122 L. L. Tieszen et al. 



The output for the canopy model was vaHdated with production 

 data (Miller and Tieszen 1972) and more recently with photosynthesis 

 data from both field cuvette experiments and an assessment of commun- 

 ity carbon dioxide exchange (Miller et al. 1976). Daily courses were gen- 

 erally similar (Figure 4-9) as were the estimates of seasonal incorpora- 

 tion. Production data were simulated for various periods throughout a 

 growing season and, in some cases, for a variety of seasons. Sensitivity 

 analyses of several environmental parameters were made with a standard 

 day (Figure 4-10) that represents the mean input data for the 5-day 

 period beginning 15 July 1971. The standard day represents midseason 

 conditions in 1971; mean temperatures and solar radiation used in the 

 model are near the means for the four years of the field program. 



Moss 



The moss simulation model is similar in concept to the vascular 

 plant model (see Miller et al. 1978b). Photosynthesis and transpiration 

 follow from the solution of the energy budget equation, with the inclu- 

 sion of appropriate physiological relations. The input climatic data con- 

 sist of solar and infrared irradiance, air temperature, air humidity and 

 wind speed. The vascular canopy is composed of leaves, stems and stand- 

 ing dead material of different species, each defined by inclination and by 

 vertical profiles of area per unit area of ground. Solar and infrared radia- 

 tion from the sun and sky are intercepted by the canopy and produce 

 profiles of direct, diffuse reflected solar, and reflected infrared radiation 

 within the canopy. The air temperature and humidity above the canopy 

 at the moss surface interact with the canopy structure, wind profile, and 

 radiation profiles to produce profiles of air temperature, humidity and 

 leaf temperature. 



At the moss surface the receipt of net radiation is balanced by heat 

 exchanges due to convection, evaporation and conduction. The convec- 

 tional heat exchange occurs by turbulent exchange of air from the sur- 

 face across a surface boundary layer and across a bulk canopy air layer 

 to a reference height in the canopy. Surface evaporation is related to the 

 turbulent exchange of water vapor across the surface boundary layer and 

 bulk canopy air layer. 



Moss photosynthesis is related to solar irradiance, tissue tempera- 

 ture and water status. Solar and infrared irradiance, air temperature, hu- 

 midity, and wind velocity affect the plant water status through their ef- 

 fect on leaf temperature and transpiration. The plant water status influ- 

 ences the rates of transpiration and photosynthesis through their com- 

 mon resistance to water and carbon dioxide diffusion. Water, in the 

 form of precipitation and dew, that is not intercepted by the vascular 



