Minimum surface temperatures at Coram and Union 

 Pass were significantly colder on the clearcut treatment. 

 Results for these two sites between the clearcut and uncut 

 follow the same pattern as at Lubrecht. Average minimum 

 temperatures are warmer for Coram than for Lubrecht. 

 This appears to be a result of topographic differences. 



At Coram the CE's following clearcutting were not as 

 frequent as at Union Pass. The frequency of CE's was low 

 at Coram, and they occurred only in May or September— 

 none in June, July, or August. The frequency of CE's 

 varied f'-om to 45 percent of the nights in July at Union 

 Pass. The effect of surface condition is discussed later. 



Comparison With Expected Results— The results 

 reported here are consistent with and explained well by 

 the energy balance equation. The major effect of the over- 

 story removal treatments was to increase energy incident 

 upon the surface (Rn) (fig. 6). Increased Rn causes an in- 

 crease in the heat flux density at the surface (G). Changes 

 in convective heat loss (H) and latent heat loss by evapora- 

 tion and transpiration (LE) are not able to dissipate the 

 added heat, which results in increased temperature varia- 

 tion at the surface. Although the temperature differences 

 were not significant for all treatments for all months, the 

 trend of increasing growing season surface temperatures 

 with increasing levels of canopy removal was observed 

 (table 3 and fig. 3). The effects of vegetation influence on 



shade will be discussed under the surface condition 

 section. 



Because the surface condition of all treatments for the 

 overstory removal comparisons is the same (Htter), the 

 thermal properties did not significantly influence the tem- 

 perature differences. Higher surface temperatures on the 

 clearcut and shelterwood treatments not only increase the 

 potential for seedling mortality due to high temperatures, 

 but also are effective in drying the surface layers, which 

 creates additional moisture stress. The drier surface condi- 

 tions in turn lower volumetric heat capacity and thermal 

 conductivity, which further increases temperature 

 variation. 



Minimum surface temperatures for the treatments are 

 also explained well by the same factors (table 4 and fig. 4). 

 The more exposed surfaces, where more of the canopy has 

 been removed, are subject to greater radiational cooling 

 and thus lower temperatures. On the shelterwood treat- 

 ment that admits about 60 percent of full sun, the canopy 

 is sufficient enough to limit radiational cooling, which ex- 

 plains the absence of CE's, and to reduce the potential 

 mortality due to low temperatures. In addition to the ef- 

 fect of the Rn term, topographic conditions at Lubrecht 

 and Union Pass are such that cold air can accumulate, 

 creating frost pockets. In these cases the H term is af- 

 fected. More will be said about this in the discussion of the 

 surface conditions. 



HOUR 



Figure 6 — Diurnal variation in net radiation by 

 cutting method on June 18, 1976, Coram Ex- 

 perimental Forest. Daily totals are given for each 

 treatment in cal cm ~ ^. 



9 



