INTRODUCTION 



Throughout much of its range, natural regeneration of ponderosa pine (Pinus 

 ponderosa Laws.) is usually slow and frequently uncertain. In addition, obtaining arti- 

 ficial regeneration is difficult and often succeeds only after rather drastic measures 

 are applied to the stand or to the site (Wellner 1970) . Practices such as clearcutting, 

 burning, and mechanical site preparation that enhance regeneration are questioned by the 

 public (U.S. Congress 1970; USDA Forest Service 1970). Successful forest management 

 requires either prompt regeneration following harvest cutting or the establishment of 

 advanced reproduction. Knowledge of the intricate factors that affect the establishment 

 of ponderosa pine reproduction is vital to obtain a complete understanding of regenera- 

 tion problems. 



Much research has been devoted to ponderosa pine regeneration problems (Hermann 

 1970). In most cases, these studies have failed to explain where in the pine stand the 

 environmental factors most influential in pine regeneration are working and how various 

 parts of the pine forest (canopies, litter, boles, etc.) affect them. The study reported 

 here was undertaken to provide this information. 



LITERATURE REVIEW 



Natural restocking principally requires a good seed crop, a low rodent population, 

 adequate available moisture, and bare mineral soil (Foiles and Curtis 1973). A bare 

 mineral soil seedbed is thought to provide seeds and seedlings with more moisture, nutri- 

 ents, and sunlight by reducing competing vegetation and by eliminating the dry, dense 

 litter-duff layer. Baker (1951) and Powells and Schubert (1951) observed that invading 

 pines were more numerous on mineral soil seedbeds than on any other sites. 



Removal of the organic layer is thought necessary because the litter and duff, 

 having a lower thermal conductivity and volumetric heat capacity than mineral soil, 

 raise daytime temperatures and lower nighttime temperatures at the litter-air interface 

 (Cochran 1970). Furthermore, pine needles compact poorly and dry rapidly, allowing 

 little moisture for seedling survival (Biswell 1973) . Fisher (1935) found that ponde- 

 rosa seed germination was best on bare or ash-covered soils and was poorest on duff- 

 covered soils. Roe and Squillace (1950) found 8 times as many seedlings and Foiles and 

 Curtis (1965) found 13 times as many pine seedlings on scarified mineral soil seedbeds 

 as on duff-covered, natural soils. 



Where mature ponderosa pine stands occur as open forest types, understory vegeta- 

 tion may be well established and vigorously compete with young pine seedlings. In the 

 presence of grass associates, ponderosa pine reproduced at a very low rate compared to 

 nearby areas lacking grasses (Baron 1962; Larson and Schubert 1969; Pearson 1934). 

 Again, many authors advocate scarification of a site to reduce this competition (Fisher 

 1935; Foiles and Curtis 1965; Van Sickle and Hickmann 1959). 



In some instances, intraspecif ic competition, especially between different age 

 classes of ponderosa pine, can affect a stand. Saplings may reduce the vigor of the 

 overstory enough to cause susceptibility to insect attack (Weaver 1955). Conversely, 

 overstory pines very often leave understory seedlings and saplings stunted and severely 

 weakened (Cooper 1960). Baker (1942) reported that in certain stands of 50-year-old pine, 

 no reproduction existed and, generally, no vegetation whatsoever grew on the needle- 

 strewn ground. 



1 



