DISCUSSION 



Western hemlock is a climax species. It is very tolerant of shade and well adapted 

 to growth as a crowded, suppressed understory in areas of high precipitation. Develop- 

 ment of Sirooooous shoot blight is favored by conditions of high atmospheric moisture, 

 mild temperatures, and low light intensities (Funk 1972; Peterson and Smith 1975; Smith 

 1973) . Spores of the causal fungus are disseminated by splashing of water drops 

 (Peterson and Smith 1975). The spores infect only juvenile needles and the disease 

 kills new shoots (Peterson and Smith 1975) . Such environmental requirements explain 

 why disease severity is greater in regeneration of suppressed and intermediate crown 

 classes than in potential crop trees. Environments of overstocked western hemlock 

 forest ecosystems in southeast Alaska are ideal for optimum development of the suscept 

 and the disease. 



The highest disease index was 0.187 for the fertilized area. This means that 

 18.7 percent of the branching has been killed by the disease. Unfortunately, there was 

 no opportunity during this investigation to sample a comparable area of western hemlock 

 regeneration that was disease-free. Neither can we find published data adequate for 

 quantitative comparisons. Therefore, we cannot express the disease index in quantita- 

 tive terms of growth reduction. 



Sirooooous shoot blight reduces tree growth because it reduces photosynthetic tis- 

 sue and causes a net loss in carbohydrates. Food for tree growth is produced by active 

 photosynthesis or may come from stored reserves. Young conifer needles are more effi- 

 cient in photosynthesis than are older ones. Likewise, branches in the upper crown 

 exposed to full sunlight are more efficient than those in the lower crown. Branches 

 in the partially shaded lower crown may not produce enough food to maintain their 

 growth. In some conifers, little or no additional food is translocated to these 

 branches and they die. This process, called natural pruning, is very common in shade- 

 intolerant species. In other conifers, food manufactured in other parts of the crown 

 is translocated to branches inefficient in photosynthesis, and they remain alive. Such 

 branches are parasitic on the tree and natural pruning is minimal. Therefore, compari- 

 sons of the effects of foliage removal by artificial or natural pruning from below with 

 equal amounts of defoliation by a foliage disease are not valid. Sirocooous shoot 

 blight is selective. It kills those tissues most efficient in photosynthesis; young 

 needles . 



When photosynthetic tissue and carbohydrate losses are so great that active photo- 

 synthesis cannot supply the food necessary for growth, then it must come from stored 

 reserves. When this condition persists for a time sufficient to deplete these reserves, 

 tree death is eminent. Levels of mortality and severe top-killing (table 1) of regen- 

 eration in suppressed and intermediate crown classes signify that the above condition 

 prevails in these crown classes. 



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