Clouds and cloud immersion alter photosynthetic light quality in a temperate mountain cloud forest

Abstract

Light quantity and quality strongly influence plant ecophysiology, especially in cloud forests. Only a few studies have investigated the quantitative effects of clouds on incident sunlight, and almost no studies have explored the specific effects of cloud immersion on light quality. In the present study, the spectral quantity and quality of sunlight during sunny (S), low cloud (LC), and cloud immersed (CI) days in open canopy and understory microsites are compared in high elevation cloud forests in the southern Appalachian Mountains, USA. Cumulative daily radiation measured on CI days was 85% less than on S days at open sites, but about the same as S days in understory sites. LC and CI conditions enhanced blue light by 5%–15% in open sites, but decreased it by 25%–60% in understory sites. In cloudy conditions, red light decreased by 6%–11% in open areas and increased by 25%–30% in understory sites. Cloudiness also resulted in important changes in spectral band ratios, and was associated with light diffusivity. Thus, cloud regimes had a strong influence on the photosynthetic light quality of incident sunlight.