A Test of A Model of Irradiance within Vegetation Canopies at Northern Latitudes

Abstract

A model of irradiance partitioning in vegetation canopies was tested using PAR irradiance profiles measured in dwarf birch canopies near Eagle Summit, Alaska. Leaf and stem area indices were between 1.4 and 3.9 m_l² · m_g^–2 and 0.3 and 0.8 m_s² · m_g^–2, respectively. Leaf and stem inclinations were more horizontal with depth. Calculated irradiances at different levels within the canopies were close to observed irradiances (r = 0.90). With increasing irradiance the agreement between calculated and observed decreased. The model predicted accurately the reflected irradiance above the canopy (r = 0.93). Measured PAR albedos were 0.03 to 0.04, lower than total solar albedos, and were insensitive to changes in solar altitude. Model sensitivity analyses indicate that the irradiance partitioning in the canopy is influenced most by total foliage area, second by foliage reflectance, and finally by foliage angle.