Is forest albedo measured correctly?

Abstract

The forest albedo is an important meteorological and ecological variable characterising a forest ecosystem. In order to measure this variable two 2n-sensors are mounted above the forest canopy. The first sensor records an upward radiant energy flux from the forest canopy into the atmosphere, and the second one a downward radiant energy flux from the atmosphere into the forest canopy. The ratio of their responses is usually interpreted as the forest albedo. This measurement method is included in standard observation programs at many research stations. In the present paper we quantify an inadequacy of this method to evaluate an albedo of a forest stand for photosynthetically active region (PAR, 400-700 nm) of the solar spectrum. We reproduced the three-dimensional PAR radiative field above a coniferous forest stand at the scientific research station "Solling" near Göttingen with high level of realism. A good agreement between simulated sensor response to the simulated radiative field and a real sensor response to the real radiative field was achieved. However, the calculated value of forest albedo was found to be systematically higher than its value derived from sensors on a clear sunny day by about a factor of 3 during morning and evening hours and by about afactor of 1.9 during day time. This deviation is mainly caused by forest heterogeneity. A daily mean of the measured solar radiant energy fluxes the ratio of which is usually assumed to represent the albedo does not average the effect of the forest heterogeneity.

Keywords

• forest ecosystem
• solar radiant energy fluxes
• waldökosystem
• göttingen
• germany