Figure 4 . --Predicted conversion 

 from blackjack pine to yellow 



pine by diameter class size 

 and site index (S) . Diameter Closs Size in Inches 



Recruitment 



The requirements for adequate natural regeneration in the southwestern ponderosa pine are 

 the presence of large, vigorous trees well dispersed over the area, a good seed crop, adequate 

 rainfall in two consecutive years, and low level of competitive vegetation (see appendix G for 

 details). This combination of conditions occurs at erratic invervals and, as a result, ade- 

 quate natural regeneration is erratic. For example, the last excellent regeneration period 

 occurred in 1918-1919 (Pearson 1950; Meagher 1950; and Schubert 1974). Cooper (1960) has 

 concluded that erratic natural regeneration is a major reason for the "unbalanced" size struc- 

 ture of the ponderosa pine forests of the Southwest. 



Ideally, a natural regeneration model should take all of these factors into consideration 

 as components of it (such as the one described by Leak and Graber 1976) . Unfortunately, the 

 data available in this study preclude the development of a regeneration model. This was 

 unfortunate because the greatest distinction between even- and uneven-aged stand development 

 is the method of regeneration. The even-aged stand usually starts as a surge of regeneration 

 over a relatively short period of time (20 years or less). Next is a stage where little or no 

 regeneration takes place. Finally, some regeneration may again occur (depending upon the 

 species and site characteristics) as the stand becomes mature (or overmature) and develops a 

 more open structure. On the other hand, the uneven-aged stand is usually characterized by a 

 more or less continuous flow of regeneration into the stand. 



23 



