Temperatures on the litter and burned surfaces are 

 expected to be similar (under full sunlight), based on the 

 energy balance equation, and the thermal contact coeffi- 

 cient (}JkC) (table 7) that influences G. This situation ap- 

 plies if we assume that (1) both surfaces are dry, thus the 

 H and LE terms should be equal; and (2) increased Rn 

 contribution to burned surfaces as compared to litter sur- 

 faces is offset by a lower \JkC value for the litter. This 

 shows that if \IKC were different for either surface, be- 

 cause of moisture, compaction, and so forth, the LE term 

 would be different, causing the surfaces to have different 

 temperatures. 



The study results reported here, and those published, 

 tend to support the expectation that burned and litter sur- 

 faces will be similar. At Coram, however, where max- 

 imums on the litter surface were warmer and minimums 

 colder on the burned surface, Rn on the burned surface 

 was apparently reduced by the rapid vegetation growth 

 that provided shade and reduced the temperature 

 variation. 



The energy balance equation and thermal properties of 

 \[KC (table 7) cause us to expect that temperatures on the 

 burned or litter surfaces will be considerably more vari- 

 able (higher and lower) than for the mineral soil surface. 

 Lubrecht data are consistent with this expectation. The 

 major factors influencing this difference are \[KC and Rn . 

 Net radiation is less on the mineral soil (fig. 13) and \IkC 

 is considerably greater for mineral soil than for litter, 

 which means that less heat to the surface is conducted 

 away more quickly than for a litter surface. This keeps the 

 surface temperature from rising as much. The same fac- 

 tors allow heat from the soil to warm the surface at night, 

 which keeps minimum temperatures of mineral soil warm- 

 er than litter or burned surfaces. Although mineral soil 

 surfaces in the clearcut were not compared with litter or 

 burned surfaces, the differences should be greater than in 

 the shelterwood. If mineral soil were more moist, the dif- 

 ferences would be accentuated even more. LE and \lKC 

 would increase for the mineral soil, which would further 

 reduce the temperature variation at the mineral surface in 

 comparison to litter or burned surfaces. 



Compared to litter and burned surfaces, the chip surface 

 is expected to have less temperature variation (lower max- 

 imums and higher minimums). The data at Union Pass are 

 consistent with this expectation. Chips have a higher 



Table 7 — Average values of albedo or reflectivity and the thermal 

 contact coefficient {\[kC) for typical surfaces. Values 

 are from Fowler (1974). 



Surface 



Albedo 



KC 



Burned 

 Litter 



(dry needles) 

 Bark (dry) 

 Chips 

 Soil 

 Air 



Water 



Percent 

 2 



6-10 

 20 

 36 

 20-35 



cal cm-^ sec-"^ °C- 

 0.0018 



.0013 

 .0031 

 .0026 

 .0111 

 .0001 

 .0361 



albedo, reducing Rn when compared to a litter or burned 

 surface (fig. 13). This lower heat flux density at the sur- 

 face, coupled with a higher \fKC (table 7) than litter or 

 burned surfaces, explains the reduced temperature varia- 

 tion. If the moisture content of the litter surface were 

 higher (relative to the chips), the temperature differences 

 would be reduced because of increased LE, which would 

 lower G. 



Figure 14 shows maximum temperatures for four sur- 

 face conditions compared to air and humus temperatures. 

 The temperatures shown are for a hypothetical site ad- 

 justed to a common set of conditions. These are typical 

 maximum summer surface temperatures on a clearcut for 

 a clear day in full sim. Burned and Htter surfaces are 

 much warmer than all other surfaces and the htimus at a 

 1-inch depth. All surface conditions are much warmer than 

 air temperature at 4.5 ft. 



At Union Pass, minimum temperatures and CE departed 

 from the expected pattern. Litter surfaces were colder 

 than burned surfaces, which should be similar, as at 

 Lubrecht, and the chip surface was colder than either, 

 where it should be warmer (fig. IIB). The basic explana- 

 tion for these reversals in minimum temperatures lies in 

 the topographic positions of the treatments. The close 

 utihzation treatment, where litter surface temperatures 

 were measured, and the chip spread treatment were in a 



700 I- 



650 - 



600 - 



W/m 



2 550 - 



500 - 



450 - 



400 - 



'^K = thermal conductivity; C = volumetric heat capacity. 



^^^^ 



Figure 13— Net radiation over several 

 surface conditions in a clearcut at Union 

 Pass at noon on July 12, 1979. Ttie dif- 

 ferences between burned and litter sur- 

 faces and between mineral soil and chip 

 surfaces probably are not significant. 



14 



