3. Determine the bulk density of live tree crowns. 



To appraise fire behavior potential of fuels one must know the weight of vegetative 

 material and its surface area. Surface area can be estimated from the size distribution 

 of biomass. Fuels of critical importance to land managers include downed woody residues 

 left after harvesting and thinning of trees, or residues created by factors such as 

 windstorms and snow breakage. To help land managers in the Rocky Mountain area appraise 

 the fuel and fire hazard of slash, a system for predicting slash weights and fire be- 

 havior potential has been developed (Puckett and others 1977) . Slash weights are ob- 

 tained from either a computer program for debris prediction-^ or a handbook that details 

 computational procedures for predicting slash fuels using tables of crown weight per 

 tree (Brown and others 1977) . The computer program requires tree inventory data as 

 input and computes weights of foliage and branchwood, unmerchantable bole tips, and 

 cull material. It is the most accurate method for predicting slash because it sums 

 weights predicted for individual trees read into the program. 



Rate of fire spread, area growth, intensity, flame length, and scorch height (Van 

 Wagner 1973) are estimated in the system primarily using Rothermel's (1972) mathematical 

 model of fire spread. Nomographs developed by Albini (1976) also provide a means for 

 predicting fire behavior in slash. 



Many studies have shown that crown weights of conifers and hardwoods can be pre- 

 dicted from bole diameter. However, except for lodgepole pine in Canada (Muraro 1966; 

 Kiil 1967; Johnstone 1970) and Engelmann spruce in Colorado (Landis and Mogren 1975), 

 only limited information exists for Rocky Mountain species (Storey and others 1955; 

 Fahnestock 1960). Also, limited information, especially size distribution of branchwood, 

 has been published on West Coast species (Kittredge 1944; Chandler 1960; Cole and Dice 

 1969; Storey 1969) . 



Storey and Fahnestock studied trees having dominant and codominant crowns ranging 

 in d.b.h. from about 2 to 40 inches. Influence of stand density on crouTi weight was not 

 studied; however, site was shown to influence crown weight relationships (Storey and 

 others 1955) . Estimates of amount of foliage and branchwood were based on only a few 

 observations. The study in this paper combines data by Fahnestock and Storey v\;ith 

 considerable additional data, especially describing dead crovm weights, size distribu- 

 tion of crown components, and live crown weights of trees less than 2 inches and greater 

 than 20 inches. 



The branchwood size classes under 3 inches correspond in increasing size to 1-, 

 10-, and 100-hour average moisture timelag classes for many woody materials (Fosberg 

 1970). These size classes are used as moisture timelag standards in the U.S. National 

 Fire-Danger Rating System (Deeming and others 1972) . A moisture timelag is the amount 

 of time for a substance to lose or gain approximately two-thirds of the moisture above 

 or below its equilibrium moisture content. Appraisal of forest fuels is greatly facili- 

 tated when data on biomass are assimilated by these size classes. Once weight of 

 foliage and branchwood by diameter classes is determined, surface area can be estimated 

 using surface area-to-volume ratios for foliage (Brown 1970) and branchwood (Bro\\rn and 

 Roussopoulos 1974) . Weights must be converted to volumes using known or assumed values 

 of density (Brown 1974) for calculating surface area from volume and ratios of surface 

 area -to -volume . 



■^Brown, J. K. , and C. M. Johnston. 1976. Debris prediction system. Unpublished 

 report on file at the Northern Forest Fire Laboratory, Missoula, Montana. 



2 



