30 



not only at seasons when trees are to be planted out but at all sea- 

 sons. It should be well protected from trespass of all kinds, includ- 

 ing damage from various forms of wild life and livestock. 



More than ordinary care is needed to eliminate sources of error 

 peculiar to this class of work. One of the greatest of these is varia- 

 tion in soil as to humic content, depth, texture, and drainage. Vari- 

 ation in humic content may affect moisture conditions and thus cause 

 inequality in growth rate and in survival on different portions of the 

 area; depth may markedly affect root development and soil tem- 

 perature, and thus survival; texture may affect the ease of planting 

 or the probability of frost heaving; drainage may affect root de- 

 velopment, occurrence of frost, and thus survival, and may limit 

 the season during which planting is possible. The topography of 

 any series of reforestation plots should be substantially uniform, 

 so that results will not be distorted by site variations, and should 

 be such as to insure good air drainage and freedom from marked 

 thermal belts, especially frost pockets. Erosion should be con- 

 trolled, except in studies involving erosion effects. Steep slopes 

 should be avoided, because soil or snow creep might affect survival, 

 form, or growth rate. 



The shape of the plot is a major consideration. Obviously, the 

 square has a smaller periphery than any other rectangular figure. 

 Use of strip plots, however, undoubtedly brings about a better 

 representation of soil and site variations. Also, strip plots in 

 which successive rows of trees differ in species, in age, or otherwise 

 offer the best possible opportunity for comparing results with the 

 different classes of stock represented. They permit such compari- 

 sons only during establishment and early life, however ; as the trees 

 develop,' the value of the plots for such comparisons is destroyed by 

 competition between adjacent rows. All things considered, probably 

 the best shape of plot is a rather long narrow strip. 



In a strip plot each row should include at least 20 trees. For a 

 reasonably good random sample of site conditions without too great 

 a periphery, the ratio between the plot's ends and sides should be not 

 more than 1 inter-row space to each 20 trees in the row, or not 

 more than 1 linear unit to 5 ; preferably, it should be not more than 

 1 linear unit to 10. 



Optimum plot size depends largely on the purpose of the study, 

 and the possible need of plots of known history for later investiga- 

 tions. In a study of planting technic, for example, if the trees are 

 alike in species, age, and condition the number required may be 

 relatively small. In such a study, which requires from 5 to 10 

 years, annual plantations of from 200 to 500 trees each will usually 

 give reliable results. If a comparison is to be made not between 

 planting methods but between seed sources, species, or species mix- 

 tures, it is best to establish a strip block of at least half an acre, and 

 preferably much larger, within which a plot of from 200 to 500 

 trees will be studied. 



Spacing has only a minor influence on the early development of 

 the stand. In many planting investigations, consequently, it is 

 permissible to set trees much more closely together than in a reforest- 

 ation enterprise. For investigating only the initial establishment 

 of trees it is preferable to adopt a spacing of 3 by 3 feet or 4 by 4 

 feet, in which case a single test will require a relatively small area. 



