136 SEEDING AND PLANTING 



Although the above are average figures of seed production for 

 an entire region, they cannot be used in judging local seed pro- 

 duction, because the age and character of the stand and differ- 

 ences in soil and climate affect the periodicity of seed years. 

 Wimmenauer, in compiling the observations made over a period 

 of 10 years at 260 stations in Germany, found that the fullness 

 of the seed crop of Norway spruce for a single year (1887) varied 

 at the different stations as follows: 



12 per cent of stations, full seed crop, expressed as 1. 

 32 per cent of stations, fair seed crop, expressed as . 

 47 per cent of stations, poor seed crop, expressed as . 

 9 per cent of stations, complete failure expressed as 0. 



This shows slightly less than half a full seed crop as an average 

 for the entire region. 



More recent methods for studying seed production have been 

 advanced by Russian foresters, notably Soboled. 1 Soboled's 

 method includes: 



a. The determination of the quantity of seed per unit of area. 



b. The determination of the quality of the seed. 



The seed crop, or x, is expressed by the formula x = ap, in 

 which a is the weight of pure air-dried seed from a unit of area, 

 and p is the germinative capacity. 



The amount of seed per unit of area is determined by means of 

 sample plots, each of which contains at least 100 trees of the prin- 

 cipal species composing the stand. The collection is made from 

 sample trees within the sample plots, and the yield of the entire 

 plot estimated from the carefully selected sample trees. This 

 intensive method carried out for a long period of years in a given 

 locality measures seed production with a high degree of accuracy. 



As yet the seed production of trees indigenous to the United 

 States has not been extensively studied. We have little infor- 

 mation on the periodicity of seed years and the average yearly 

 production per unit of area for different regions and different 

 species. From ocular studies Cox 2 estimates the following as a 

 full seed crop for a number of western species: 



1 See reviews and discussions of these methods by Zon, Dana and others. 

 (Proc. Soc. Am. For., vol. VI, pp. 145-152; and vol. VIII, pp. 117-130.) 



2 Cox, W. T.: Reforestation on the national forests. (U. S. Forest Service 

 Bui. 98, p. 13. 1911.) 



