PACK RAT AS AN ENEMY OF NATURAL REPRODUCTION 423 



ing in the open and those growing in the chaparral, since it requires 

 15 years for a tree in relatively open conditions — such as in sage- 

 brush, sumac, and rose thickets — to reach this height, while in the 

 dense chaparral — oak, huckthorn, manzanita, and chemise — the tree 

 requires 23 years. Most trees growing in the chaparral are just be- 

 ginning at this age to push their heads above the top of the brush, and, 

 as the bark does not begin to break into plates until the tree attains 

 this age, it is in danger for about 7 more years, or until it reaches the 

 age of 30. 



Not being out of danger for this length of time, it would require 

 repeated poisonings on each area to prevent the rat from again be- 

 coming a pest, since only a few rats would be necessary to repopulate 

 the area as thickly as before in 10 years' time. The study of the 

 sample plot showed that 24 per cent of the trees were killed and 19 

 per cent w^ere so badly damaged that it would require but little further 

 work to kill them. In other words a total of 43 per cent of the stand 

 had been placed in jeopardy or killed by 9 rats in a period of but 

 2 years as a study of the callosus disclosed. It would appear, 

 therefore, that at least three treatments in addition to the initial one 

 would be necessary- on an area artificially stocked, in order to keep 

 the rats so in control that the loss would not be severe. This would 

 mean a large totail cost, possibly as high even as $10 an acre, for pro- 

 tection alone during the first 30 years in the life of the stand. 



