Biophysical Processes and Primary Production 85 



cause such measurements involve difficult logistic problems in the low 

 canopy of the coastal tundra vegetation. The humidity difference should 

 be close to the difference between the saturation vapor density at the 

 temperature of the ground surface and the humidity of the air. 



Heat and humidity of the canopy air are exchanged vertically by 

 processes of turbulent transfer, which are related to wind and to foliage 

 area in a complex manner (Monteith 1974). The Carex-Oncophorus 

 meadow canopy, with its small leaf area at the top and larger leaf area at 

 the bottom, has a rapid exchange of air near the top of the canopy and 

 slower exchanges near the ground. Weller and Holmgren (1974a) meas- 

 ured wind profiles through the canopy with a hot wire anemometer. Foli- 

 age area indices were not measured at the same time, but assuming that 

 the foliage area index was between 1 and 2, and that wind decreases ex- 

 ponentially with the foliage area index, an extinction coefficient for wind 

 of 1 to 1.5 (foliage area index)"' is appropriate. Wind has been shown to 

 affect stomatal opening, photosynthesis, and transpiration (Caldwell 

 1970a,b, Grace and Thompson 1973), and has been imphcated in reduc- 

 ing growth by lowering plant temperatures (Warren Wilson 1966a). 



Effect of Plant Properties and Environmental 

 Factors on Leaf Temperatures 



The influence of plant form on plant temperature was suggested pre- 

 viously. Krogg (1955) found that willow catkins with transparent hairs 

 reflected solar radiation to the inner surface while trapping the infrared 

 radiation. The inner surface was dark, increasing the absorption of solar 

 radiation. As a result, catkin temperatures were several degrees above 

 ambient temperatures. Hocking and Sharplin (1965) noted that flower 

 shapes are sometimes parabolic, focusing the sun's rays into the center of 

 the flower. The warmer center may then attract pollinators or speed de- 

 velopment of reproductive parts. The influence of plant properties on 

 plant temperature can be simulated by defining the environmental vari- 

 ables and solving the energy budget equation for plant temperature. Such 

 simulations indicated that leaf temperatures may increase 0.07 °C per 

 percent change in leaf absorption, 0.2 °C per mm change in leaf width, 

 and 2 to 3 °C per s cm'' change in leaf resistance at low leaf resistances 

 (Figure 3-12). Leaf temperatures can be expected to rise with decreased 

 wind speed at low wind speeds, but be relatively unaffected by changes in 

 air humidity. 



