For Rocky Mountain conifers, loadings of slash can be predicted from relationships 

 between tree crown weight and tree characteristics such as d.b.h. and height (Brown 1978). 

 In the USDA Forest Service Northern Region these relationships have been installed in a 

 computer program that obtains input from tree inventories and predicts potential debris 

 as output. 3 ' 4 In addition to predicting debris, a method to predict fuel depth is needed 

 to appraise fire potential. The objectives of this study were: 



1. Determine the relationship between fuel depth and loading of slash and the 

 extent to which species, age of slash, method of skidding, lopping, and other factors 

 influence the relationships. 



2. Determine the relationships between the easily measured high intercept depth 

 and bulk depth. 



When using analytical models such as Rothermel's (1972), fuel depth is a critical 

 parameter because it determines bulk density of the fuel array for given fuel loadings. 

 Rate of fire spread is very sensitive to bulk density (Williams 1977). Fuel depth is a 

 measure of the vertical extent of fuel in the zone that is actively involved in the 

 spreading flame front. Conceptually, the bottom of this zone is the forest floor and 

 the top is the height where fuel ceases to exist or is too sparse to affect propagation 

 of the flame front. Fuel depth that is compatible with fire modeling can be difficult 

 to measure because locating the top of this hypothetical zone in the fuel array requires 

 judgment . 



Fuel depth in slash and other downed woody material has been measured primarily on 

 a high intercept basis (Brown 1974) . In this procedure, the top of the fuel is defined 

 by the highest particle to intersect a vertical plane about 1 foot wide. Although this 

 procedure is easy to learn and to use, it does include large void spaces in the fuel 

 array whenever they occur. This permits overestimation of an effective fuel depth and 

 underestimation of bulk density required for fire modeling. A procedure for measuring 

 effective fuel depth--called "bulk depth"--has been developed by William Frandsen (1974) 

 at the Northern Forest Fire Laboratory. It allows observers to account for void spaces 

 in the fuel arrays and provides a measure of effective fuel depth appropriate for fire 

 modeling . 



The merchantable top diameter relates to depth. Small top diameters result in more 

 lopping of supporting branches and removal of bolewood than large top diameters; hence, 

 the slash is more compacted. Species may influence depth of slash due to differences 

 in branch stiffness and branching habit. In a study by Roussopoulos and Johnson (1975), 

 loading and depth of slash were directly related; however, as loading increased, depth 

 increased at a reduced rate. Probably as more tree crowns are added to a fixed area, 

 overlapping of branches occurs and the weight may cause compression. Methods of felling 

 and skidding trees should influence depth because the amount of trampling and breakage 

 depends on if and how merchantable boles are removed. 



Settling with age has a most significant influence on slash depth. For some 

 western conifers over a 5-year period, depth of lopped and unlopped slash was reduced 

 to one-half of the original value (Fahnestock and Dieterich 1962; Kiil 1968). Rate of 

 settling varied; for some species, depth actually increased slightly during the second 

 year before settling. In a study of piled slash over a period of 29 years, Wagener 

 and Offord (1972) found that piles continued to settle; however, 50 percent of the 

 settling occurred during the first 5 years. 



3 Users' guide to debris prediction and slash hazard appraisal. 1977. USDA Forest 

 Service Northern Region, Division of Fire Management, Missoula, Mont. 



^Brown, James K. , and Cameron M. Johnston. 1976. Debris prediction system. Office 

 report on file at the Northern Forest Fire Laboratory, Drawer G, Missoula, Mont. 



2 



