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

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. Key words: diffuse light. Southern Appalachians, spectroradiometry. La qualite el la quantite de lumiere influencent fortemenl l'ecophysiologie ties plantes, surtout dans les forets ennuagees. II n'existe que quelques etudes traitant des effets quantitatifs des nuages sur la lumiere solaire incidente, et presque aucune etude ayant explore les effets specifiques de l'immersion nuageuse sur la qualite de la lumiere. Les auteurs comparent la quantite et la qualite spectrales de la lumiere solaire au cours de journees ensoleillees (S), de faible nebulasite (LC) et d'immersion nuageuse (CI), dans la canopee ouverte et sur des microsites, et les comparent dans des forets en haute elevation dans le sud des montagnes Appalaches. aux Etats-Unis. La radiation quotidienne simulee mesuree en jours CI est 85% moindre par rapport aux jours S sur les sites ouverts, mais presque la meme que celle des jours S sur les sites en sous-etages. Les conditions des jours LC et CI augmentent la lumiere bleue de 5 a 15% dans les sites ouverts, mais la diminuent de 25 a 60%, sur les sites en sous-etages. Sous des conditions uuageuses, la lumiere rouge ditninue de 6 a 11% dans les aires ouvertes el augmente de 25 a 30% sur les sites en sous-etages. La nebulosite entralne egalement d'importantes modifications dans les ratios des bandes spectrales et s'associe a la diffusivite de la lumiere. Ainsi, les regimes nuageux influencent fortement la qualite pholosyniheiique de la lumiere solaire incidente. Mots-cies : lumiere diffuse, sud des Appalaches, spectroradiometrie. [Traduit par la Redaction]
Introduction Cloud cover can have both direct and indirect impacts on plant biology. These include changes in microclimatological conditions and alterations of the spectral quality of light, all of which [...]