172 THE FOUNDATIONS OF FOREST MANAGEMENT, 



(3.) The yield of intermediate cuttings depends frequently on 

 events, which do not occur regularly, or ^Yhich cannot 

 he foreseen, so that it is almost impossible to estimate 

 it correctly beforehand ; for instance, in the case of 

 wind-break, snow-break, death caused by disease or 

 insects, etc. 



Hence, it is desirable to confine the regulation of the annual 

 yield to the final cuttings, and to be satisfied with an approxi- 

 mate equalisation of the intermediate returns, such as will 

 naturally happen, if the final cuttings are systematically 

 equalised ; provided always, that the thinnings are not made 

 so heavy as to affect the subsequent final returns. 



If a forest is to yield a return, either annually or periodi- 

 cally, it must be in a certain state. In order to determine, 

 what this state should be under a given set of conditions, it is 

 useful to construct an ideal pattern, such as would be presented 

 by a forest, which has grown uj) uninfluenced by external 

 interfering circumstances. The ideal state difi'ers, of course, 

 for every different method of treatment, in accordance with 

 the objects at which the management aims. In all these cases, 

 a forest, which corresponds in every way to the objects of 

 management, is called a normal forest. It enables the forester 

 to study the laws, which must govern the management, and it 

 serves as an ideal to be aimed at, though it may never be 

 altogether reached, and at any rate not permanently maintained. 



The normal state of a forest, under a given set of conditions, 

 depends chiefly on the presence in it of — 



(1,) A normal increment, 



(2.)- A normal distribution of the age classes, 



(3,) A normal growing stock. 



By noniud increment is understood that, which is possible, 

 given a certain locality, species and rotation. An abnormal 

 increment may be caused by faulty formation, faulty treatment, 

 injurious external influences and also, for the time being, by 

 a preponderance of certain age classes. 



By a iionnal distribution of (Hjc elnsscs is understood a 



