Effect of Cloudcover on Photosynthesis and Transpiration in the Subalpine Understory Species Arnica Latifolia

Analysis of solar radiation data for three summers (1978—1980) revealed that cloud—cover was present for at least 10% of the daytime on 85% of the days in the Medicine Bow Mountains of southeastern Wyoming and occurred most frequently from 1000 to 1800. Although cloudcover caused a major reduction in direct—beam irradiance as well as a 30% decrease in daily integrated photon flux density at the forest floor, substantial increases in diffuse photon flux density were measured. The understory herb Arnica latifolia had a 37% greater carbon gain on representative cloudy days than on clear days with an 84% reduction in transpiration, which resulted in more than a sevenfold increase in water use efficiency when compared to clear days. The increase in carbon gain may have been the result of photosynthetic rates that were still near saturation even under the diffuse light of cloudy days. Thus, photosynthesis and water relations of A. latifolia may be enhanced by the natural occurrence of afternoon clouds which prevent potential increases in transpiration without decreasing in carbon gain.