Light Extinction

Abstract

Light extinction is a central process in ecology. Light is the main source of energy for ecosystems and a key factor in a large number of ecological processes, such as photosynthesis and evapotranspiration, but is partly extinguished when traveling across air, water, and vegetation. Therefore, it can be of fundamental importance to quantify the fraction of solar radiation effectively reaching the Earth's surface, ecosystems, and living organisms. Here, the process of light extinction is described taking an ecological perspective. First, the decrease that light intensity undergoes when passing through the Earth's atmosphere is described. Such decrease is due to absorption and scattering by aerosols and atmospheric gases and is linked to hour, day, latitude, and radiation wavelength. Then, given that visible light is one of the main factors driving primary production, light extinction in aquatic and terrestrial ecosystems is illustrated. In water bodies, light intensity is reduced by depth and turbidity, but also by the presence of primary producers through self-shading effects. In terrestrial ecosystems such as forests, light extinction is linked to tree structure and canopy density, which can be measured using the Leaf Area Index. Simple modeling relations are presented, the main one being the first-order kinetic Lambert–Beer law, which can be applied to the atmosphere, water bodies, and forest ecosystems, by using a single attenuation coefficient to account for the contribution of the different factors causing extinction.