ACTA BOTANICA NEERLANDICA, Vol. 4 (3), 1955 Plant Growth under Desert Conditions BY F.W. Went (Earhart Plant Research Laboratory, California Institute of Technology, Pasadena, Calif, U.S.A.) [received July 6th, 1955) the last I have visited desert in different of During years areas parts I I the world, and have found that they were differing more than had expected from published descriptions. I would, therefore, like to stress some of the differences which were observed. The deserts visited include: (1) The fog desert along the Peruvian coast in the neighborhood of Lima. Dr. Ferreyra and Engineer Postigo were kind enough to of take me around and show me some of the typical aspects this desert including the so-called Lomas. (2) The dust desert which was found in the Northern Sahara around Beni where Killian the of the Ounif Dr. kindly put facilities Station Biologique Saharienne at my disposal. (3) The clear but dry deserts of Southern California and Israel. The latter were visited through the help of Drs. Boyko, Dr. Zohary and Dr. Evenari. The following table gives some comparison between a number of typical localities. TABLE I Comparison between the vegetation of different deserts. Altitude Rel. Daily Radi- Precipi- Vegeta- in Humi- temp. ation tation tion meters dity fluctuât. Peru, coast 0 low high small 25 mm 0 Lomas 300 low v. small 125 high „ good Beni Ounif 800 in low desert ter- v. 125 Algeria, great „ med. Elat 0 low 25 desert v. v. Israel, high great „ Subeita 500 v. low 125 desert- high great „ steppe -50 low 35 desert So. Cal. Death high v. v. Valley great „ 29 Palms. 800 low 125 desert- high great „ steppe is that far It evident as as precipitation is concerned the Loma de Lachay in Peru, Beni Ounif, the central Negev and Twentynine Palms 482 F. W. WENT in Southern California have approximately the same total precipitation measured with rain Their altitude is also as a gage. approximately the same. However, there are enormous differences in vegetation. with desert far At Beni Ounif we are dealing a typical as as plant growth is concerned. On the flat desert plains less than 1 % of the surface is covered with perennial plants. To these belong Anabasis arietoides, Limoniastrum Feei, Zilla macroptera, Launaea spp. and a few other low shrubs. Along the smaller washes Zizyphus Lotus is the only larger shrub which grows up to 2 meters high but for the rest the area mountains there but is barren. Against was slightly better vegetation the surface coverage was still considerably under 10 %. There had been a fairly rich vegetation of annuals because of heavy Autumn rains in but all the time 1953, these annuals were entirely dry at of visit in 1954. of my June Near villages plants Peganum Harmala and remained because of their were bright green uneaten by sheep extreme toxicity. The only other lush green plant was Capparis spinosa found of cracks in rocks which was growing out with no apparent root connection with soil. Again, because of extreme bitterness, this plant was not eaten by animals. The area around Twentynine Palms which has the same altitude Beni Ounif and the is different far as same rainfall, entirely as as vegetation is concerned. The desert floor there is covered with fairly shrubs which 2 whereas in the washes large grow up to meters height, shrubs and reach of 5 At least trees a height or more meters. 20 % of the soil surface is covered with perennial plants, mainly consisting of Larrea divaricata, Franseria dumosa, Hymenoclea Salsola and Krameria canescens. Along the washes Dalea spinosa and Acacia Greggii are abundant. A similarly rich shrub vegetation is found in the Negev at the same altitudes and with similar rainfall. This vegetation cannot be character- but is also in the of desert ized as desert-like, nature a steppe. Still Lomas the more extreme is the vegetation of the along coast of Peru. Whereas immediately along the coast the rainfall is less than 25 which results in without mm absolute desert any plants at all, as one further inland into the foothills of the the rainfall goes Andes, increases sand. Also and soon big plants of Tillandsia grow in the several of Trichocereus covered with species appear, amazingly enough lichens and small Tillandsia species. Shrubs of Nicotiana and of Croton and in with rainfall of 125 Ruizianus start to appear finally areas a of mm shrubs and small trees grow and there is about 50 % coverage the surface with The and perennials. trees are Caesalpinia Capparis prisca with a few specimens of Carica candicans. Most amazing was the the dense growth of mosses on the branches of the trees, especially on of Caesalpinia, and numerous epiphytes Peperomia crystallina were standing others upright on the branches. Among the perenials there were among Hyptis sidaefolia, Pitcairnia ferruginea, Piqueria peruviana, Trixis paradoxa, Loasa urens Oenothera Malvastrum Heliotropium , laciniata, peruvianum, lancelotum, and Cestrum kediundinum. that these three From these short descriptions it can be seen types of desert differ enormously in the kind of plant growth which occurs. PLANT GROWTH UNDER DESERT CONDITIONS 483 The difference does not lie in the amount of rainfall because this was the same in the four localities discussed. There are, however, big differences in the relative humidity. At the Loma de Lachay the while relative humidity is 100 % all through the night and even the sun is shining it does not drop to below 90 %. At 17:30 the humidity 98 while the is the was again % sun was shining. This humidity same found in rain forest and the that as a tropical epiphytic growth proves dense the humidity remains high throughout the year. Therefore the due vegetation on the Lomas along the coast of Peru is largely to the the form water available to plants in of fog or dew. The degree to the which fog is of importance can be seen immediately along coast. Along the seaward side of the road where the road edge protrudes somewhat from the rest of the slope and where consequently more fog be Suaeda can caught some plant growth occurs, namely foliosa, Cynodon dactylon and Chenopodium murale. Similar fog conditions seem the to occur along western coast of South Africa where also fog is held back by coastal mountains and where plant growth occurs in the greatly excess of amount of precipitation available. The humidity in the other deserts is low at practically all times and there is of the difference no trace any epiphytic vegetation. Therefore, between the Sahara, on the one hand, and the Southern California and Israel deserts on the other hand should be accounted for in some Beni in other manner. During my stay at Ounif Southern Algeria, on be- the Northern edge of the Sahara, the sky was continuously dusty cause of frequent gusts ofwind which often reached a velocity to move also sand grains and become sand storms. This wind also causes a curious surface pitting of stones which I had not observed in other deserts. These winds come very suddenly and stop equally suddenly. This is of local for thebarometer does largely a question very heating, these and not change abruptly during wind storms, and during night the air is The of flat early morning very quiet. enormous expanses terrain without mountain such as found in intervening high ranges winds. the SouthernCalifornia deserts, must help in building up strong In deserts it is with and easy to recognize areas frequent strong winds; such areas are characterized by sand dunes which are found in only few places in Southern California or in the Negev, whereas they are fre- very frequent in the Sahara. In deserts where the flat areas are limited lateral air quently intercepted by mountains, only very move- ments develop, which are indicated by so-called ‘dust devils’ or whirlwinds. The dustiness of the air in the Sahara is best characterized by the the whole which looks rather than high luminosity of sky gray blue, the haze which hides the and grayish more distant objects also is the cause of the lack of dew which was observed by many desert travelers of this such as Fitting. Instead considering lack of dew a characteristic of the Sahara he claimed that there was no dew in deserts. However, and Duvdevani have shown Volkens clearly the frequent occurrence of dew in Israel and other desert areas, and it also can be observed not infrequently in the Southern Californian deserts. In these places 484 F. W. WENT distances look clear because of mountains, even at great usually very a lack of dust and haze. rainfall of is With a only 25 mm permanent plant growth impossible except where water collects immediately after infrequent rains. In the Death of Valley area shrubs Larrea divaricata, Peucephyllum Schottii and are and are able to with Atriplex Hymenelytra very widely spaced grow less than of rainfall. After 50 mm a long drought period, however, these active and sometimes dead. plants are hardly any more seem With less than 50 mm of precipitation the vegetation in Peru is also and consists of Tillandsia and Trichocereus. Near very scanty exclusively Elat in Israel only one plant was found growing on top of a ridge with no apparent source of water except from rain. This plant was which Capparis spinosa was completely green and apparently in excellent condition after without This a year any precipitation. plant is also found in areas with much higher precipitation such as in Jerusalem with 500 mm and the Ruisseau des Singes in the Atlas de in with rain.