INTRODUCTIOIM 



Pi-nus ponderosa Laws, is found throughout the western United States from Canada 

 to Mexico making it the most widely distributed pine species in North America (Curtis 

 and Lynch 1965) . Although it grows under a variety of environmental conditions and 

 different forest types, ponderosa pine is most commonly associated with sites that 

 are characterized by low soil moisture and high surface temperature at some time during 

 the growing season (Pearson 1951, Foiles and Curtis 1973). Because of its drought 

 resistance ponderosa pine can survive in areas that are too dry for other conifer 

 species (Foiles and Curtis 1973) . Only on the driest sites does ponderosa pine be- 

 come the climax species. 



Observations indicate that virgin ponderosa pine stands were uneven-aged stands 

 composed of even-aged groups (Cooper 1960, Weaver 1943) . The age class structure of 

 the stand was maintained by a series of continually occurring natural events. A few 

 trees or groups of trees would be killed or die from insect attack, disease, or old age. 

 Fires, which are known to have been frequent in virgin pine (Weaver 1959), would consume 

 the remains of these trees, often leaving an opening and good seedbed for young ponder- 

 osa pine seedlings. Because of the optimum conditions, a large number of seedlings 

 would likely become established on this seedbed if a seed source was available. Subse- 

 quent fires would remove the smaller trees from the dense areas of reproduction until 

 the most vigorous and rapidly growing saplings were no longer susceptible. The result 

 was a group of even-aged trees that continued their growth until the cycle was repeated 

 (Weaver 1943) . Seedlings occasionally became established beneath the larger mature and 

 overmature trees, but they seldom survived the fires that burned the deep layers of 

 litter and debris. The elimination of reproduction within the pine stand left an open 

 forest floor, usually covered with nutritious vegetation and providing a park-like 

 appearance to the forest as described by early travelers (Cooper 1960, Biswell 1973) . 



Seedlings that grew beneath mature trees were very susceptible to fire because of 

 stunted growth and poor vigor. Cooper (1960) reported that even seedlings that grew 

 within an open canopy of mature ponderosa pine grew very slowly and were stunted and 

 unhealthy. The poor growth was attributed to competition for soil moisture rather 

 than light. The effect of light should not be overlooked, however, because light 

 shading of pine seedlings will reduce growth compared to those in full sunlight, and 

 heavy shade can cause death (Pearson 1936, 1940) . McDonald (1976) discovered that 

 seedling height growth was significantly reduced by the presence of mature seed trees 

 20 feet or closer, and the greater the seed tree density the poorer the height growth. 

 Also, removal of the seed trees did not immediately eliminate the inhibitory effect. 

 Moisture depletion by the seed trees was considered the major limiting factor, but it 

 did not explain continued inhibition after the seed trees were removed. Possible ex- 

 planations were nutrient depletion and toxic substances produced by decaying seed-tree 

 roots. 



The following report describes the results from five experiments (a test for vola- 

 tile inhibitors, a test for water soluble toxins, the toxicity of plant leachates on 

 soils, a field soil bioasssay, and the effect of seedbeds on germination) that were 

 conducted to determine if phytotoxins are released from mature ponderosa pine trees or 

 the decomposition of its litter, and if these phytotoxins drastically reduced the 

 germination or growth of ponderosa pine seeds and seedlings. 



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