Influence of sky conditions on carbon dioxide uptake by forests

Sky conditions play an important role in the Earth’s climate system, altering the solar radiation reaching the Earth’s surface and determining the fraction of incoming direct and diffuse radiation. Sky conditions dictate the radiation distribution inside plant canopies and also the carbon dioxide uptake by forests during the growing season. On the long term these diffuse conditions may have a positive influence on forest growth in Northern Britain during the last 50 years. We compared the quantity (amount) and quality (spectral distribution) of direct and diffuse radiation above, inside and below a forest stand under sunny, cloudy and overcast conditions in a thinned Sitka spruce [Picea sitchensis (Bong.) Carr.] forest (28 years, with an leaf area index (LAI) of around 5 m2m-2). Similar radiation properties (sky conditions) were used for analysis of light response and canopy conductance measurements in the same and also in a different spruce forest of the same species (33 years, LAI of around 7 m2 m-2) over the growing season 2008 in order to compare canopy activity under these conditions. In order to integrate short-term and longterm studies, we were looking at how far these conditions are influencing forest growth over several decades. To do so, we used freshly cut tree discs of Sitka spruce from a felled forest (planting year 1953) in southern Scotland and solar direct and diffuse radiation along with other meteorological data from the nearest meteorological station. Our analysis show that the amount and quality of solar radiation is distributed differently inside forest stands under various sky conditions, leading to an enhanced carbon dioxide uptake and canopy stomatal activity under diffuse cloudy and overcast conditions. Furthermore we demonstrated which factors have influenced diffuse radiation distribution over the past 50 years and how these are correlated with forest growth in southern Scotland.