84 P. C. Miller et al. 



The incoming solar radiation was partitioned between the canopy 

 and the soil for the Carex-Oncophorus meadow during the growing sea- 

 son. The incoming solar radiation was relatively high in June and became 

 lower as the growing season progressed. The fraction of incoming solar 

 radiation (300 to 3000 nm) absorbed in the canopy varied because of the 

 different live and dead leaf area indices, but increased through the season 

 as the Hve leaf area increased (Figure 3-11). The lowest interception oc- 

 curred in the coldest and wettest year (1973) because of the small leaf 

 area index in that year. The reflected fraction decreased through the 

 growing seasons as the darker colored live leaf area extended above the 

 lighter colored dead leaf area. The ground surface absorbed 50 to 60% of 

 the incoming solar radiation at the beginning of the season and 40 to 

 50% by the end of the season. Under an evergreen shrub canopy, with its 

 more even seasonal course of leaf area, the ground surface would have a 

 more uniform seasonal course of radiation absorption. 



Air Temperature, Humidity, and Wind Profiles 



Air temperatures within the vegetation canopy differ from air tem- 

 peratures above it because of the heat exchanged between the air, soil 

 surface, leaves and stems. The vertical profile of air temperatures 

 through the canopy depends upon the absorption of radiation vertically 

 through the canopy, which depends on the solar altitude and the profiles 

 of leaf and stem area. With low solar altitudes, when the solar radiation 

 is intercepted mostly near the top of the canopy, the air is warmest at the 

 top of the canopy. With high solar altitudes, solar radiation penetrates to 

 the ground surface and the air near the ground surface is warmed more 

 than the air higher in the canopy. These patterns are consistent with 

 measurements of Weller and Holmgren (1974a) at Barrow and Larcher et 

 al. (1973) in the Austrian alpine tundra. Because net radiation is positive 

 through the 24-hour arctic summer day, surface temperatures and air 

 temperatures near the ground are usually higher than air temperatures 

 above the canopy or near the top of the canopy, in contrast to the diurnal 

 patterns of air temperatures in lower latitudes. The difference between 

 surface and above-canopy air temperatures can be up to 20 °C in the 

 Carex-Oncophorus meadow and up to 30 °C on drier beach ridges 

 (Kelley and Weaver, unpubl.). 



Similarly, air humidities within the canopy differ from those above 

 it. Evaporation from the ground surface increases the humidity near the 

 ground surface and transpiration from leaves increases the humidity of 

 the air within the canopy. The exact profile has not been measured be- 



