90 



P. C. Miller et al. 



TABLE 3-3 



The Effect of a ±20% Change of Initial Conditions 

 after 100 Simulated Years, Expressed as Percentage 

 Change from the Standard Case 



roots of plants for water and minerals decreases. The depth of thaw on 1 

 August decreased about 1.3 cm per unit increase in foliage area index in 

 simulations, compared to 1.5 to 0.1 cm per unit foliage area index in field 

 measurements. It is possible that without a periodic removal of the 

 standing dead material by lemmings the vegetation composition would 

 shift to shallow-rooted species such as mosses, lichens and Dupontia or 

 to evergreen forms requiring less nutrients. Decreased thaw depth and in- 

 creased moss has occurred in long-standing exclosures in the vicinity of 

 the Biome research area. 



Some of the interactions between foliage area index, thaw depth, 

 phosphorus availability, plant growth and accumulation of soil organic 

 matter were explored in a simplified ecosystem model (Miller 1978). Co- 

 efficients relating the annual change in each compartment to the state of 

 the system were estimated from the understanding at the time of the pro- 

 cesses involved. The initial values for the organic mat thickness and thaw 

 depth influenced several variables after 100 simulated years, but changes 

 in initial values for standing dead and live vegetation had little effect 

 (Table 3-3). The simulations support the notion that subtle changes in the 

 tundra may persist for many years, although in its grosser features the 

 system appears unchanged. Parameters defining phosphorus cycling 

 were critical in influencing the system. Changing the initial amounts of 

 standing dead and live plant biomass by ± 20% had no effect but chang- 

 ing the initial thickness of the organic mat by ± 20% caused long-lasting 

 changes in several variables. Changing the initial thaw depth by ±20% 

 changed the peak season thaw depth, phosphorus release, standing crop, 

 foHage area index, and organic mat thickness. The state of the system 

 after 100 years was influenced by changing parameters defining trans- 

 fers, especially those affecting phosphorus release. Periodic clipping of 

 aboveground standing crop by lemmings stabilized the system, although 

 varying the period between clippings from 3 to 5 years had little effect. 



