44 BULLETIN 1059, U. S. DEPARTMENT OF AGRICULTURE. 



investigation it is obviously necessary to use spectroscopic methods. 

 Since the growth of a tree requires many years, and even complete 

 suppression in the densest forest is seldom accomplished in less than 

 two or three years, it is evident that in the forest minute examination 

 of every variation in sunlight is unnecessary. An examination cov- 

 ering the entire period of the activity which is being studied must, 

 however, be obtained. The nearly ideal and still practicable arrange- 

 ment would seem to be provision for continuous observation of the 

 total energy available as radiation throughout the period of plant 

 activity, with sufficiently frequent spectroscopic observation of the 

 composition of this energy to establish not only an average quality 

 analysis for the whole period but also to show the variations which 

 occur from season to season and year to year and their relations to 

 the functioning of the plant. 



Unfortunately, spectrum analysis by present common methods does 

 not permit an examination of either the ultra-violet or infra-red 

 spectra. For this reason it has been suggested that all spectrum 

 analyses might be better conducted by means of energy measure- 

 ments (e. g. thermal effect) than by optical comparisons. This is an 

 almost unexplored field and presents infinite possibilities for the in- 

 vestigator who will devise a satisfactory method of measuring the 

 energy of all parts of the spectrum under both laboratory and field 

 conditions. 



HOEIZONTAL AND VERTICAL EXPOSURES. 



It is perhaps well to point out at this stage that, particularly in 

 forest studies, light measurements of whatever kind may be on two 

 distinct bases. In forestry the growth of an individual tree is rarely 

 spoken of, or even if it is, no practical significance is attached to it, 

 because the individual can rarely be separated from the influence of 

 other individuals. Forest growth, in any practical sense, must be 

 growth (volume increment) per unit of land area. Similarly, if an 

 attempt is made to find a relationship between growth and available 

 light, it is certain that the energy must be expressed in terms of a unit 

 of area inclined at the same slope as the ground. The total energy 

 available to the crowns of trees on a northerly exposure of given 

 gradient, for example, can not be more than that which would be inci- 

 dent upon a plane of exactly the same aspect and gradient. Land 

 areas, however, are always measured in terms of their horizontal 

 projections. It therefore follows that the measurements must be re- 

 duced to horizontal areas, and the simplest means for reduction is to 

 expose a given area horizontally for the original determinations. 



Determinations of total energy available for growth, however, 

 will rarely be made in ecological studies, which are much more likely 



