Orientation and its consequences for Copiapoa (Cactaceae) in the Atacama Desert.

Three species of the barrel cactus Copiapoa (C. cinerea, 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 (30°-40°C) relative to air temperatures (15°-20°) 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 detrimental 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.