If we take the air-dry value of 24.5 g. , adjust it to ovendry (23 g.)> and add a 

 probable 15 percent (erring on the high side) for annual wood growth, and another 5 

 percent for leaves, seeds, and small annual weeds that undoubtedly were lost before 

 yield estimates were made (in the fall), we calculate the net annual increment of aerial 

 growth to be around 28 g./m.'^. If we are correct in assuming a steady state of minimum 

 organic reserve, the average period of turnover for the 400 g. of recognizable 

 aboveground plant material must be about 400 v 28 as 14 years. Rate of litter decomposi- 

 tion depends upon the composition of tissues, the most ligniferous tissues being the 

 last to lose their identity (Van der Drift 1965). That breakdown of litter is slow 

 in an arid climate was apparent in our harvest of aboveground material, where four age 

 classes of dead stems of the succulent annual weed Salsola were distinguishable by 

 degree of weathering. Salsola was not present on the site in 1966, so some of this 

 herbaceous material had weathered at least five winters without losing its identity. 



Underground Productivity and Turnover 



Decay of organic matter is more rapid below the ground surface where humidity is 

 high for a longer period than above the ground and the habitat therefore more favor- 

 able for micro-organisms and the small animals active in litter breakdown. Because of 

 the difficulties encountered in assessing the underground operations of ecosystems, 

 little information is available regarding subterranean productivity (Dahlman and Kucera 

 1965). Most of what has been reported is incomplete or, as is true here, is based in 

 part on experience or assumption (Newbould 1968) . Based on our estimate that 75 per- 

 cent of the root mass is made up of roots less than 0.5 mm. in diameter, a rate of 

 turnover of underground material two, three, or perhaps more times that of the above- 

 ground parts seems plausible. If we take 2.5 as the factor (i.e., 0.4 of the turnover 

 time of aboveground decomposition) , the average period of breakdown of root-derived mat- 

 ter would be about 6 years. Then, if minimum mass of organic reserve is at equilibrium 

 between productivity and decomposition, the average annual net underground productivity 

 is in the neighborhood of (1,235 g. root + 135 g. root-derived litter) v6 = 230 g./m.^. 



Comparisons With Otiier Studies 



Most studies of productivity of terrestrial ecosystems with which we are familiar 

 approach their determination by measurement of growth, often for the separate plant 

 organs, through the growing season (Whittaker 1962; Ovington, Heitkamp and Lawrence 1963; 

 Wiegert and Evans 1964; Dahlman and Kucera 1965; Pearson 1965; Chew and Chew 1965; and 

 others). For the subterranean fraction, they are either incomplete or depend to some 

 degree on assumptions. An exception is the study by Dahlman and Kucera, who were con- 

 cerned specifically with root productivity of a prairie. These and the studies summa- 

 rized by Rodin and Basilevich (1968) analyze the organic material in a number of ways, 

 which makes it difficult for us to make direct comparisons among their results and ours. 



Litter seems to be an ill-defined fraction of organic matter accumulation (Rodin 

 and Basilevich 1967, p. 5). Some reports recognize only 1 year's litter fall; others 

 ignore it. Underground litter from aerial plant organs, never mentioned in the litera- 

 ture, may be an exceptional phenomenon, peculiar to the desert. As in our study, the 

 living root mass is usually not separated from the dead, although by our terminology, 

 at least the dead roots no longer attached to living plants should be considered under- 

 ground litter. At present, we have no clearcut concept of what constitutes aboveground 

 litter. Certainly any loose-lying material is litter, and so is the last year's 

 herbaceous material, even that still standing. (Shoot material of some desert herbs 

 can accumulate for several years without falling; perhaps it disintegrates in place.) 

 If old, standing dead, herbaceous matter is litter, then also are standing dead shrubs. 

 And, if dead shrubs are litter, then so, too, are dead branches and twigs on living 

 plants, and perhaps even dead wood in living branches. For perennial woody communities. 



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