than 1 percent annual mortality in Colorado as compared to the 3.6 percent rate in the 

 Gros Ventre. Furthermore, the Gros Ventre rate is perhaps two to four times greater 

 than reported recently by Manion and Valentine (1971) in New York. Also, in Ephraim 

 Canyon of central Utah, Dr. K. T. Harper 4 found about 2.7 percent annual mortality in 

 a sample of 2,091 aspen larger than 4 inches d.b.h. Harper's technique involved reexam- 

 ination in 1970 of trees originally examined and tagged as being dead in 1969. When 

 examining Harper's sample trees in autumn of 1970, we found that many would not have 

 been tallied as 1970 mortality by the criterion used in the Gros Ventre study; these 

 were trees that had not maintained dead leaves and were assumed to have succumbed during 

 the 1969-1970 dormant season. From this we conclude that the 3.6 percent annual mor- 

 tality measured in the Gros Ventre probably is an underestimate of actual mortality. 



In the Gros Ventre, it appears that increased mortality has been caused by big game 

 "barking" combined with pathogens and injurious insects that invade the wounded trees. 

 Most of the 1,061 trees included in the "Barking, weak parasites, pests, etc." category 

 of the tabulation on page 7 had scars resulting from big game chewing and some had been 

 completely girdled. "Barking" damage was most injurious to the smaller trees; this 

 partially accounts for the especially high rate of mortality in the 0- to 6-inch class. 

 Probably though, the higher mortality in smaller trees was caused mostly by partial 

 suppression which reduced their resistance to weak pathogens and stem-boring insects that 

 characteristically follow "barking." Cytospora ahrysosperma probably is a primary 

 contributer to the demise of such trees. This fungus was thought to be a major factor 

 in the aspen dieback associated with similar elk wounding of trees in Rocky Mountain 

 National Park (Packard 1942), and "...it is generally considered a weak parasite on 

 declining or dying trees" (Hinds 1964). Although encountered infrequently, Crypto- 

 sphaeria populina seemed to replace Cytospora as an invader of wounded bark in the 

 dead aspen trees encountered in some of the Gros Ventre plots. It is not known whether 

 this fungus is parasitic on aspen, but these observations suggest that it may be a weak 

 parasite on this species and similar in aggressiveness to Cytospora. 



The sooty bark fungus (Cenangium singulare) is an aggressive pathogen that can 

 enter wounded tissues and girdle and kill aspen trees within 2 to 4 years (Hinds 1962). 

 In the Gros Ventre, this fungus appears to be frequently aided by Agrilus beetles. 

 Although Hinds (1964) found that elk wounds on aspen in Colorado seldom served as 

 entrance points for canker organisms, the sooty bark fungus in the Gros Ventre was more 

 frequently associated with big game wounds than would be expected by mere coincidence. 

 Perhaps this results from more severe wounding in the Gros Ventre, or possibly wounding 

 occurs at a more favorable time for fungus infection. In this respect, it should not 

 be overlooked that moose are abundant in the Gros Ventre and might be "barking" some 

 aspen trees during the growing season. In this study, a few fresh "barking" wounds 

 were found in late September (fig. 8) in an area where only moose sign was fresh. In 

 contrast, other observations in the Gros Ventre sample area suggest that most of the 

 elk "barking" occurs in winter between December and May. 



Harmful fungi and insects were not particularly severe on aspen foliage during 

 1970, but it is suspected that in years when they are epidemic they might complement 

 the aggressiveness of other pathogens, or as Mielke (1957) suggested for Marssonina , 

 some aspens might even die from heavy attacks. In spite of the appeal of this logic 

 the Gros Ventre data failed to support a role by Marssonina in aspen mortality; the 13 

 plots showing leaf blight had similar mean numbers of live and dead trees (20.5 and 

 14.8, respectively) as did plots without Marssonina (23.7 and 17.1, respectively). The 

 hypothesis might better be tested during and after epidemic conditions. 



^Personal communication, Dr. K. T. Harper, University of Utah, Salt Lake City, 

 based on research while employed by Intermountain Station. 



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