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,[35]
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.
From the observations last given it appears that sufficient moisture
persists in the soil to tide perennials over the long periods of
drought, although not in sufficient amount to permit active growth
during the dry seasons. This is not an uncommon occurrence with
desert plants. For example, whenever in the Tucson region the arid
seasons are uncommonly long, or there has been a relatively small
rain, as 25 per cent less than usual, much of the vegetation may
remain dormant. Under such conditions an evergreen like _Covillea
tridentata_ drops all save the youngest and smallest leaves and
maintains this nearly defoliate condition for long periods, with
little other change. The plant behavior noted is always connected
with insufficient moisture.
The soil in the oueds and probably also in the reg, as well as
that of the dayas, is of considerable depth. The soil depth, or
rather the depth before solid rock is encountered in the valley
of the Oued M’Zab at Ghardaia, is from 20 to 30 meters, or even
more; at the Daya Tilrempt the depth to water in one of the wells
is about 95 meters. Although there is much gravel and sand in the
fill of these depressions, it is likely that they afford the most
favorable soil conditions in the desert for the development of
a large root-system. Be that as it may, it is certain that large
plants, such as _Tamarix_ and _Pistacia_, occur only in such places.
Where there is most soil on the hamada, as in certain pockets near
the old hill town of Ghardaia, abandoned several centuries since,
one finds also the most and largest plants of this, the hamada,
formation. Moreover, at the time in November when growth was noticed
in the oued plants, it was also taking place in these favoring
situations, about 50 meters above the level of the valley of the
M’Zab and much above any possible permanent water-supply such as
is afforded by a water-table.
The various habitats, therefore, not including the oases, are
naturally closely associated with soil differences. These are few
and, in nearly each case, to name is sufficient definition. There
are dunes (areg), hamada or stony desert, reg or alluvial desert,
the daya, and the flood-plain of the oueds (reg?). Modification
of the hamada, reg, and oued flood-plain occurs whenever white
hard-pan (caliche) is present. The soil of the reg, often that
of the oued flood-plain and that of the daya, is fine alluvial
and is relatively or actually deep. The hamada has the poorest
soil condition, being underlaid by rock, and often or always by
hard-pan as well. Large stones and boulders are embedded in the
soil or lie on the surface. A modification of the hamada occurs
whenever sand is strewn over its surface, even if the sand is only
a few centimeters in thickness. This acts as an effectual mulch,
increasing the retentive capacity of the soil, and very strikingly
changes the character of the vegetation. The final habitat to be
mentioned is the salt spot, or chott, where gypsum constitutes an
important salt.
As regards their relative importance the habitats in southern Algeria
can probably be grouped in the following sequence, a relation which
very possibly holds good for the Sahara taken as a whole: Hamada,
dune, oued, flood-plain, reg, daya.[36]
The habitats are unlike as regards the relation to the rainfall
and its effects. This is in part due to the differences in soils,
or their depth, and in part to topographical differences. By the
latter the low-lying areas receive a relatively large amount of
water; and since their soil is relatively deep the water is retained
longer than on the hamada, for instance, where the soil is shallow.
The habitat preferences of the plants of southern Algeria are
marked, as would be expected from the striking differences in
the habitats. On the dunes, for example, we find drinn (_Aristida
pungens_) as possibly the most commonly occurring and the most widely
distributed sand-plant. One finds on sandy areas also _Tamarix_ sp.,
_Euphorbia guyoniana_, _Ephedra_ sp., _Retama retam_, _Limoniastrum
guyonianum_ (zaita), and other forms in smaller numbers. On the
oued banks there are _Tamarix_, _Nerium oleander_ (Laghouat),
and, near the oases, date palm and other introduced plants. On the
flood-plains will be found a large number of species, among the most
typical of which are _Peganum harmala_, _Retama retam_, _Ephedra_,
_Genista_ sp., and _Haloxylon_ sp. The typical plants of the dayas
are _Pistacia atlantica_ and _Zizyphus lotus_, the latter occurring
on flood-plains and the reg as well. On the chotts we find mainly
such halophytes as _Anabasis articulata_, _Halocnemon strobilaceum_,
_Salsola_ sp., _Limoniastrum guyonianum_, and others. The flora of
the reg, so far as my observations show the conditions obtaining, is
essentially like that of the flood-plains, which would be expected
from the relation of the two habitats. South of Biskra, however,
where the reg is probably not of fluvial origin, one finds a forest
of _Zizyphus lotus_, and much _Ephedra_ sp. among the most striking
forms. Finally, the flora of the hamada, which has a peculiar stamp,
can be briefly characterized.
On the hamada are to be found the fewest species and the smallest
individuals. Probably most perennials of the hamada are under 30
cm. in height. Among the species characteristic of the hamada are
_Artemisia_, _Teucrium_, _Deverra_, _Centaurea_, _Acanthyllis_,
_Thymelæa_, _Echinops_, _Henophyton_, and _Haloxylon_. The last
named is possibly the most widely distributed, occurring in other
habitats as well.