ACCRETION OR INCREMENT. 35 



more saw timber than in the young grove above considered. 

 For this reason, a good thicket of pine 60 years old may not cut 

 much more than 6 M. feet per acre, since much timber is under 

 sized, but the same stand at 120 years old would easUy cut 15- 

 20 M. in spite of the fact that over half the trees that were 

 found in the 60 year grove have died before this age is reached. 

 From this it would appear that 100 feet B. M. per acre and year 

 on sapling timber is probably a safe estimate for the growth 

 in this region. About the same conclusion will be reached if 

 a grove of old hardwoods is considered. Such a grove, which 

 may cut say 6 M. feet per acre,i will be found to consist lai^ely 

 of young trees, and among these 20-30 good sized older trees. 

 If examined, it will be found that the age of the oldest is not 

 over 150 years, so that here about one-third or less of all the 

 trees standing on the acre have produced in 150 years the 6 M. 

 feet B. M., which we are taking for lumber. The whole acre, 

 therefore, may well be assumed to be able to produce this quan- 

 tity in one-third this time, or in other words the same acre might 

 be logged over for 6 II. feet about every 50 years. Such an 

 assumption is fully supported also by comparing the cross-sec- 

 tions of the pine and hardwood. These show that, though the 

 rate of growth of hardwoods in Wisconsin is rather slow, yet 

 the growth of oaks, basswood, etc., equals and excels that of piue. 

 If, then, 100 feet B. M. per acre and year be assumed as an 

 average estimate of growth for this region, the total annual 

 growth over the whole may be set at about 900, million, feet B. 

 M. and is distributed among the different kinds according to 

 their ascertained acreage as follows: 



Million feet. 

 White and red (Norway) pine . . 250 



Jack pine 30 



Hemlock 75 



Tamarack 30 



915 



Of this growth the greater part is balanced by decay or nat- 

 ural waste, which in all wild woods necessarily equals growth 

 when large areas and long periods are considered. For white 



Million feet. 



Cedar 20 



Spruce and balsam 10 



Hardwoods 500 



