Typical crown ratios are displayed in table 1. The fraction of the canopy layer 

 that is occupied by tree crowns (F above) we take to be 0.4 for dense stands and 

 0.1 for open stands. Table 2 gives the volume-filling fractions (f above) resulting 

 from the products of the crown ratios of table 1 and these closure factors. In terms 

 of the volume fraction, f, equation (17) provides a relationship for the windspeed under 

 the canopy, U^, and the windspeed at the top of the canopy, U^: 



1/2 



(19: 



Table 1 . --Typical crown ratios^ percent 



Stand 



: Shade- 



-tolerant trees : 



Shade - 



intolerant trees 



stocking 



: Young 



: Mature : 



Young 



; : Mature 



Dense 



80 



60 



40 



20 



Open 



90 



70 



70 



50 



Table 2 . --Volume filling fractions (factor f)j percent 



Stand 

 stocking 



Young 



Tolerant 



Intolerant 



Mature 



Young 



Mature 



Dense 

 Open 



32 

 9 



24 

 7 



16 



7 



This expression can be reduced to a simpler form by using nominal numerical values for 

 the parameters that are essentially constant. The value of K can be taken as 0.4, C^^ 



approximately 1.0 (Schlichting 1968), and the product ga can be treated as a constant^ 



equal to 3.26 ft ''^ . So for H measured in feet. 



/ 2t 



6a fH 



0.4 /— 



V(i 



[3.26) fH 



/fH 



:20) 



Values of U /U for three tree heights are given in table 3, based on the f values of 

 table 2. ^ 



1 The authors are indebted to Dr. James K. Brown of the Northern Forest Fire Labora- 

 tory for providing these typical quantities. 



2 This is an average value for 10 Rocky Mountain conifers. Data were taken from study 

 material by J. K. Brown (on file at NFFL, Missoula) used in establishing crown foliage 

 weight relationships (see Brown 1976) . The standard deviation of the product 60 for 



these species is 1.08 ft \ 



8 



