Oecologia (Berl.) 46, 63 67 (1980) Oecologia 9 by Springer-Verlag 1980 Orientation and its Consequences for Copiapoa (Cactaeeae) in the Atacama Desert J. Ehleringer1, H.A. Mooney 2, S.L. Gulmon 2, and P. Rundel 3 1 Dept. of Biology, University of Utah, Salt Lake City, Utah 84112 2 Dept. of Biological Sciences, Stanford University, Stanford, Ca. 94305 3 Dept. of Ecology and Evolutionary Biology, University of California, Irvine, Ca. 92717 Summary. Three species of the barrel cactus Copiapoa (C. cinema, C. columna-alba, C. haseltoniana) were investigated in their native habitats along the cool, arid coastal regions of the Atacama Desert in northern Chile. All species orient towards the north with a high degree of precision. Two consequences of adaptive value result from this northerly orientation. First, tissue temperatures of the meristematic and floral regions on the tip of the cactus receive high solar radiation loads which result in high temperatures (30o-40 ~ C) relative to air temperatures (15~ ~ during winter and spring months when adequate soil moisture for growth is available. Second, absorption of solar radiation by the sides of the cactus is minimized, which reduces both the potential detrimen- tal effects of light and heat load on the cactus and probably balances daily quanta absorbed for photosynthesis with nighttime CO2 uptake rates during drought stress periods. Introduction The Atacama Desert of northern Chile is perhaps the driest desert in the world. Along the coastal plains and canyons which drain into the Pacific Ocean, the vegetation of this arid zone is dominat- ed by succulents (Rundel 1976; Rundel and Mahu 1976; Rundel et al. 1979). Members of the genus Copiapoa (Cactaceae) are the principal succulents in this area. In certain parts of the Atacama Desert, species of Copiapoa form essentially monospecific stands (See Gulmon et al. 1980, for a stand photograph) and attain densi- ties of 2 plants per square meter (Gulmon et al. 1979). Copiapoa are frequently found along the coast both below and above but not within the fog zone. From casual observations, species of Copiapoa are intriguing because these barrel cacti appear to be orientated in a non-random fashion, generally facing north to the equator. Mooney et al. (1977) have made initial observations of this orientation and of cactus surface temperatures for C. haseltoniana. They reported that the range of cactus body orientations were within 30~ of due north. A similar phenomenon occurs in plants found in the Sonoran Desert in North America. Here species of the barrel cactus Fero- cactus generally face south to the equator, although quantitative data are not available (Humphrey, 1936). The purposes of this study were to first quantify the orientation Offprint requests to: J. Ehleringer of species of Copiapoa in the Atacama Desert, second to measure the consequences of such an orientation, and third to determine the possible adaptive values of this phenomenon. Methods and Materials Species of Copiapoa were studied in their native habitat along the coast in northern Chile. Populations of Copiapoa cinerea Brit- ton and Rose were sampled 10 km northeast of Paposo (longitude 70~ 30' W, latitude 25~ 5' S) and at Tal Tal (longitude 70~ 34' W, latitude 25~ 25' S), C. columna-alba Ritter (= C. cinerea v. columna-alba (Ritter) Backebg.) at Pan de Azucar (longitude 70o 37'W, latitude 25~ ' S), and C. haseltoniana Backebg. at Papo- so. Measurements of azimuth and angle of the cactus body, length, and width were collected on 50 to 100 randomly chosen cacti at each site. Additionally, the absorptances to photosynthetically active radiation (400 700 nm) by different parts of the cactus body were measured using an Ulbricht integrating sphere (Ehleringer and Bj6rkman 1978). Detailed measurements of microclimate and cactus surface temperatures were measured on C. columna-alba at Pan de Azucar during September, 1978. Diurnal measurements of cactus surface temperatures were made by inserting 24 gauge copper-constantan the~'mocouples just be below the epidermal surface. Thermocou- ples were placed under the surface in the four compass directions as well as on the base of the hairs on the top of the cactus. Solar radiation components were measured with a net radiometer, quantum sensor, and solar radiometer. Humidity was measured using an aspirated wet bulb psychrometer and wind speed with a cup anemometer. Simulations of solar radiation absorption by Copiapoa were based on equations describing the path of solar radiation through the day and throughout the year from List (1968) and Gates (1963). Calculations of the solar quantum flux absorbed by the cactus assumed the cactus to be an octahedraI cylinder with a top perpendicular to the sides of the cactus. Results Measurements on randomly selected individuals of C. columna- alba revealed a strong tendency for the body of the cactus to face north (Fig. 1). Out of a sample of 100 individuals, the orienta- 0029-8549/80/0046/0063/$01.00