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.