Biophysical Processes and Primary Production 79 



canopy solar irradiance decreases because of interception and absorption 

 by the leaves, stems and dead material, while infrared irradiance com- 

 monly increases because the leaves usually radiate more than the sky 

 does. Most of the photosynthetically active radiation is absorbed and not 

 reflected or transmitted (Stoner et al. 1978a). Thus, from the point of 

 view of the irradiance absorbed for photosynthesis, only the solar irradi- 

 ance penetrating to a leaf without interception is considered. But from the 

 point of view of the total energy exchanged by a leaf and an analysis of leaf 

 temperature, the reflected solar and infrared radiation must be included. 



Simulation models for irradiance in vegetation canopies have been 

 developed for lower latitudes for vegetation with well developed, homo- 

 geneous canopies (deWit 1965, Anderson 1966, Duncan et al. 1967) and 

 have been applied to the tundra (Miller et al. 1976, Ng and Miller 1977, 

 Stoner et al. 1978c, Tieszen 1978c). Stoner et al. (1978a) showed that the 

 simulation model used previously (Miller et al. 1976, Ng and Miller 1977) 

 predicted the vertical distribution of photosynthetically active radiation 

 well in northern latitudes, so that the basic equations appear valid. For 

 canopy energy exchange, incoming shortwave radiation was divided into 

 downward streams of direct beam and diffuse, and an upward stream of 

 reflected radiation. Infrared radiation was divided into downward and 

 upward streams. Canopy properties affecting the interception and 

 penetration of solar radiation in the canopy included the inclination of 

 the leaves from the horizontal, the distribution of leaf and stem area, and 

 the reflectivities of leaves, stems and dead material. In addition the alti- 

 tude of the irradiating source affected the interception and penetration 

 of radiation. 



The simulation models were used to estimate the partitioning of 

 solar and infrared radiation in the canopy. Of the incoming direct solar 

 beam, about 86% was intercepted in the canopy; the rest passed through 

 to the soil or moss surface (Figure 3-7). The canopy appeared more trans- 

 parent to diffuse solar radiation because of the scattering and downward 

 reflection of direct beam radiation. The diffuse radiation reaching the 

 soil-moss surface was 36% of the incident diffuse above the canopy be- 

 cause of the additional loss to scattering. About 18% of the incoming 

 solar was reflected back, most of the reflected amount coming from the 

 canopy rather than the soil-moss surface. Some absorbed solar radiation 

 was emitted as infrared. Infrared radiation was lost from the canopy 

 both upwards and downwards. However, the canopy received more in- 

 frared from the soil-moss surface than it lost to the sky. The net radia- 

 tion in the canopy was about twice that of the soil-moss surface. 



Of the net radiation absorbed by the canopy, 80 to 90% was lost by 

 convection and 10 to 20% was lost by evaporation. Bowen ratios — 

 convectional heat loss divided by evaporative heat loss — for the canopy 

 were 4 to 9. Transpiration was low because of the nearly saturated air 



