STANDARD FOR "NORMAL" DENSITY OF STOCKING 397 



306. Standard for " Normal " Density of Stocking. In selecting 

 plots for a yield table, in natural stands, it is neither possible nor advis- 

 able to seek areas which show the maximum theoretical density of 

 stocking, either as to crown canopy or number of stems per acre. Nor 

 should any effort be made to select plots which represent the empirical 

 average of stocking. The standard should be to exclude from the plots 

 all larger blanks caused by destructive agencies or failure of stocking 

 and to select areas reasonably well stocked, with comparatively complete 

 crown canopy. This standard of selection should be such that a suf- 

 ficient number of plots can be readily obtained from the larger areas, 

 without refinements either in size or in location. If too high a standard 

 is set, the plots conforming to this standard will be found to be either 

 located exclusively on the better portions of each site, or the area of 

 the plots' will be too small for safe results. In natural stands this ten- 

 dency will lead to the selection of plots containing too great a number 

 of trees, which will result later in over-stocking. 



The average yield obtained from plots selected on this basis is 

 termed the normal yield, though it may be exceeded by the best plots, 

 or by stands grown under management. 



307. Age Classes. The area of a plot should include but one age 

 class. Where stands are actually even-aged over considerable areas, 

 plots are easily and rapidly located. Where there is difficulty in dis- 

 tinguishing the age classes, and in locating areas which exclude all 

 trees but those belonging to the class desired, it may be necessary to 

 include a few scattered trees of a different age class in order to obtain 

 plots of a suitable size. The net area of the plot can then be found 

 by deducting the space occupied by these trees, which can be based 

 on the area covered by their crown spread, modified in open stands 

 to include a proper proportion of the gaps in the crown cover. 



Stands whose period of reproduction is from ten to thirty years, 

 depending on site and climatic factors, but which may still be classed 

 as even-aged stands ( § 259) will be measured as such and their average 

 age determined. 



308. Area of Plots. The value of a single plot in indicating normal 

 yield increases with its size, within the limit which permits of securing 

 a uniform stocking and crown cover conforming with the standard 

 sought. Since one plot represents but a single age and one shade of 

 site quality, and the cost of measurement increases with size, it is better 

 to limit the size of plots for a yield table and obtain a greater number 

 more widely distributed. 



The size of plots should increase with the size and age of the trees 

 to be measured. The greatest danger in measuring small plots is 

 failure to coordinate the quantitative site factors utilized in producing 



