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

Reinhardt, Keith; K.Smith, William; A.Carter, Gregory
May 2010
Botany;May2010, Vol. 88 Issue 5, p462
Academic Journal
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. La qualité et la quantité de lumière influencent fortement l'écophysiologie des plantes, surtout dans les forêts ennuagées. Il n'existe que quelques études traitant des effets quantitatifs des nuages sur la lumière solaire incidente, et presque aucune étude ayant exploré les effets spécifiques de l'immersion nuageuse sur la qualité de la lumière. Les auteurs comparent la quantité et la qualité spectrales de la lumière solaire au cours de journées ensoleillées (S), de faible nébulosité (LC) et d'immersion nuageuse (CI), dans la canopée ouverte et sur des microsites, et les comparent dans des forêts en haute élévation dans le sud des montagnes Appalaches, aux tats-Unis. La radiation quotidienne simulée mesurée en jours CI est 85 % moindre par rapport aux jours S sur les sites ouverts, mais presque la même que celle des jours S sur les sites en sous-étages. Les conditions des jours LC et CI augmentent la lumière bleue de 5 à 15 % dans les sites ouverts, mais la diminuent de 25 à 60 %, sur les sites en sous-étages. Sous des conditions nuageuses, la lumière rouge diminue de 6 à 11 % dans les aires ouvertes et augmente de 25 à 30 % sur les sites en sous-étages. La nébulosité entraîne également d'importantes modifications dans les ratios des bandes spectrales et s'associe à la diffusivité de la lumière. Ainsi, les régimes nuageux influencent fortement la qualité photosynthétique de la lumière solaire incidente.


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