INTRODUCTION 



Natural seeding is normally the preferred method of regenerating conifers in the 

 Northern Rocky Mountains (Schmidt and Shearer 1973) . Some seedbed preparation is 

 usually necessary for successful natural regeneration. Shade intolerant species such 

 as western larch {Larix oooidentalis Nutt.) regenerate best on bare mineral soil. 



In northwestern Montana, broadcast burning for seedbed preparation is commonly 

 practiced in western larch--Douglas-fir [Vseudotsuga menziesii var. glauoa [Beissn.] 

 Franco) stands. Prescribed fire is preferred over mechanical scarification because: 

 (1) it normally costs less (Boyd and Deitschman 1969) ; (2) it recycles nutrients back 

 into the soil, ultimately resulting in increased tree growth rates (western larch trees 

 up to 13 years old grew about one-third faster on broadcast burned areas than on dozer 

 scarified seedbeds (Schmidt 1969); and (3) it is usually the most practical method for 

 site preparation on steep slopes. 



The amount of seedbed preparation desired (or achieved) is best expressed by min- 

 eral soil exposure. However, no conclusive quantitative data on the spatial distribu- 

 tion of mineral soil exposure by broadcast burning are available. Instead, research 

 has generally focused on measurements of duff layer consumption. The postfire seedbed 

 condition can be more meaningfully expressed by an estimate of both duff reduction and 

 the spatial distribution of exposed mineral soil. Research has not provided adequate 

 guides for managers to prescribe fires that expose a specified percent of mineral soil. 

 A method is needed to simply and accurately predict the quality of seedbed preparation 

 by burning. 



The study reported here is part of a Research and Development Program evaluating 

 alternative residue utilization and disposal treatments, one of which is burning. The 

 objective of this study was to develop regression equations, using only preburn measure- 

 ment input, for both duff reduction and the amount of mineral soil exposed by prescribed 

 fire for clearcut and shelterwood si Ivicultural systems. The prescription goal for the 

 fires was a 2-inch reduction of the duff layer. This would have resulted in an overall 

 mean duff depth reduction of about 50 percent, and provided the minimum 30 percent min- 

 eral soil exposure desired. Prescribed conditions deemed necessary to achieve the 

 objective were as follows: 



SHELTERWOOD BLOCK 



RANGE 



0-1 inch (0-2.54 cm) diameter 



dead fuel water content 

 Upper duff water content 

 Windspeed 

 Air temperature 

 Relative humidity 

 Time of day 



16%-18°6 

 40%-60% 



0-5 mi/h (0-8.05 km/h) 



70°F (21.1°C) or less 



25% or above; steady or rising 



1400 or later 



CLEARCUT BLOCKS 



RANGE 



0-1 inch (0-2.54 cm) diameter 



dead fuel water content 

 Upper duff water content 

 Windspeed 



Relative humidity 



12% -15% 

 25% -40% 



0-10 mi/h (16.09 km/h) ; 



steady to diminishing 

 25% or above, steady or rising 



1 



