The Herbivore-Based Trophic System 379 



The idea that large quantities of nutrients may be tied up in organic 

 matter during the summer following a lemming peak (step 4, Figure 

 10-13) is less tenable. Lemmings reduce the aboveground standing crop 

 of vascular plants by about 50% at midsummer of a peak year (Figure 

 10-14) so nutrient storage in standing dead at the end of summer is also 

 reduced. Furthermore, 15% of the phosphorus put into live biomass may 

 be removed by translocation in late summer and by leaching the follow- 

 ing spring (Chapin, pers. comm.). Although a few lemming carcasses 

 may be found at snowmelt, large numbers of carcasses do not accumu- 

 late on the tundra, apparently because they are eaten by predators and 

 scavengers (Mullen and Pitelka 1972). Nutrients from all tissues other 

 than bones are probably returned rapidly to the soil. Phosphorus and 

 calcium are concentrated in lemming bones, but only 10 to 20% of the 

 total phosphorus and calcium in lemming forage is retained in bone. In a 

 high year lemmings consume about half of the aboveground vascular 

 plant production, so no more than 5 to 10% of the total phosphorus and 

 calcium content of forage could be sequestered in lemming bones. 



In some spots, where lemmings have grubbed for rhizomes, the 

 standing crop of vascular plants may be reduced 90% (Schultz 1964, 

 Dennis 1968). Thinning of the plant canopy does increase depth of thaw, 

 although Schultz does not provide quantitative data. A simulation model 

 suggests that complete removal of the canopy increases maximal depth of 

 thaw by about 20% of normal, or about 5 cm, when the surface is satu- 

 rated with water (Ng and Miller 1977). If the moss layer is drier, the ef- 

 fect on thaw is somewhat less. When lemmings are excluded from 

 patches of tundra for long periods of time, standing dead plants continue 

 to accumulate, and thaw depth is reduced as much as 25% (Batzli 

 1975b). Thus, the fifth and sixth premises are supported, although the ef- 

 fect on depth of thaw does not seem to be large. 



Schultz (1964) presented evidence that total calcium and phosphorus 

 decreased with depth in tundra soils; however, soluble soil nutrients do 

 not necessarily follow the same pattern (Chapter 7), although soluble in- 

 organic phosphorus usually does (Bar^l and Barsdate 1978). The decline 

 in nutrient absorption rates of temperate plants for 7 to 10 days after 

 grazing, reported by Davidson and Milthorpe (1966), might also support 

 the nutrient-recovery hypothesis. But the nutrient absorption rates of 

 tundra graminoids increase following grazing under field conditions 

 (Chapin 1980b). Moreover, the direct impact of grazing upon nutrient 

 absorption rates would not last long. 



Schuhz's idea that roots would exploit greater soil depths does not 

 seem likely when one considers that the plants could use the more con- 

 centrated nutrients in the upper soil horizons. In fact, all roots of Du- 

 pontia fisheri and the secondary absorbing roots of Carex aquatilis, the 

 two most important forage plants for lemmings, are found in the upper 



