284 JOURNAL OF FORESTRY 



annual increment, this might he more than offset by new trees growing 

 into merchantable size. 



A glance at the table at once reveals a wide range of increment on 

 various plots. In accounting for these differences, a great many fac- 

 tors must be taken into account. These factors fall into two general 

 classes, namely: Site quality, or the physical conditions affecting 

 growth ; and growing stock, comprising density, distribution, age, and 

 condition of the trees. Method of cutting is important only in so 

 far as it alfects the above factors. 



A comparison of plots 3 and 4 is interesting, since both were cut in 

 the same year, under Forest Service regulations and according to the 

 same silvicultural system. Plot 3 shows the higher increment in both 

 board feet and cubic feet. This relation is in accordance with what 

 may be expected from a comparison of the two sites, since plot 3, 

 on the basis of available information, offers the better conditions for 

 growth. The table, however, also shows plot 3 to be more fully 

 stocked than plot 4. In fact, when the two plots are compared on the 

 basis of increment per cent, or increment in proportion to growing 

 stock, the advantage is on the side of plot 4. 



The annual board-foot increment from 1909 to 1914 is only 2.07 per 

 cent for plot 3 against 2.40 for plot 4, while the cubic increment per 

 cents are 1.91 and 2.07 respectively. This means that plot 4 is yield- 

 ing a higher return for the capital invested than is plot 3, which con- 

 trary to what might be expected from a comparison of the sites. 



The probable explanation of the above discrepancy is to be found 

 in the condition of the stand. The difference in age classes, as indi- 

 cated by the proportion of yellow pine and black jack, is in favor of 

 number of trees as against 21.6 per cent for plot 4. The real ex- 

 planation appears to be in the proportion of unhealthy trees. On plot 

 3, 10.8 per cent of the trees are classed as unhealthy, while on plot 4 

 only 3.8 per cent are thus classed. A comparison of healthy and un- 

 healthy trees of all diameters on an "intensive" plot shows the basal 

 area accretion in the latter to be from 30 to 50 per cent lower than in 

 the former. A similar comparison of mistletoe-infected trees of all 

 diameters and degrees of infection with normal uninfected trees on the 

 same plot showed the infected trees to be 40 per cent below the unin- 

 fected. Mistletoe is reported on 34.9 per cent of all the trees on plot 

 3, while on plot 4 there is no mistletoe whatever. 



Both the lowest and the highest yields are recorded on the old cut- 

 tings, plots I and 2. The former shows an annual increment of only 

 31. 7 board feet and 5.44 cubic feet per acre as against 145.8 and 29.61 



