feed on sapwood and heartwood can reduce structural 

 quality, making these trees somewhat more suscep- 

 tible to falling (Kimmey 1955; Schmid and others 

 1985). 



A primary activity associated with successful insect 

 attacks is the introduction of fungi that occurs most 

 often on insect bodies, but also by air-borne means 

 (Lowell and others 1992). First to infest the sapwood 

 generally are stain fungi that slow water transport 

 within the tree but cause limited structural decline 

 (Kimmey 1955). Decay fungi, which generally follow 

 stain fungi, destroy wood cells. This results in reduced 

 bole strength, thereby increasing the likelihood of tree 

 fall (Keen 1955). 



A tree's resistance to insect and disease attack is 

 determined largely by the intensity of the attack and 

 the tree's chemical defenses. The intensity of the 

 insect attack is generally related to the severity of the 

 damage, with greater injury resulting in greater at- 

 tacks (Miller and Keen 1960; Ryan 1993). However, 

 trees with fire-consumed crowns and extensive bark 

 charring do not normally attract bark beetles. The 

 capability of the chemical defense mechanism is deter- 

 mined largely by tree vigor (Kaufmann and Stevens 

 1984; Matson and others 1987) and, perhaps, pheno- 

 logical state (Lorio 1986). 



Conifers respond to pest attacks primarily by syn- 

 thesis and translocation of resin that can physically 

 flush out or isolate attackers and may contain toxic 

 monoterpenes (Berryman 1972; Lieutier and Berryman 

 1988; Raffa and Berryman 1982). The synthesis and 

 movement of large quantities of resin and monoter- 

 penes are energy demanding (Raffa and Berryman 

 1982, 1983). Waring and Pitman (1985) reported that 

 allocation of carbohydrates for chemical defense likely 

 has the lowest priority, occurring only after produc- 

 tion for foliage, buds, roots, stem elongation, and 

 storage reserves. Because the amount of photosyn- 

 thate determines the amount of carbohydrates allo- 

 cated for each function, reducing crown volume by 

 scorching would have a great impact on resources for 

 defensive action (Gerry 1931; Harper 1944). 



Injured trees with low vigor as well as those with 

 high vigor are capable of at least initiating resinosis of 

 infected tissue (Owen and others 1987; Raffa and 

 Berryman 1982). Therefore, even if the injured tree 

 dies, some resin movement has likely occurred. Be- 

 cause resin delays fungal spread and, therefore, cell 

 deterioration, higher resin concentrations and larger 

 resin-soaked areas would greatly retard stem decay 

 and probably lengthen the time a tree would stand 

 after death. 



In this study, trees that died with low crown scorch 

 but survived for 2 or 3 years after injury had only a 

 27 percent probability of falling within the 10-year 

 study. This compares with greater than 75 percent 



probability of falling for those with low scorch and 

 first-year mortality, and those with high scorch re- 

 gardless of years of survival after injury. 



Insect attacks were noted on all dead trees, but the 

 intensity of the attack was not estimated. With 80 

 percent or greater crown scorch and, therefore, high 

 loss of photosynthetic tissue, the high probability of 

 early falling was likely due to the limited ability to 

 produce quantities of resins sufficient to resist insect 

 invasion and stem decay. With low crown scorch but 

 first-year mortality, early falling may have resulted 

 from minimal resinosis due to initial low tree vigor, 

 limited response time, or massive insect attacks, that 

 can overwhelm even healthy trees (Raffa and Berryman 

 1982). The lower probability of falling for trees with 

 low scorch and 2 or 3 years of postinjury survival may 

 have been the result of sufficient resin synthesis from 

 the higher volume of residual photosynthetic tissue 

 and sufficient time for resin translocation to a large 

 area of the stem. Therefore, wood decay before and 

 after mortality would be retarded. 



Management Implications 



If stand and prescribed burn conditions resemble 

 those of this study, low mortality could be expected. Of 

 the trees that die, a relatively high fall rate could be 

 anticipated within 10 years of fire injury. When man- 

 agers are evaluating tree mortality from the viewpoint 

 of potentially enhanced wildlife values or added woody 

 debris, they should expect that 75 to 80 percent of the 

 trees that die in the first postburn year will fall within 

 10 years. 



Trees that have an extended wildlife value, espe- 

 cially for nesting and perching, will be those with 

 moderate crown scorch and that remain alive for 2 

 years or more after injury. Mortally injured trees with 

 these characteristics will not be distinguishable in an 

 immediate postburn evaluation. A survey during the 

 second or third postburn year will allow better deter- 

 mination of long-term snag presence in the stand. 



References 



Berryman, A.A. 1972. Resistance of conifers to invasion by bark 

 beetle-fungus associations. Bioscience 22: 599-601. 



Bull, E.L. 1978. Specialized habitat requirements of birds: snag 

 management, old-growth, and riparian habitat. In: DeGraaf, 

 Richard M. , tech. coord. Proceedings of the workshop on nongame 

 bird habitat management in the coniferous forests of the West- 

 ern United States; 1977 Feb. 7-9, Portland, OR. Gen. Tech. Rep. 

 PNW-64. Portland, OR: U.S. Department of Agriculture, Forest 

 Service, Pacific Northwest Forest and Range Experiment Sta- 

 tion: 74-82. 



Dahms, W.G. 1949. How long do ponderosa pine snags stand? Res. 

 Note No. 57. Portland, OR: U.S. Department of Agriculture, 

 Forest Service, Pacific Northwest Forest and Range Experi- 

 ment Station. 3 p. 



Gerry, E. 1931. Oleoresin production from longleaf pine defoliation 

 by fire. Journal of Agriculture Research 43(9): 827-836. 



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