128 



L. L. Tieszen et al. 



140 



16.7 25. 



Radiation 



FIGURE 4-15. The 



simulated response of 

 daily total net photosyn- 

 thesis to daily irradiance 

 and saturation irradiance 

 (indicated below curve). 

 Also indicated is the fre- 

 quency of daily irradi- 

 ancesfor 15-30 June, Ju- 

 ly and August 1970-73. 

 (Other input = standard 

 day.) Simulations of the 

 effects of varying re- 

 quirements for satura- 

 tion are also shown. 

 (After Miller et al. 1976.) 



remain so even if they saturated at 140 J m"^ s"'. In the alpine similar 

 response curves and equivalent daily total irradiance result in less carbon 

 dioxide uptake because at solar noon, when the irradiances are substan- 

 tially greater, plants become saturated, and water stress often develops. 



Vapor Density, Soil Water Potential, 

 and Root Resistance 



Cuvette data and the simulations indicate that water stress develops 

 to a sufficient extent to cause some stomatal closure and therefore occa- 

 sional reductions in the photosynthesis of vascular plants. However, the 



