376 



G. O. Batzli et al. 



RECOVERY 



14) Reproduction improves and 

 population builds over two 

 or three years 



13) Plant roots shallow^wtiere 

 nutrient concentration 

 greater,and forage quality 

 improves 



12) Increasing standing dead 

 increases insulation of soil 

 wfiich decreases depth of 

 thaw 



1 1 ) Poor reproduction so popula- 

 tion remains low the year follow- 

 ing crash 



10) Low grazing over winter and 

 poor nutrient availability 'during 

 following summer 



DECLINE 



HIGH LEIVIMING 

 DENSITY 



1) Intensive winter and early 

 summer grazing 



2) Nutrient release from feces, 

 urine and litter at snow melt 



3) Uptake high during early summer 

 so high nutrient content in early 

 forage 



4) Nutrients locked in standing 

 dead at end of summer 



LOW LEMMING 

 DENSITY 



5) Live biomass and standing 

 dead material reduced 



6) Low insulation leads to 

 increased depth of thaw 



7) Plant roots grow deeper 

 in soil where less phosphorus 



8) Low nutrient uptake produces 

 poor quality forage in late 

 summer 



9) Poor quality winter forage (leaf 

 sheath bases) so no winter repro- 

 duction and dechne continues 



FIGURE 10-13. Summary of steps in the nutrient-recovery hypothesis. 

 (Adapted from Schultz 1964, 1969.) 



duces the amount of standing live and dead plant material during the 

 summer of a lemming decline. This in turn reduces the albedo of the soil 

 and its insulating cover, and the depth of thaw increases. As the depth of 

 thaw increases, plant roots penetrate more deeply to where soil nutrient 

 solutions (particularly phosphorus) are more dilute, and the available nu- 

 trients are distributed throughout a larger volume of soil. The low nutri- 

 ent uptake of these roots leads to low nutrient concentrations in late sum- 

 mer growth, so that the leaf sheath bases produced then provide poor 

 winter forage for lemmings. Because of the poor quality forage little lem- 

 ming reproduction occurs during the winter, and the decline continues. 

 There is little winter grazing, so the nutrient pulse in the spring is weak, 

 and the nutrient quality of the vegetation stays low during that summer. 



