14 CTRCTJIiAB 333, U.S. DEPAEIMEX1 OF AGRICULTURE 



them. Small openings in the stand such as often result from the 

 action of wind, insects, diseases, or fire, from culling, and from 

 other causes may be included if they are typical of the stand and 

 site, except that they probably should not be included in yield plots. 



Proper location of a plot within a stand calls for the exercise of 

 good judgment and requires considerable preliminary work. In 

 choosing a plot location the investigator should at all times guard 

 against personal bias. When it has been determined by careful 

 examination that the area meets the conditions of the experiment 

 in a general way. a more detailed and thorough scrutiny is in order. 

 This may involve a preliminary tally of diameters by species, to check 

 composition and diameter distribution : the preparation of a height- 

 diameter curve, or several such curves in various portions of the 

 stand, to check site: borings to determine age: a check of history. 

 especially tire history: and a preliminary study of the soil 9 as to its 

 texture and depth (especially, the depth of the A and B horizons) 

 and the character of the subsoil. An example of soil conditions 

 having an important influence on tree development is shown in 

 plate '2. 



The next step is to establish tentatively the boundaries of the plot 

 or plots. This can be done by running twine along the proposed 

 boundaries, taking care only to keep the lines approximately straight. 

 The plots so laid out are again examined to make sure that they are 

 in general comparable. Often it is necessary to establish subplots 

 and to analyze the data for these in order to be positive that the 

 plots are homogeneous, that statistically no significant differences 

 exist among them, and that they are thoroughly comparable. Some 

 shifting of lines may be necessary to eliminate a portion of a plot 

 where the number of trees per unit is below the acceptable standard, 

 to avoid a rock outcrop, or to get the plot arrangement that is the 

 best possible from all points of view. 



Time spent in the preliminary survey of a plot is well spent, if 

 it assures due consideration of all the factors concerned. 



SURVEYING AND DEMARCATING PLOTS 

 SURVEY OF PLOT 



The accuracy of all data on an areal basis derived from a sample 

 plot depends upon the accuracy with which the plot is surveyed. 

 Conversely, the accuracy required in surveying plots depends upon 

 the accuracy desired in the areal data to be obtained from study of 

 the plots. On permanent plots additional need of accuracy arises 

 from the fact that it may sometime be necessary to relocate a sec- 

 tion of the boundary or even a corner. 



Absolute accuracy in sample-plot work is of course out of the 

 question, because 'biological phenomena incapable of absolute 

 measurement are involved and because perfect sampling of the 



9 A knowledge of soils and of methods of classifying soils is greatly needed in forest 

 research in order to make sure that plot conditions are uniform. Through recent 

 developments in the technic of soil analysis it has become possible to make an approxi- 

 mate check of soil characteristics with relatively little labor. The Bouyoucos hydrometer 

 method (11) enables one to determine the main features of soil texture in about 15 

 minutes with relativelv simple laboratory equipment. Microchemical methods make 

 possible rapid analvsis'of soils for important plant nutrients (53, 76). Classification 

 of soils, according to percentage of sand. silt, clay, and various combinations of these, 

 is easily effected ~by use of the Whitney diagram or some variation of it (17). 



