Disauss-Lon. --The possible factors influencing the growth of ponderosa pine seed- 

 lings, like most other plants, include light (quantity and quality), temperature, soil 

 moisture, nutrients, competition, and phytotoxins. Potential phytotoxins were prob- 

 ably not naturally effective because in high concentrations they did not influence 

 seedling growth in laboratory experiments (Kelsey and Harrington 1979) . 



Seedling productivity on the fire treatments was much greater than that on any other 

 treatment. Comparing the environmental conditions of the fire treatment with those of 

 the opening-cleared and opening-litter treatments, seedlings from the former had no ad- 

 vantage due to optimum temperatures, sunlight, or soil moisture, and experienced greater 

 competition. The fire seedlings were, however, exposed to higher nutrient levels which 

 was most likely responsible for the rapid growth. The lower light levels probably did 

 not hurt the fire seedlings' growth because if soil conditions (nutrients and moisture) 

 are near optimum then seedlings can, perhaps, tolerate exposure to much less light 

 (Bates 1925). 



The growth of seedlings in the opening-cleared and opening- litter treatments was 

 nearly equal. Slight differences were likely due to the mulching effect of the litter 

 which permitted higher soil moistures and probably lower soil temperatures. 



It is clear, however, that the seedlings from the two opening plots discussed above 

 grew better than those from the opening-natural plots. The physical characteristics of 

 the opening-natural treatment were comparable to those encountered on the opening-cleared 

 and opening-litter treatments in all respects except competition. These opening-natural 

 plots had all of the native grasses and forbs present throughout the year and their pres- 

 ence can be considered most influential in reducing seedling growth. 



The litter plots had environmental conditions which approximated those conditions 

 found on the opening treatments and consequently, the growth was similar, being some- 

 where between that of the opening-cleared and opening-natural plots. 



The reason for the poor growth in the stemflow plots is not apparent when examining 

 the physical characteristics. Temperatures, light, and soil moistures in these plots 

 were all comparable to those found in the openings and the nutrient levels were slightly 

 higher. Time and rate of germination of the stemflow seeds were similar to those on 

 the opening treatments. However, the seedlings on the stemflow plots experienced sub- 

 stantial mortalities due to cutworms and birds or small mammals early in the growing 

 season. Many of the seedlings received cotyledon damage and did not die, but suffered 

 growth impairment. It was also observed during the excavation of stemflow seedlings that 

 some of their roots had grown into the outer corky layer of the large roots of the cen- 

 ter tree. When this happened, there was a considerable reduction in the size of the 

 seedlings' root systems which certainly affected shoot sizes. 



The remaining three treatments- -canopy, near, and natural--had some very similar 

 characteristics which resulted in similar seedling growth responses. Soil moisture and 

 nutrients appeared in sufficient amounts so as not to limit growth. However, surface 

 temperatures were low, except for periodic intervals on the natural plots. The amount 

 of light received on these three treatments was less than 50 percent of that received 

 on the opening treatments. The reduced light and temperatures made themselves evident 

 by causing later and slower germination rates, which reduced the growing period and, 

 coupled with low temperatures and light levels, resulted in small seedlings. Larson 

 (1967) also showed that ponderosa pine seedling epicotyl lengths, root penetration, 

 numbers of lateral roots, and dry weights were positively correlated to number of 

 degree-hours. 



The natural plots differed from the canopy and near plots by the presence of a 

 litter layer and understory vegetation. The effect of the litter layer on seedling 

 growth was probably twofold. It reduced mineral soil surface temperatures and delayed 



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