150 BULLETIN 1233, U. S. DEPARTMENT OF AGRICULTURE. 



system, may be able to drain the moisture of the soil more completely, 

 because from the outset its roots are more finely divided, and they 

 more completely reach all of the moisture. The tree accustomed to 

 a dry soil insures its moisture supply by reaching into an extensive 

 area. 



There has been shown to be, however, a very great difference be- 

 tween the species in photosynthetic efficiency, in ability to utilize 

 whatever of light may be available, in sensitivity to high tempera- 

 tures and drying which may result from direct insolation, and, finally, 

 in the efficiency with which water is used. The relations of the sev- 

 eral species to light are important silviculturally. The fact that one 

 species may utilize 10 per cent of the energy of incident sunlight in the 

 production of carbohydrates, and 90 per cent in the wasteful evapora- 

 tion of water, while another utilizes only 5 per cent of the energy of 

 incident sunlight, is of the vary greatest importance economically. 

 The species which nature has developed to serve as the climax of any 

 forest succession is the most highly developed plant organism of that 

 association, and in all probability is the species which may most fully 

 serve human needs, at least in the sense of quantity production. For 

 a great many regions, as well as for the one here discussed, this species 

 will probably prove to be spruce. 



The most obvious application of the results of this study is that in 

 the reforestation of denuded areas or the afforestation of such areas 

 as our sand-dunes, it is possible to advance nature's process by at 

 least one successional stage. For example, a denuded, strongly inso- 

 lated site, which is known to have borne m the past a stand of Douglas 

 fir, but which, from direct or indirect knowledge it is known will first 

 be stocked with pine, is under no necessity whatever to go through 

 the pine stage. The area may be immediately stocked with fir, with 

 very great assurance of success, because the 3-year-old trees which 

 may be planted are not susceptible to the heat and drought of the site 

 which would be fatal to young seedlings. To a certain extent, on the 

 same basis, a given species may be planted on a somewhat lower, 

 warmer, and drier site than would naturally, at the same stage, repro- 

 duce that species. 



Not only are these things possible in reforestation, but they are 

 obligatory upon foresters, in order that the land may be put to the 

 highest use, and the moisture, almost always limited in quantity, may 

 be utilized by the most efficient of the organisms capable of existing 

 under the given conditions. Exception must of course be made 

 where a certain species is most desirable for its technical value. 



In the management of forests for continuous production, which 

 always implies the securing of natural reproduction, the realization 

 that the nature of the reproduction secured will depend almost en- 

 tirely on the conditions of insolation must be very helpful in deter- 

 mining the weight and character of the cut ting. It should be kept in 

 mind that, while the cutting of undesirable species may reduce the 

 number of seedlings of that species which will appear, it is no guaran- 

 tee whatever as to the identity of the seedlings that shall survive. 

 That will be determined by the light and heat conditions which result 

 from the cutting in the aggregate. In general, all cutting in climax 

 forests must tend to encourage 1 the species of the successional order 

 next below the climax, and in clear cutting the result may be a rever- 

 sion of two or three stages. The maintenance of the climax forest may 



