Periodical Literature. 105 



mean soil deterioration. A complementary species such as beech 

 must, therefore, be introduced into the stand if half the solar en- 

 ergy is not to be wasted. 



Wisely Wagner (unlike his namesake in Tubingen!) refrains 

 from making world-wide deductions on inadequate premises or 

 recommending his findings as being of universal applicability. 

 Very rationally he confines the use of his spectralphotometer and, 

 indeed, of all photometric methods in the forest, to the realms of 

 research. In practice, e.g. in the marking of thinnings', the find- 

 ings of investigators can be applied. The essential thing is to 

 utilize the solar energy just as completely as possible by secur- 

 ing the maximum of absorption in the crown cover. Besides this, 

 the crown spread and hence the growing space of the individual 

 tree must be larger in northern than in southern latitudes if 

 all the solar energy is to be absorbed, since in northern latitudes 

 the sun's rays fall less vertically. Of course, the exact grow- 

 ing space depends also on age, species' and site quality. This 

 adequacy of future crown spread is not always properly re- 

 garded in marking, so that stands approaching maturity have 

 often been too severely thinned in their youth. This means 

 enforced isolation of crowns in mature trees ; often breaks in the 

 crown cover with attendant loss of solar energy. 



Wagner has introduced a new aspect of Conservation: i. e. 

 Conservation of Elemental Energy. That this energy is limited 

 is a thought strange to even our era of conservation. 



In this line of research Wagner admits science has only made 

 the barest beginnings. The solar energy and the composition 

 of solar rays are inadequately known. The analysis of chloro- 

 phyll composition is far from completed. Physics and chemistry 

 have not yet, determined the exact chemical eflFect of light in the 

 forest. 



Wagner concludes these exhaustive studies with the modest 

 assertion that the future will see the study of solar energy play 

 as important a role in forest management as it already does in 

 medicine and in general technology. 



"Die Sanncncnergie iui Walde." Allgenieine Forst- und Jagdzeitung, 

 June, July, September, October, 1913. Pp. 185-200, 225-242, 297-319, 333- 

 3SI. 



