6 BULLETIN 10 "fl, IT. S. DEPARTMENT OF AGRICULTURE. 



their reactions on the organism, reducing to precise terms relations 

 between environment and life which may be already understood in 

 general terms. In such processes no distinction will be made between 

 a condition which is a direct result of the climate or site, one which 

 is the result of cumulative effects of the presence of the plant forma- 

 tion, and one which may represent the current influence of the pres- 

 ent plant formation. Thus, while recognizing in principle and in 

 the application of results historical conditions and the so-called 

 social relations which are particularly important in forest aggrega- 

 tions, it must be clearly understood that, in the current measure- 

 ments with which this bulletin has to deal, the source of a given con- 

 dition has no bearing on the method of its determination. 



SAMPLE PLOT METHOD COMMON TO BOTH ECOLOGICAL AND STATISTICAL STUDIES. 



A method common to both ecological and statistical problems is 

 the method of sample plots. The details of the sample plot method 

 vary with the purpose of the problem which is being investigated. 

 The plot may vary in size from a square foot to an entire section. 

 It may have all possible geometrical forms — circle, square, quad- 

 rangle, strip, or triangle. It may be used in the study of herbaceous 

 vegetation, of seedlings in a nursery, or of virgin forests; for the 

 purpose of studying the evolution of the vegetation, for bringing out 

 the effect of a definite condition, for determining the growth of the 

 vegetation, or for observing any other change that takes place in 

 the plant association, whether it be grass, brush, or forest. The prin- 

 ciple, however, remains everywhere the same; namely, the use of 

 areas representative of a given type of vegetation for intensive obser- 

 vation over a long period of time. 



NEED FOR A PERMANENT ORGANIZATION IN FOREST INVESTIGATIONS. 



The great variety of forest stands, the difference between stands in 

 different regions, and the longevity of trees make it difficult for an 

 individual to complete any investigation on the life of the forest. 

 This difficulty is now universally recognized. A permanent organi- 

 zation charged with such investigations has been formed in practi- 

 cally every country in which the care of the forests is a matter of 

 national concern. This permanent organization consists of investi- 

 gators assigned to forest experiment stations. 



FOREST EXPERIMENT STATIONS. 



Since it is practically impossible to follow all the changes which 



take place in a stand during its entire life of 100 years or more, the 



usual procedure is to carry on a number of observations simultane- 



By distributing the observations over stands of the same 



