step 3: Compute 

 Years Since Last 

 Disturbance 



Step 4: Calculate 

 the Increment in 

 Stocking 



Step 5: Estimate 

 Number of Trees 



For each plot, the number of years since last disturbance is computed — ^the 

 result is dependent on the method of site preparation. Untreated plots were 

 last disturbed at the time of harvest. Mechanically disturbed plots were last 

 disturbed at the time of mechanical site preparation, and burned plots were 

 last disturbed at the time of burning. 



The increment in stocking is the increase in the probability of stocking from 

 one time period to another. Figure 14 illustrates the increment in the prob- 

 ability of stocking for successive time periods of 5, 10, 15, and 20 years. The 

 number and length of time periods can be altered by controlling cycle lengths 

 in the Prognosis Model. Whenever there is a cycle boundary in the Progno- 

 sis Model and a disturbance has occurred in the previous 20 years, a regen- 

 eration tally will be predicted by the regeneration model. 



A calibration feature in the regeneration model is invoked when the stand 

 inventory is taken within 20 years of a requested tally of regeneration. Calibra- 

 tion adjusts the probability of stocking curve to coincide with the actual inven- 

 tory at the time the inventory was taken. Then the increment in stocking from 

 the time of inventory to the end of the cycle boundary is calculated. Details 

 of the calibration feature are explained in a later section. 



The number of trees per plot is conditional on the plot being stocked. In the 

 first tally of a tally sequence, the number of trees on the plot is determined 

 from Weibull cumulative distribution functions developed from the data. A 

 uniformly distributed pseudo-random number is used to choose the actual 

 number of trees from Weibull distributions. The number of trees is rounded 

 to the nearest integer. 



10 15 



Years Since Last Disturbance 



Figure 14 — Increments in probability of stocking at 5, 10, 15, and 

 20 years time since disturbance. 



26 



