Figure 14— Maximum temperatures expected in a clearcut of a 

 hypotlietical site for four surface conditions— burned, litter, mineral soil, 

 and chips— humus at 1-inch depth and air temperature at 4.5 ft. 



relatively flat area where cold air accumulates at the toe 

 of a slope that is below the burned surface (fig. 12). The 

 chips treatment is the lowest. In this case H was reduced 

 because of restricted airflow and colder, more dense air 

 accumulated in pockets. This aided radiational cooling, 

 thus lowering the surface temperatures. Additionally, 

 where chips were spread, basins were created in the chips 

 to allow seedlings to be planted. These additional depres- 

 sions trapped more cold air. Surfaces with lower \/KC 

 values are more susceptible to lower temperatures and 

 frosts, particularly in frost-pocket situations. 



Air Temperature 



Air temperatures above the surface conditions were 

 measured only at Union Pass. Surface condition did not 

 have any effect on maximum air temperature at 4.5 ft or 

 8 inches above the surface. Because surface temperatures 

 were higher on burned and litter surfaces than on chip 

 surfaces, we expected that air temperatures above these 

 surfaces might be different. However, prevailing winds 

 and upslope thermals likely caused enough mixing that 

 surface differences did not significantly influence max- 

 imum temperature at these levels. 



Minimum air temperatures were significantly different 

 at both 4.5 ft and 8 inches. Temperatures above the chip 

 surface were significantly colder than above the litter or 

 burned surfaces at both heights. At 8 inches above the 

 litter surface, temperatures were also significantly colder 

 than above the burned surfaces. As with the overstory 

 treatments, these differences are due, at least partially, to 

 cold air drainage caused by slope differences. 



Soil Temperature 



Temperatures at depths of 2 inches and greater were 

 measured only at Union Pass. Higher surface tempera- 

 tures of some surface conditions at Union Pass caused 



greater soil temperatures at depths of 2, 8, and 16 inches. 

 Mean temperatures (at all depths) were significantly 

 warmer below the burned surface than below the litter 

 and chip surfaces (fig. 9). Root zone (2- and 8-inch depths) 

 temperatures ranged from 58.1 to 54.5 °F under the 

 burned surface, 54.3 to 51.8 °F under the litter surface, 

 and 50.1 to 45.5 °F under the chips. Temperatures under 

 the burned and litter surfaces are probably not different 

 enough to cause differences in root growth, plant develop- 

 ment, or microorganism activity. The colder temperatures 

 under the chips are likely to decrease root growth, tran- 

 spiration, and»microorganism activity. Temperatures under 

 the chips increase much more slowly in the growing sea- 

 son than under the other surfaces. Near-freezing tempera- 

 tures and frost were often observed in June under the 

 chips. Spreading the chips shortened an already short 

 growing season at this high elevation. 



Summary 



Temperatures at the ground surface, to depths of 16 

 inches, are warmer for burned and litter surface condi- 

 tions than for mineral and chip surface conditions. The 

 results are explained rather well by the energy balance 

 equation and the thermal properties of the surfaces. Dif- 

 ferences in temperature between the burned and litter 

 surfaces seem to be influenced by the rate of vegetation 

 recovery. This is consistent with other published results. 

 Minimum temperature differences departed from the ex- 

 pected pattern due to slope position that affects cold air 

 drainage. Differences in the soil mimic the differences at 

 the surface for the surface conditions studied. 



Surface condition does not appear to influence air tem- 

 perature. Wind movement likely caused enough mixing to 

 mask any differences in maximum temperature. Differ- 

 ences in minimum air temperatures are apparently caused 

 by differences in slope position. 



15 



