88 P. C. Miller et al. 



convection. However, convection is more sensitive to turbulent exchange 

 than is evaporation. Increasing the foliage area index decreases solar ir- 

 radiance at the surface, increases the absorbed infrared, and diminishes 

 turbulent transfer. Decreased soil moisture results in higher surface tem- 

 peratures and decreases evaporative heat losses. 



Simulations of the seasonal course of the depth of thaw with differ- 

 ent environmental conditions, using 1973 environmental data as stand- 

 ard conditions, indicated decreasing sensitivity, i.e. centimeter change in 

 thaw depth at peak season per unit change in the environmental variable, 

 in the order (Figure 3-14): diffuse solar radiation, total solar radiation, 

 infrared radiation, vapor density, air temperature, and turbulent diffus- 

 ivity. The thaw depth at peak season increased with increased solar ir- 

 radiance, infrared irradiance, and air temperature, but decreased with 

 turbulent exchange and air humidity. The seasonal course of thaw depth 

 with different values for ecosystem properties indicated that the thaw 

 depth was most sensitive to changes in thermal conductivity of the or- 

 ganic layer, air resistance near the ground, thickness of the organic layer, 

 and leaf inclination, and was least sensitive to the reflectance of the sur- 

 face under the vascular canopy. Expressed in terms of the expected ac- 

 curacy of the instruments used to measure the variables, the sensitivities 

 were in the order (most sensitive to least sensitive): thermal conductance 

 of the organic layer, thickness of the organic layer, vapor density, diffuse 

 solar radiation, total solar radiation, infrared radiation, leaf inclination, 

 air temperature, ground air resistance, turbulent diffusivity, leaf area in- 

 dex, and ground surface reflectance. Changing the vertical distribution 

 of the foliage area index had little effect on the air temperature and 

 humidity profiles. 



The general trends in sensitivity were the same in another set of sim- 

 ulations involving a plot from which the canopy was removed. The depth 

 of thaw was more sensitive in the clipped plot than in the control plot to 

 solar radiation, ground surface reflectance, boundary layer resistance at 

 the ground surface, and the thickness of the organic layer. Thaw was less 

 affected by infrared radiation in the clipped plot than in the control and 

 showed about the same sensitivity in the two plots with regard to air tem- 

 perature and thermal conductivity. These changes in sensitivity relate to 

 the attenuation of solar radiation and turbulent transfer by the canopy. 

 The thaw development under horizontal leaves, as contrasted with more 

 vertical leaves, decreased because of the decreased penetration of solar 

 radiation with horizontal leaves. The thaw deepens with increasing 

 boundary layer resistance or vapor density because both suppress evapo- 

 ration, and increase surface temperature and conduction. 



The simulations indicate that as the vascular canopy develops, and 

 standing dead or live and dead moss material accumulates, the depth of 

 thaw decreases and the potential volume that can be exploited by the 



