314 P. W. Flanagan and F. L. Bunnell 



narrow-peaked response of respiration versus moisture can be obtained 

 only by having a, equal 02. 



Despite an uneven data base, trends in coefficients a, and 02 are re- 

 vealing. With increasing age and pitting within the substrate, the mois- 

 ture range for effective respiration appears to broaden (see E. angusti- 

 folium. Table 9-2). Bunnell and Tait (1974) stated that the volume of 

 water relative to the amount of organic matter was critical. Thus, they 

 predicted that the moisture range over which respiration was uncon- 

 strained would broaden with age in aboveground substrates and narrow 

 with increasing depth and bulk density below ground. The trend below 

 ground has been documented most rigorously for aspen forest floor in 

 the taiga, and does show a gradual decrease in the effective moisture 

 range with depth (Bunnell et al. 1977a). 



Coefficient a^ represents the respiration rate at 10 °C when moisture 

 and oxygen are not limiting. It is assumed to be a measure of substrate 

 quality and as such should decline with the age of the substrate. Bunnell 

 et al. (1977a) documented the expected pattern within the taiga forest 

 floor; among substrates of the Biome research area it is evident among 

 the E. angustifolium age classes (Table 9-2). The exponential response of 

 respiration with temperature, defined by coefficient a^, assumes Q,o 

 values ranging from 2.2 to 8.8, with younger substrates showing a higher 

 Qio than older substrates (Table 9-2). There are two possible reasons. 



1) Newly senescent substrates have not experienced a winter and may not 

 be well colonized by psychrophilic organisms; thus they would show 

 lower rates of respiration at lower temperatures and higher Qio values. 



2) Younger substrates contain greater proportions of constituents of low 

 molecular weight which appear to have higher Qio values associated with 

 their utilization as discussed earlier in this chapter. Over the range 0° to 

 10 °C the weighted average of Qio values for all aboveground substrates 

 tested at the Biome research area is 3.65. 



Observations suggest that while younger aboveground substrates 

 have the chemical potential for higher respiration rates than do older 

 substrates, respiration is more likely to be constrained by temperature 

 and moisture. The high Qio values from younger substrates imply a pop- 

 ulation poorly adapted to low temperatures. The narrower moisture 

 range suggests that drying by wind frequently may reduce realized respir- 

 ation. These environmental constraints act to ensure that nutrients pres- 

 ent in newly dead standing vegetation are not released into the system un- 

 til the spring thaw. 



The same clear pattern of carbon dioxide evolution with tempera- 

 ture and moisture is not observed for decomposition processes below 

 ground. The best fit of the gresp function to Gilson respirometry meas- 

 ures of wet meadow soil accounts for only 10 to 20% of the variation. 

 The computed Q,o is near 2.0 for a variety of soils and the optimal mois- 



