218 P.J. Webber et al. 



perturbations, and their severity can be predicted from an understanding 

 of the controls over ecosystem processes in natural tundra. Vehicle tracks 

 that decrease canopy cover but do not destroy the vegetation or depress 

 the soil surface create an impact comparable to intensive lemming graz- 

 ing and the tundra may recover within a few years, or it may not. Dis- 

 turbances that depress or destroy the vegetation mat initiate a series of 

 changes in physical environment comparable to those occurring in the 

 thaw lake cycle, and result in a corresponding recovery time, probably 

 thousands of years. 



