regeneration that occurs during the cycle. The new trees are added to plots 

 that correspond to the inventory plots in the Prognosis Model. The use of 

 plot information provides some spatial resolution to the regeneration model 

 because each plot can have unique attributes such as slope, aspect, habitat 

 type, site preparation, overstory density, and overstory species composition. 



New regeneration is added to the Prognosis Model tree list at the end of the 

 cycle in what is called a regeneration tally. A tally of regeneration is similar 

 to the results of taking a regeneration survey for a stand; the regeneration 

 process continues for a number of years before it is quantified by a regeneration 

 survey. 



The regeneration period is the number of years during which regeneration 

 becomes established as a result of a disturbance. For modeling purposes, the 

 regeneration period is set at 20 years. Any time there is a Prognosis Model 

 cycle boundary during the 20 years of a regeneration period, the regenera- 

 tion model will be invoked to predict new regeneration. Multiple tallies during 

 these 20 years are called a tally sequence. 



Regeneration that occurs after the regeneration period is called ingrowth. 

 Ingrowth is both the result of succession by shade-tolerant species and the 

 continued regeneration of trees into gaps in the tree canopy. 



STEPS IN THE REGENERATION MODEL 



The computer steps for the regeneration model are depicted in figure 13. 

 The computer code is written in FORTRAN. Each step is briefly described 

 in this section. 



Step 1: Initiate 



Regeneration 



Model 



The regeneration model is invoked at the end of a Prognosis Model cycle. 

 At this point in the Prognosis Model, silvicultural prescriptions for the cycle 

 have been implemented, growth has been predicted, mortality has been ac- 

 counted for, and plot/stand statistics have been updated. 



Before the regeneration model is invoked, the tree list is checked to ensure 

 the tree list has enough space to hold new records. Space is provided by 

 compressing the tree list. This feature of the Prognosis Model combines 

 similar tree records as discussed by Wykoff (1986). 



In the regeneration model, computer storage is allocated and prepared for 

 use. Plot replication, if needed, is done at this time. Plot replication ensures 

 there are an adequate number of plots (default = 50) to dampen the stochas- 

 tic effects of any one plot on the projection. The effect is to predict regenera- 

 tion closer to the mean expectation. Fifty plots are enough to average out 

 the effects of unusual events, yet is stiU efficient use of computer time. Model 

 users can change the default minimum number of plots to as few as 20. 



Step 2: Determine 

 Plot Site 

 Preparation 



Each plot is assigned a site preparation — mechanical, burn, or untreated. 

 The percentages of site preparations are either specified by the user or are 

 determined from default equations derived from the data. User-specified 

 percentages take precedence over default equations. Once percentages are 

 determined, site preparations are assigned at random to the plots. 



Users have two ways to assign site preparations to specific plots. Site prepa- 

 ration can be coded on the inventory records, or it can be specified on special 

 computer records at the time the Prognosis Model is initiated (see Ferguson 

 and Crookston 1991). A category for roads is included. Road site prepara- 

 tion is all road cuts, all road fills, and unmaintained roadbeds. 



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