The Peruvian-Chilean Desert

B The Peruvian-Chilean Desert

Introduction

In South America there are two very exten-sive subtropical deserts. One is the Monte Desert in the Argentine (p. 255); the other is the Peruvian-Chilean desert which forms a

Fig. 3.37. Map of the Pacific coast of Ecuador and, from 8 S to 28 S, the coastal desert in Peru and Chile. Fine hatching shows the foot of the Andean chain

narrow strip along the Pacific Coast, extend-ing from about 8 S across the tropic of Capricorn, to about 28 S (Fig. 3.37). It lies in the rain shadow of the high Andean chain, which, rising to more than 6000 m, shelters this desert from the SE trade wind. Another important factor determining the character of this desert is the cold Humboldt or Peruvian current off the west coast of South America. The rotation of the earth results in a west-ward deflection of this current, so that north of 30 S it is fed by cold water brought from the depths of the ocean to the surface. The current leaves the co ast only at ab out 4 S, at Cabo Blanco, and flows westwards to the Galapagos Islands. The temperature of the water on the Chile an and Peruvian coasts is thus relatively low, 17C in winter and 24C in summer.

The older literature on this area includes the geographically-oriented works of Ber-ninger (1925) and Troll (1930); the climate was described by Dberitz (1967). the vege-tation of the Chile an part by Reiche (1907) and Schmithsen (1956), that of the Peruvian part by Weberbauer (1911) and Rauh (1985). A more recent review on Peru has been pre-pared by Koepcke (1961). The description which follows is based mainly on the latter, since we have no personal experience of this region.

The Peruvian Desert

1 Climate

Under the influence of the cold Humboldt current the climate of this desert is always cool. Mean monthly temperature range is 5.T-7.7C. March is the warmest month, September the coolest.

The Peruvian-Chilean Desert

Monthly means: Callao (on the coast)

September 17.9C

Lima (30 km inland) 23C 16.6C The latitude has little influence on the

temperature: the temperature difference be-tween Callao and the harbour Mollendo, 5 further south, is only 5C.

Rainfall along the coastal strip is almost too small to be measured. At Casa Grande (7 45' S) it fluctuates between 8 and 33 mm, but rain in measurable quantities fell on only 17 of 69 rainy days. In Lima the measurable rainfall from 1943 to 1947 was between 11 and 51 mm. Mean values calculated over many years are thus not very useful. This is especially true of the driest part in northern Chile.

Schmidthsen (1956) gives the following data for rainfall: Arica: In 39 years there were only 4 years with more than 2 mm of rain (annual mean O.6mm). Iquique: In 49 years there were only 17 years with more than 2mm of rain (annual mean 1.9mm).

TRUJ l lLO(60m) [~ J

...... .. .. .. .. .. ..

.................. L..._._._._._ .... -.... : .. :.:.: .. : .. : .. : .. : .. : .. : .. : .. : .. : .... .. .. "." ......................................... .

:::::::::::::::::::::::::::::::::::::::::::: : ............................................

.. .. .. .. .. .. .. .. .. .. .. .. .. .. ..........

"." .................. + .. "

LlMA(I58m) 115- 18J

263

Tocopilla: In 16 years there were only 6 with more than 2 mm (annual mean 3.8 mm).

This must be borne in mind when the climatic diagrams are studied (Fig. 3.38).

The seasonal distribution of the rare bouts of rain is shown in Table 3.11: in the most northerly parts of the desert the rain falls in summer, the central region has rain at any time of the year, while in northern Chile, if rain falls at all it is during the winter months.

The rare "nifto" (= Christ Child) years are an exception: these are years when, from Ecuador to far into the desert, rain falls at Christmas. In such years there is stronger air circulation in the tropics so that the cold Peruvian current is replaced by a warm equatorial current trom the Gulf of Guayaquil and extremely heavy rain falls on the coast. The water flows off the hard sur-face crusts of the desert soils, causing vast floods. The last such catastrophic year was 1982, when the floodwaters in the most northerly part of the desert even washed away concrete bridges over gullies that are normally almost completely dry.

ANTOFAGAS TA ( 9~ m ) [9 - In

.. ............ .. .. ................ ..

L..:~-~~-~~-~~""'~~crtIIrIItt, Fig. 3.38. Climatic dia grams for stations in the coastal desert of SW Peru and N Chile (Antofagasta). The rainfall data are very different from those in Table 3.11; this is commonly the case in arid areas when data are presented for different years

Table 3.11. Rainfall (in mm) recorded at meteorological stations from north to south of the Peruvian coast: stations situated somewhat inland are shown in brackets (from Ellenberg 1959)

Station m Summer Winter months Summer Year NN (total)

Jan Feb Mar Apr May June July Aug Sept Oet Nov Dee

Chicalayo 31 0 0 0 0 0 0 1 0 5 Trujillo 26 0 0 0 0 0 0 0 0 0 0 0 3 Paramonga 6 0 0 0 6 3 2 0 0 0 15 (Lirna) 137 0 0 3 5 6 7 6 2 34 Ganete 36 0 0 9 3 4 3 3 1 12 40 Pis co 6 0 0 0 0 0 0 0 0 0 0 2 Lomas 10 0 0 0 0 0 0 0 0 2 0 0 3 (Tacna) 457 3 2 0 2 4 3 3 7 8 6 3 2 43

264 s.W.

Zonobiome III: Subtropical Deserts (The Arid Zonobiome) N.E.

Over 20'C

800 Rare

- ... .. _-:. \000

'100

200 Rare (a~d only lasting untll morning)

Aboutt7C . 1~~~~~ __ ~~~~~~O~rt o '=-- TIliandsias : Pacific -

Fig.3.39. Schematic profile through a Peruvian coastalloma to show the freque ncy of fog (garua) and distribution of vegetation during the southe rn winter (after Ellenberg 1959)

Rain is, however, less important for the vegetation of this coastal desert than fog, known as "garua". This forms above the cold ocean current and is blown inland by the south-westerly winds that arise as a re-sult of the greater warming of the land mass. In some places the cordilleras rise to 1000 m dose to the coast and here the fog is carried upwards across the slopes. As it rises, the air cools and the fog becomes denser and satu-rated with even larger water droplets. Thus the wetting effect, small on the flat beach, increases with altitude up to 700 m NN; thereafter it decreases as the greater part of the water content of the air has condensed (Fig. 3.39).

These wetting fogs occur during the winter months, May to October, but espe-cially in July and August, while in the sum-mer the air over the land warms up so rapid-ly that there is hardly any condensation. This garua or winter fog makes possible the development of the "Ioma" vegetation, so that this region is less desertlike in character. Since the fogs rarely pass over the coastal cordillera, the extreme, barren desert begins behind this mountain range. Further north, on the coast of Ecuador, fog also occurs (Diels 1937), but in this rainy area its effect is less noticeable.

2 Soils

As in other deserts, these are mostly raw soils.

3 Sub division of the Vegetation

3.1 Peruvian Desert

The sub division of the vegetation of the Peruvian part of the desert is shown in Fig. 3.40. The following components can be dis-tinguished :

1. An area of desert along the co ast, con-sisting of the flat coastal plain and the west-ern slopes of the coastal mountains; this re-gion is affected by the fog.

2. An inland, extreme desert, lying be-tween the coastal mountains and the lowest, western slopes of the Andes; this area is not reached by the garua fog, has almost no rainfall, and is a barren rock desert. On the western slopes of the Andes rain fall in-creases with altitude and the desert is re-placed by Orobiome ur, which extends to the nival belt of the highest peak (p . 272).

The islands lying off the coast are the breeding grounds of sea birds and are cover-ed with guano; for this reason they have no vegetation.

3.1.1 Subdivision oi the Fog Desert

This is shown in Hg. 3.39. On the coast itself, fog is rare, occurring only at night and form-ing dew.

The sand of the beach is barren except on dunes, where narrow strips of large species of Tillandsia (Bromeliaceae) occur. The Til-landsia rosettes lie loosely on the sand and


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265

1 ___ - .... 1 Comumd~dej d~ p/4"f~$ ~/mqhld,U4US ${JMwIJ'1 It"'y-.... v I P'!lOfM/d~ AJnff lLi!j FS/IIp.l dtgf'imr"'4S l(1q~r~US'(}S (flSP~r1~S

~ Sm.nl'p.nIS

266 Zonobiome III: Subtropical Deserts (The Arid Zonobiome)

Tlilandsla straminea

JiXIIJ

8 .'11 .7 July 1958

Fig. 3.42. Water balance of Tillandsias in the Peru-vian coastal desert during July 1958. A Weight at 09.00h, after dew fall: B weight at 17.00h (after Alvim and Uze da, personal communication). For further explanation, see text

water. It follows that the Tillandsias can only absorb water through their leaves when they are weUed directly.

Water gained and lost can easily be determined in the natural habitat. Plants weighing about 3 kg were placed on a plastic sheet and left to grow in situ. Successive weighings were done at 09.00 h, when the dew had already disappeared, and at 17.00 h, after the plant had transpired the whole day. The investigation was made in July under the most favourable conditions. Fog and dew are particularly frequent at this cool season of the year. The mean temperature was 14C and the mean vapour pressure was 12 mmHg. The results are shown in Fig.3.42. The increase in weight during the night and its loss during the day can be clearly recognized.

Calculation of the mean fresh weight increase during July gave a growth rate of 0.17'Yo day'. Thus the plants grow at an extremely slow rate even in this favourable season. This investigation shows that the Tillandsias survive entirely on the water which they absorb from condensation of fog or dew with their absorbing scales and they are thus able to live in a practically rainless fog desert.

The Tillandsias reproduce mainly vegeta-tively, from parts that are broken off and scattered by the wind. Their arrangement in rows, at right angles to the direction of the wind, suggests that this orientalion is espe-cially favourable for condensation of fog or dew, which occurs here at night and in very small quantities only. In some the growing tips are oriented to the sea, from which the

fog comes, while the parts of the shoot that are directed inland die and turn black. They have colonized a habitat on which other plants, even cryptogams, cannot grow.

Tillandsias are also found in another biotope in this fog desert, behind the coastal cordilleros, in finger-shaped stretches where the fog penetrates the mountain barrier: this is indicated in Figs. 3.39 and 3.40.

As can be seen in Fig.3.39, the crypto-gamic zone begins a few kilometers inland, where the land begins to rise and condensing fog wets the ground more heavily. Accord-ing to Ellenberg (1959), thick, jellylike masses of blue-green algae, Nostoc cummune and others, are found here. They cover at most one-third of the surface. Unlike the Tilland-sias, the algae cannot stand being covered with blown sand. South of Lima, on stable sand, there are large colonies of lichens (species of Cladonia). while colourful crus-tose lichens are found on stones. The effect of fog and the distribution of the vegetation are closely related to the relief of the terrain.

Rauh has pointed out that amongst the Til-landsias there may be smaller or larger stands of cacti; these are low creeping forms which are spherical or have short columns. According to Ellenberg (1981). these "cactus lomas" occupy special, localized biotopes, where at night dry winds blow from the mountains and disperse the garua fog. The tender loma herbs cannot survive this periodic drying out, but it is actually favour-able to cacti for it prevents cactus seedlings from being infected with fungi and also eliminates competition from the herbs (Fig. 3.43).

South of Trujillo, in the more humid north, groups of Haageocereus repens occur; its 2-m-Iong shoots are flat, have roots on the lower surface and are partially covered with sand: only the erect tips emerge and point seawards. The water economy of these cacti has not yet been investigated. It can, how-ever, be assumed that they have diurnal acid metabolism, so that the stomata are open only during the humid nights, and that water uptake is through the roots. Near Chala in southern Peru species of Islaya form their own community ne ar to the coast, in an area which otherwise includes only cyanophytes. In northern Chile the Copiapoa cinerea cactus community is characterized

The Peruvian-Chilean Desert 267

NE

SW LOMAS OF MOllENDO

.. MOLLENDO .........---... ~,as~,;,-,:'$::=:.,::5;=-_--__ -_ lOOOL~ __ _ ~ ~ rare ~~:--... . .. .. ---:-~ /t'J dry land breeze

800 . ______ - - - .- -- - . : 'opE,n lOg ~eg,;lallon (tlerbs) . : . ' .. : . ' " . ~ (at night)

600 -- -- ~s~~u:~ - --_::.;~~~-j~-:~~~:;:~!;-:0iSr .. /:E2> GaruafrequentweHlng :. :.:::':.


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Fig.3.45. Soil moisture conte nt (%) beneath an open Eucalyptus stand in the lomas of Lachay in 1957. In this year the fog season began only in June (after Ellenberg 1959)

PrecpltBl3O~mm , So'.-\gs,. aV8I'age

CUlt1eme" Oryytar1e57

PI~e:~~~I.~:;ed 59 .. rnrn

NO veQe!lIIhon

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LJm.lcl

RooI.

Fig.3.46. Quantity of precipitation and soil mois-ture in sandy soils beneath different vegetation in the lomas of Lachay (after Ellenberg 1959)

ately cloudy days evaporation, measured with Piche evaporimeters, reached a maxi-mum of 0.45 ml h -\ only on dry days, with an air temperature of 28C, did it exceed 0.7ml h- 1.

The natural loma vegetation on the lower slopes consists entirely of annuals. These are various indigenous Nolana spp. (Nolanaceae, aff. Solanaceae, Convolvulaceae). At first there are no grasses, but they become more frequent with greater soil humidity higher up the slope; the herbs still dominate, how-ever, forming a thick carpet, 60 cm high. This is comprised of the tender-Ieaved Nico-tiana paniculata, Galinsoga, Palaua malvi-folia (Malvaceae). Nama dichotoma (Hydro-phyllaceae). Drymaria weberbaueri and Spergularia collina (Caryophyllaceae). Bow-

lesia palmata (Apiaceae). Plantago limensis, Loasa urens, which is covered with stinging hairs, and many others. Plants regarded as weeds in Europe are also found here, includ-ing Sonchus oleraceus, Stachys arvensis, Erodium cicutarium, Stellaria media. None oi these species can be regarded as fog plants, for water uptake is mainly through the raots from the well-moistened soil. A possible small uptake of water through the leaves is of no ecological significance.

Apart from the annuals, a number of geo-phytes (Iridaceae, Solanum, Oxalis and others) are also characteristic, as are the large-Ieaved narcissus Hymenocallis aman-coes, the Amaryllidaceae Zeyphyranthes and Stenomesson coccineum; the latter developed only leaves in the foggy winter, while the orange-red flowers develop from the bulb only in summer. Semi-succulent species of Calandrinia (Portulacaceae) occur in large numbers in some places.

Woody plants are found in places where the fog is thickest, at 450-600 m NN; they are usually isolated specimens, however, growing on rocks; they include Acacia macracantha, Carica candicans, Capparis ptisca, Caesalpinia tinctoria and the poison-ous Croton shrubs. Near Atiquita which is close to Chala (Fig. 3.32) there a small patch of forest with Eugenia ierreyrai, which has hard, evergreen leaves and a thick crown canopy above 5-8-m-high trunks. The branches of these woody plants are thickly covered with liverworts and mosses, that hang down in long beards. Even the small semi-succulent Peperomia crystallina occurs as an epiphyte, and so do species of Polypodium. In the forest shade there are several hygromorphic ferns.

Ellenberg (1959) believes that such forests were at one time widespread, but have since been destroyed by man; the reason for this view is that during the main period of growth of the loma vegetation at 300-500 m NN, Indians drive their cattle out of the dry mountains into the loma region and use the trees for firewood. Rauh (1985) argues, how-ever, that the loma trees nowhere form a closed forest belt, occurring always as is-lands in places where much fog collects, separated by wide expanses of desert. Man has, however, probably caused a certain amount of degradation.


In northern Chile the coastal cordillera re-treat from the coastline, so that there are wide sand flats between the mountains and the sea, and no fog is formed. A loma vege-tation is found here only far from the coast, at 1000-1200m NN.

The last spurs of fog vegetation, consisting of colourful crustose lichens, are found on stone heaps at 800-1000m NN. Higher up, often to 2000 m, the mountain slopes are al-most completely barren.

4 Loma Fauna

There is, of course, a characteristic fauna as-sociated with the loma vegetation in Peru (Koepke 1961). Flies swarm around the flowers, carabid bettles are very numerous; the spider Lactrodectus and the scorpion Hadruroides lunutas also occur here, the latter being found on the Galapagos Islands as weH; woodlice further the process of soil formation with their excrement. Several varieties of Bulinus snails are found on the separate loma islands. This is true also of the bird, Geositta peruviana, which seeks shelter in holes in the sand. There are also lizards and a smaH fox, a species of Dusicyon.

5 Oases

The valleys of the 41 rivers of the Andes are densely populated oases. In the Peruvian part of the desert they penetrate through the coastal cordillera and flow into the sea. Salix humboldtiana and the pepper tree, Schinus molle, grow on the banks of the rivers and along their edges the tall grass, Gynerium sagittatum. On the broad terraces crops are planted and irrigated; besides grain crops and vegetables, cotton, sugar cane and lucern are planted. In southern Peru there are olives, in central Peru citrus fruits and higher up the valley, near the rock desert, apples, pears and plums. Viticulture is mainly concerned with the production of spirits.

Along the coast north of Lima as far as Chidayo (see Fig.3.37) there are large irri-gated areas with cotton and sugar cane

269

plantations; these stretch beyond Trujillo. The Tinajones project has made it possible to lead large quantities of water into the desert and extend the areas under irrigation.

The North-Chilean Desert Conditions are far more extreme in the part of the desert which lies in northern Chile, because there is no coastal fog here and rain falls on very rare occasions in the winter.

The region is almost completely barren of flowering plants. Between Iquique and Arica there is, at the most, a grey-green tinge which doser examination shows to be fructose lichens Anaptychia leucomelaena and Ramalina cerruchis and air alga Trente-pohlia polycarpa. Follmann (1965) has de-scribed window lichens from the Atacama Desert, between 22 Sand 27 S, members of the families Buelliaceae, Dermatocarpaceae, Lecideaceae and Acarosporaceae. Their only source of water is the nightly dew; they lie buried in the sand, only the disc-shaped fruiting bodies projecting just above the grains of sand. It is possible to cultivate some blue-green algae from sand taken near Caldera and Antofagasta - Schizothrix atacamensis, Calothrix desertica. There are also two forms of Cyanidium which are endolithic and occur in cracks in diorite. This is also an area where the migratory lichens and earth cacti are found (p. 267).

There is no loma vegetation in this region. If rain falls in the winter, the desert becomes carpeted with flowers for abrief period.

Kohler (1967) described such an event in the north-Chilean Atacama Desert in 1965. By the 18th of July, 22 mm rain had fallen. Since temper-atures are relatively low in the winter months, growth was slow. The main flowering of Nolana baccata and Hippeastrum thus took place only at the beginning of September, that of Calandrinia and Cristaria at the beginning of October; the seeds were ripe by the end of October. Amongst the ephemerals, a distinction must be made be-tween annuals and geophytes (ephemeroids). The former survive the rain-free period as seeds, the latter as tubers or bulbs underground. The geo-phytes appear to require more water far their development, for where the relief is undulating, only annuals are found on the elevations; here they have a cover of 10-40'X" and their roots penetrate to a depth of 10-20 cm. Geophytes are found only in the hollows, where they occur to-gether with a dwarf shrub, Skytanthus acutus


(Apocyn.). Besides species of Nolana there are several other annuals, a yellow Viola, for exam-pIe, and even Cuscuta, which parasitizes these ephemerals. Calandrinia, which has succulent leaves, is able to prolong its growth period by utilizing the stored water.

On the Pampa deI Tamarugal of the Salar de Pintados, south-east of Iquique, there are large stands of Prosopis tamarugo, a relative of P. julif1ora, the mesquite of the Sonoran Desert. In this extreme desert these trees seem especially striking.

Went (1975) has reported the astonishing inves-tigations of F. Sudzuki, which purported to show that, at night, the leaves of Prosopis bushes take up large quantities of water from the vapour-saturated air and conduct this to the roots; these excrete the water, so that the soil around the roots becomes thoroughly wetted to a depth of about 1 m, while the soil above and below remains com-pletely dry. This reserve water, it was suggested, is then available to the transpiring plant during the day.

This remarkable idea, untenable on physical grounds alone, has been shown to be mistaken by Mooney et al. (1980).

Chile an hydrologists have found that beneath the Pampa deI Tamarugal there is a groundwater lake, the level of which had varied and had dropped in the years prior to the investigation, owing to the demands made on it by Prosopis. Formerly there was a lake here which, owing to the high evapora-tion in the desert climate, became covered with a crust of sodium salts which have been exploited as a source of salpeter.

The underground water lake is fed by groundwater that flows from the tops of the mountains through the detritus which lies above the bedrock on the slopes; at the low-est point of the Pampa deI Tamarugal the lake rises almost to the soil surface (Fig. 3.47). As a typical phreatophyte, the mes-quite is limited to places where there is groundwater and its tap root can grow to considerable depths to reach this water. There can be no doubt, but that the water requirements of Prosopis tamarugo are met by the groundwater lake. The leaves of the plant show absolutely no anatomical fea-tures that would allow uptake of large quan-tities of water. It is, however, not easy to ex-plain the thick mass of roots at 1 m depth or the accumulation of water in the soil with an osmotic potential of -15 bar. The mean

Fig. 3.47. Profile through the Pampa deI Tamarugal (after Klohn, modified from Mooney et al. 1980)

8~~IO~~12--1~4--16~-18~2~O--2~2~2~4~2~~4--~1--8~-IO~~12 Hour

Fig.3.48. Curves of air temperature (0C), vapour pressure deficit (VPD in mbar) and water poten-tial (- bar) of Prosopis tamarugo near Canchones (Chile) on 5-6, Oct. 1978 (after Mooney et al. 1980)

osmotic pressure of the cell sap of the leaves was -21.3 bar. In the Sonoran Desert we measured values of - 20 bar in Prosopis juli-flora var. velutina, and during the dry sum-mer period of -30 to -34 bar; these are thus closely similar to the values for P. tamarugo.

The nightly change in direction of the vascular flow of water from the shoot to the roots observed by Sudzuki may indeed occur temporarily during the spring, when, at night, the water potential of the leaves rises to -10 bar and that in the roots is -15 bar (Fig.3.48). Normally, however, the flow of water is from the roots to the leaves.

Mooney et al. (1980) suggest that the wetting of the soil in the region of the roots


might arise when, due to the lower transpi-ration at night, some of the water absorbed from greater depths by the tap root is ex-creted by the root felt, and acts as areserve when transpiration increases during the day. This is, however, entirely speculative. It seems to us that the root felt, at 1-1.5 m depth, might have been formed at a time when the groundwater was at this level, and that recent endeavors at reforestation have led to its rapid fall. The P. tamarugo stands grow in the lowest part of the terrain, where the groundwater level is only a little below the surface.

The question arises as to how the tap root of a Prosopis seedling reaches the ground-water if the soil above is dry. This takes place, as in all phreatophytes, only when there is an exceptionally rainy year; the soil is then wet from the surface to the ground-water and the tap root grows rapidly to the depth of the groundwater before the soil dries out.

A rainy year of this sort occurred in this area at the turn of the century and again in 1977, when the water masses from the mountains flooded the whole of the Pampa deI Tamarugo, washing away the salt so that the salt-sensitive seedlings could take root. The old P. tamarugo trees should thus be about 75 years of age, but this has not been checked. In many extreme habitats the stands consist of a few generations only.

Detailed investigations in the desert a1-ways revea1 a simple explanation for obser-vations which those not acquainted with the special situation at first find inexplicable, and are thus often led to unjustifiable con-c1usions.

In reforestation with P. tamarugo today, seedlings are planted in a 40-cm-deep hole which goes be-neath the salt crust; thorough irrigation then en-sures that the soil is wetted as far down as the groundwater. The tap roots of these phreato-phytes grow rapidly in length, without producing lateral roots, and form a dense felt of lateral roots only when they have reached the capillary zone above the groundwater. Too dense afforestation can result in a fall in the groundwater level, for the water utilization by phreatophytes in the de-sert is extremely high.

The extreme desert or "Atacama" extends in northern Chile over a vast area and is one of the most desolate and barren areas on

271

earth. Rauh described a transit from Toco-pilla in the west to Calama in the east as fol-lows: "Only along small erosion gulleys does one find isolated specimens of Adesmia ata-camensis (Leguminosae), Coldenia ataca-mensis (Ehretiaceae, afL Boraginaceae) and Cristaria divaricata (Combretaceae). The lowest hill range of the Sierra Domeyko has a slightly denser cover of thorn bushes. Be-hind these hills, in the valley of San Pedro de Atacama, are oases with irrigated planta-tions. Here there are the leguminous trees, Prosopis juliflora and the "Chanar" Gourliea decorticans. South of this is the 2270 km2 Salar de Atacama and other salt lakes which are covered with thick salt crusts. On their banks are open stands of Atriplex atacamen-sis, Ephedra andina, Lippia trifida, Teneria absinthioides and Distichlis meadows, strewn with the small chenopodiacean Nitro-phila axillaris".

In Peru, however, the extreme desert is only a small area of the lower land at the foot of the western slope of the Andes, where it occurs as rock or stone desert. The upper surface of the stone heats up to 70C in the sun and the humidity of the air is extremely low. Isolated bushes are found only in places where, after exceptionally heavy rain, water flows off the slopes and is stored in the soil.

6 Orobiome III on the Western Slope of the Andes

As altitude increases on the slopes of the Andes above the extreme desert, the cactus desert begins. The floristic composition of the stands of cactus changes markedly from north to south (Rauh 1958). It starts in north-ern Peru with the largest column cactus of Peru, the 8-10-m-high Neoraimondia gigan-tea; this cactus is distributed along the whole length of the almost rain-free western slopes of the Peruvian Andes. It has a shal-low, horizontal and very branched root sys-tem. Growth in height is very slow: in cen-tral Peru it was estimated to be 20 cm in 50 years.

The most extreme habitats are occupied by the more spherical genus Melocactus (see p. 190 and Fig.2.91 on p. 212). Its deli-


cate root system penetrates into the finest cracks in the rocks and is said to absorb dew, but no detailed investigation has been made. These cacti can endure high tempera-tures of about 45C in the central water tis-sue; they shrivel up completely when it is dry, refilling their water reservoir immediate-ly should rain happen to fall. Still higher up the slope other column cacti replace those already named; amongst them is the climbing species Hylocereus venezulensis. Higher up, above the cactus desert, there may be an open deciduous forest of Bombax discolor, the crowns of which are thick with epiphytes - Tillandsia spp., Vriesea cereicola, Guz-mania monostachya and others - an indica-tion that dew probably falls frequently.

The detailed floristic composition of the cactus desert is, however, very varied, as is that of the other altitudinal belts. Rauh (1985) has described this in the Valley of Chillon, north of Lima. Between 400 and 700 m NN the rock desert is without vegeta-tion. Above this, from 700 m to 1700 m NN is the cactus desert. The first species to occur is Armatocereus procerus. On the basis of floristic composition, three different cactus altitudinal belts can be distinguished, al-though they are not sharply demarcated from one another. On the upper part of the slope are also succulent shrubs such as Jatropha macrantha and Carica candicans, succulent species of Peperomia and the suc-culent Pilea serphyllacea (Urticaceae); there are also poikilohydric ferns - species of Cheilanthes and Pellaea, and, as well, Selaginella peruviana. The higher slopes, from 700 to 2400m NN, are especially dry. Still higher up on these western slopes of the Andes, large banks of cloud form from rising air masses so that rain falls here, the quan-tity depending on the latitude.

The limits of the altitudinal belts differ greatly, depending on exposure and on the orientation of the valleys.

For central Peru the following altitudinal belts can be distinguished:

O~ 300 m Weak effect of fog: barren sand desert or Cyanophyta lichen com-munity or non-rooting tillandsia stands or coastal cacti 300~ 700 m Strong effect of fog: loma forma-

tion of annuals and geophytes or woody plants or cactus stands. At

the upper limit, the effect of fog is again weak and in places non-rooting Tillandsias occur 700~1400m Lower dry belt: only large col-

umn cacti without shrublike companion flora 1400~2400 m Upper dry belt: low column cacti

with succulent shrubs and bushes 2400~3000 m Summer rain belt: column cacti

with raingreen woody plants and annuals, and shrubs as com-panion flora 3000~3800 m Cold summer-rain belt: there are

no cacti, apart from two opun-tias; evergreen shrubs; grasses and shrubs increase with altitude 3800~4500m Summer-green grass puna:

above 4500 m only a few detritus and rock plants

Rauh lists the following fm nmthern Peru in the Valley of Rio Sana (7 S). that is, within the reaches of zonoecotone 1II/1I

O~ 100m

100~ 500m 500~ 900m

900~1500m

1500~2400m

2400~3000m

3000~3200m

Semi-desert with scattered woody groups Cactus belt Dry forest with cacti Dry forest hung with Tillandsia usneoides Evergreen montane forest with Ficus and palms High montane, evergreen forest with Podocarpus and Drymis Ericaceae-Melastomaceae shrub belt

above 3200 m Paramos, here known as Jalla This altitudinal belt series is very closely similar

to that in northern Venezuela (Vol. I, p. 206). The first two altitudinal belts are the equivalent of the Espinar-Cardonal, the next two of the Selva decidua belts. Mean annual rainfall measured over 4 years at 1750 m NN in these northern-Peruvian altitudinal belts was 1370 mm.

We will not describe here the vegetation of the deeply incised, often desertlike inner-Andean valleys. They do not belang to the Peruvian-Chilean desert, but form part of the huge meridionally oriented mountain range of the Andes (Fig. 2.52). Their vegeta-tion is greatly influenced by the strong di-urnal winds.

7 Zonoecotone III/ll

Zonoecotone IIIIIV between the Chile an Desert and the winter rain area of central


Chile will be dealt with in Volume 4, in the section on zonobiome IV. Here we will con-sider only zonoecotone III/H, the most north-erly coastal region of Peru and the most southerly part of Ecuador. There are no mangroves along the reach of the Peruvian coast which is under the influence of the cold Humboldt current. The most southerly habitat of this sort on Peruvian soil is the delta of the Rio Tumbles, on the Gulf of Guayaquil, a little north of 4 S. Here all the typical mangrove species are to be found, such as Rhizophora mangle, Avicennia tomentosa, Laguncularia racemosa and Conocarpus erecta. Further south the tidal zone consists only of a salt marsh, with species such as Sesuvium portulacastrum, Salicomia fruticosa, Batis maritima and the halophilic grasses, Distichlis, Sporobolus.

North of 80 S light summer rain falls and the desert becomes a rain-green grassland with mainly annual grasses. These include Aristida adscensionensis, Anthephora hermaphro-ditica, Bouteloua disticha, Chloris virgata and Eragrostis spp.

In the Peruvian part of the desert, north of 6 S, there are no long-lasting winter fogs. Instead, a heavier summer rain falls about every 5-12 years, following which the sandy soil becomes covered for a few weeks with an ephemeral vegetation. Elsewhere, the plant cover consists of a mosaic of barren sand dunes, open shrub and tree savanna with a parkland-like distribution. On the western slopes of the coastal cordillera on rocky biotopes is the huge column cactus, Neoraimondia gigantea; in addition, there are sm all tree species such as Prosopis juli-flora, the .. algarrobo" with hard wood and husks which are rich source of nutrients for cattle, the evergreen Capparis angulata, the green-barked Cercidium praecox (Sapo-tacaea) and Parkinsonia aculeata. There are also more shrublike forms, Capparis cordata and C. ovalifolia, Grabowskia boerhavii-folia (Solanaceae). Galvesia limensis (Scro-phulariaceae). Monina pterocarpa (Poly-galaceae). Scypharia spicata (Rhamnaceae). and others.

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A striking annual is Luffa operculata (Cu-curbitaceae) which forms a dense thicket after good rain.

In the Ecuadorian part of the region there remains only a small, desertlike area with a sparse bush vegetation at Cape Puntavilla near Salinas on the Santa Elena peninsula. Elsewhere rain-green shrub changes to savanna and rain-green forests, in which Ceiba (Bombacaceae) with its mighty barrel trunk and buttress roots is especially strik-ing. Cacti are represented by species of sev-eral genera. In the undergrowth Ipomoea carnea is often dominant; this is, of course, the zonal vegetation of zonobiome H.

Galapagos Islands Before leaving the Peruvian-Chilean desert areas, we wish to deal briefly with the Galapagos Islands which, although they lie on the equator and are 1100 km from the mainland, are also washed by the cold Hum-boldt current. As a result, the coastal region is arid. Annual rainfall is only 100 mm, falling in the months January to April. Thus, at low altitudes there is a typical Espinar-Cardonale vegetation, as on the arid north coast of Venezuela (Vol. I, p. 206).

The islands are of volcanic origin and rise to 1000 m. Everywhere above 200 m NN is shrouded in low stratus clouds even during the dry season, providing moisture by con-densation. The flora consists of 463 species of flowering plants, of which 204 are endemie. At high altitudes the islands are forested with woody species such as Psidium galapa-gicum, Scalesia pedunculata, Pisonia flori-bunda, Xantoxylum fagara, Piscidia erythrina, bushes of Miconia robinsoniana with pterido-phytes and moss undergrowth.

Charles Darwin visited these islands in 1835: here the large number of finch species later pro-vided some of the evidence for his theory of evolu-tion. The large Galapagos tortoises are also well-known; they have had to be protected from exter-mination by man. Although the islands are under nature proteetion, the many tourists are a great threat.

The Peruvian-Chilean Desert

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