one notable exception, namely,
the influence of animal grazing on the distribution of the betoum
(_Pistacia atlantica_). The betoum, which is the largest arboreal
species in the Sahara, is confined to the region of the Dayas;
that is, to the country immediately south of Laghouat. The tree
is unarmed and is eagerly sought after by all herbivorous animals
for its foliage and tender twigs. Owing to the presence of such
animals, wild and domesticated, the young tree would have no chance
to survive were it not that, growing in association with it, is
the jujube (_Zizyphus lotus_), which is armed and is not eaten by
any animals. The jujube affords safe protection for the seedling
betoum, and in its capacity as nurse prevents predatory attacks by
animals during the critical period. The survival (and probably the
distribution as well) of the betoum is mainly conditioned on the
presence of its protector.
When I first visited southern Algeria it seemed improbable that any
portion of it, or at least any portion that I should be likely to
see, would exhibit the possibilities of plant growth as unaffected
by herbivorous animals. But finally there were found two classes of
plant formations in which animals either had not intruded at all,
or not to a harmful degree. One of these is the wide flood-plain
of the Oued M’Zab, or one of its tributaries, lying about 50
kilometers east of Ghardaia. Despite the fact that the flocks are
very numerous in the vicinity, and that the flood-plain is on the
regular caravan route between Ghardaia and Ouargla, there are few
signs of grazing. The entire plain is so well covered by shrubs
that the vegetation gives the tone to the landscape—a rare thing
in the desert. Here one finds _Retama retam_, _Genista saharæ_,
and _Ephedra_ sp., as well as other species in fair abundance. Thus
the plants are not only numerous, but are of a good size. It should
be observed that on either side of the flood-plain, on the hamada,
scarcely any vegetation may be found.
The second formation referred to is that of protected areas,
especially at Ghardaia, which have been little disturbed at any
time, and portions of them not at all disturbed for centuries. These
are the cemeteries. Such areas are situated not only in the floor
of the M’Zab Valley, where the plant conditions are relatively
favorable, but also on the hamada, where they are relatively very
unfavorable. Both in the valley and on the hamada, as well as on
the valley wall between the two wherever there chances to be soil,
the plants are relatively numerous and of fair size. This fact has
been detailed under the section on Ghardaia and need not be more
than mentioned here.
From these two general observations (exceptions to the usual
conditions), that on the flood-plain of the Oued M’Zab and the
cemeteries at Ghardaia, it is concluded that the grazing of animals
has had a very marked influence in modifying the flora of southern
Algeria. So far as could be told from the limited opportunity to
observe, the modification has gone along on at least two lines,
which are, of course, closely related. The size of the plants eaten
is much under normal for the particular locality, and at the same
time the capacity of the plant for reproduction has been greatly
lessened. That such species as are not touched by animals have not
spread more rapidly, or even have not become the dominant forms,
is an interesting problem, and one that would have to be worked out
for each species. It can be suggested, however, that the restricted
distribution probably lies in the fact that the struggle of desert
plants is mainly with an adverse physical environment rather than
with one another, and that such conditions would not be affected
by grazing animals. For example, large shrubs do not occur on the
hamada, hence _Tamarix_, _Zizyphus_, and _Rhus_, as well as _Peganum
harmala_, are limited to situations where the soil is fairly deep
and the water relatively favorable. _Euphorbia guyoniana_ also
occurs only in sandy soil, and the number of plants limited in
their distribution to soil characters is necessarily a large one.
COMPARISON OF SOME GENERAL FEATURES OF THE VEGETATION OF SOUTHERN
ALGERIA AND OF SOUTHERN ARIZONA.
When we compare the most striking characteristics of the vegetation
of the Algerian Sahara with that of the Tucson region where
the Desert Laboratory is situated, we find some interesting
differences, which may be summed up in the terms “desert”
and “semi-desert,” as applied to the two widely separated
regions. What is meant by these terms will be apparent from the
following short characterization:
Passing into the Sahara from the Saharan Atlas, over the route which
I followed, one encounters a great variety of topography, of which
the most extensive may for the moment be classed as plains. The
plains are divided into three well-marked regions, that of the
dayas, the Chebka, and the Gantara (hamada). The topography is
further diversified by oueds and their flood-plains and by low,
flat-topped mountains. On the northern portion of the plains one
encounters a sparse population of low perennials, and as Ghardaia
is approached the plains vegetation becomes continuously poorer
until at Ghardaia there appears to be none. On the hamada between
Ghardaia and Ouargla areas are to be crossed, several kilometers
in width, where perennials are wholly lacking. The decreasing plant
population of the plains, until it entirely disappears, is entirely
due to the increase in aridity as one goes from the mountains to
and across the Ghardaia-Ouargla country. The low mountains are
almost entirely barren. The flood-plains of the oueds, however,
support a surprisingly luxuriant population of perennials.
Should we contrast the topography and vegetation of the Algerian
Sahara with that of southern Arizona we would find little that
is similar and much that is different. The wide-stretching plains
(bajada) of southern Arizona are well covered with perennials of
good size. The water-courses are fringed with trees, and often an
open forest is to be found on the flood-plains. The low mountains
have a fairly dense plant population, partly of trees, and the
lower mountain slopes are often covered with a mixed flora of
shrubs and trees. It may be said that there is probably no large
area in southern Arizona, where the soil conditions are favorable
for plants, where the water conditions are too meager to support a
perennial flora of some sort. The greater aridity of the northern
portion of the Sahara is evident, therefore, from the great contrast
in its flora.
In crossing the plains of southern Algeria one is likely to call a
region barren when close inspection will show that this is not the
condition. In fact, it was found that areas on the plain, 16 by
16 meters in extent, carried as many as 330 or more perennials,
although a casual glance did not reveal the presence of any
conspicuous vegetation.[39] Both of these conditions are the immediate
result of the small rainfall. The reason for the large number of
plants in certain areas, as above noted, lies in their small size,
since it would probably be difficult to find an equal number on
this area were the plants as large, for example, as in southern
Arizona. The fact that the perennials are inconspicuous is in part
because they are small and in part because the leaves are either
absent or greatly reduced.
When viewed somewhat more closely, one finds other features
in which the flora of southern Arizona and of southern Algeria
are unlike. Travelers, botanical as well as non-botanical, have
described the armed condition of the Saharan plants until the
impression is general that such plants as persist from season to
season are usually well provided with spines. What may be the
proportion of armed to unarmed plants in the northern Sahara I
do not know, but to a person familiar with the plants of southern
Arizona, where spinose forms are very numerous, the Algerian plants
do not appear especially well protected. As this appears to be a
general condition, it is scarcely an accident that the spines of
the American species of the genus _Zizyphus_, for example, are much
better developed than are those of the Algerian representative of
the same genus. From the circumstance that grazing by wild as well
as by domestic animals is very destructive in Algeria, apparently
more so than in Arizona, where the results of grazing are scarcely
to be noted, the general facts regarding spininess in plants, as
given above, suggest the really small influence such animals play
in shaping such a character in desert plants.
Finally, it need only be remarked that plants with a water-balance
are wanting in southern Algeria, and that they constitute one of
the striking features of the flora of the southwestern United States.
[Plate 2:
Illustration: Fig. 4. Shoot-habit of Acanthyllis tragacanthoides.
Laghouat.
Illustration: Fig. 5. Shoot-habit of Zollikoferia spinosa. Laghouat.
Illustration: Fig. 6. Acanthyllis tragacanthoides on sandy plain.
Laghouat.
Illustration: Fig. 7. Zollikoferia spinosa in habitat, plain
(hamada). Laghouat.]
[Plate 3:
Illustration: Fig. 8. Detail of north slope of Nomad Mountains where
Zollikoferia spinosa is the dominant species. Laghouat.
Illustration: Fig. 9. Vegetation of plain (hamada) at Tilrempt. The
conspicuous shrub is Haloxylon articulatum.
Illustration: Fig. 10. The Daya of Tilrempt from the plain, showing
the character of the depression.]
[Plate 4:
Illustration: Fig. 11. Near view of the Daya of Tilrempt. The
fortified stage station, bordj, and a nomad camp are to be seen. The
flattened, level, lower surface of the trees is the effect of
grazing, mainly by camels.
Illustration: Fig. 12. Jujube (Zizyphus) shrubs and betoum (Pistacia)
at the Daya of Tilrempt.]
[Plate 5:
Illustration: Fig. 13. A young specimen of betoum in the midst of a
protecting jujube, at the Daya of Tilrempt.
Illustration: Fig. 14. Leaf and shoot habit of the jujube (Zizyphus
lotus). Daya of Tilrempt.
Illustration: Fig. 15. Leaves of the betoum (Pistacia atlantica),
from the Daya of Tilrempt.]
[Plate 6:
Illustration: Fig. 16. South wall of the valley of the Oued M’Zab at
Ghardaia.
Illustration: Fig. 17. Detail of an eroded bank of the Oued M’Zab at
Ghardaia. The overhanging stratum is hardpan similar to the “caliche”
of southwestern United States.]
[Plate 7:
Illustration: Fig. 18. Shoot-habit of Haloxylon articulatum, from the
plain near the Daya of Tilrempt.
Illustration: Fig. 19. Agriculture at Ghardaia. The fields are
divided into plots about 3 by 4 feet in size, for the more economical
use of water. Young barley is shown growing (November).]
[Plate 8:
Illustration: Fig. 20. Vegetation in upper part of a “draw” on plain
north of the M’Zab Valley Ghardaia.
Illustration: Fig. 21. Situation of square No. 2. on plain north of
valley at Ghardaia.]
[Plate 9:
Illustration: Fig. 22. Capparis spinosa at base of valley wall at
Ghardaia.