2 BULLETIN 1233, TJ. S. DEPARTMENT OF AGRICULTURE. 



points of view and make clear what conditions of environment 

 respond directly to physiological requirements and thus most vitally 

 affect the composition of the forest, and in what terms these condi- 

 tions must be measured and expressed, it is felt that an advance will 

 have been made. But it is believed that a further progress has been 

 made. In these two approaches to the subject the understanding 

 of the relative qualities of the forest trees considered may become 

 clarified for the reader, and thus assist foresters in reaching a proper 

 understanding of the technical problems which are met almost 

 daily. This belief is held notwithstanding the fact that the methods 

 in ecological study of the environment are in some respeets very 

 crude and in these respects still afford opportunity for much inten- 

 sive and fruitful work. 



A brief review of the physiological work and of the conclusions 

 which have already been presented is here given. 



The historic concept of a difference between tree species as to "shade toler- 

 ance" or photosynthetic capacity has been borne out by observations on the 

 growth rate during growing seasons and the sap densities at the end of these 

 seasons, in which the several species were subjected to identical conditions. 

 The six species considered seem to be photosynthetically effective in the follow- 

 ing decreasing order: Engelmann spruce, Douglas fir, lodgepole pine, bristlecone 

 pine, yellow pine, and limber pine. 



As a direct result of greater photosynthetic activity, the water use per unit of 

 growth is reduced; or, in other words, in relative "water requirements" the most 

 "tolerant" or effective species take the lowest positions. 



This .also affects the absolute requirements, as expressed by transpiration per 

 unit of leaf area or exposure, indirectly, through the relative evaporating rates 

 of solutions of different densities, so that in a general way the most active species 

 are absolutely the smallest users of water. It also appears in this connection 

 that certain species may be much better adapted than others to resist transpira- 

 tion on account of the thickening of the epklermis and the reduction of the 

 stomata. This places them in a lower category with respect to the absolute 

 transpiration rate, but does not appreciably affect their water requirements, 

 because such economics can be effected only through means which exclude 

 carbon dioxide or light from the functioning cells. Thus, Engelmann spruce, 

 Douglas fir, and yellow pine appear in increasing order as absolute water users; 

 but limber pine, bristlecone pine, and, to a lesser extent, lodgepole pine, take 

 lower positions than would be expected, all being "weedy" trees which sacrifice 

 growth rate for protection against great water losses. 



In view of the behavior of weaker and denser solutions, it is conceived that 

 the species which are most effective photosynthetically may be satisfied with 

 the coolest environments, because, with their relatively dense cell saps a smaller 

 proporation of the energy of the absorbed sunlight is used in the process of evapo- 

 ration. In other words, a dense solution is not so rapidly cooled by its own 

 evaporation as a lighter solution. Hence the possessor of the dense solution 

 may more readily maintain leaf temperatures which will be conducive to effective 

 photosynthesis. That the temperature of the leaf is important in this process 

 can hardly be questioned. Thus the relative light requirements and heat require- 

 ments of the several species are seen to be identical. On the basis of light re- 

 quirements the species would be zonated in the order already given, with spruce 

 occupying the situations of lowest air temperatures. 



It is also conceived that, as the most effective species is least readily cooled 

 by the evaporation of its cell sap, it may not only require less heat for effective 

 growth, but may most readily become superheated to the point at which direct 

 protoplasmic injury results. For this reason no species may become established 

 in a temperature /one or in an exposed site whose temperatures exceed a certain 

 maximum. It must, however, be admitted that, the direct evidence on this 

 point is extremely meager. It. is also apparent that the maximum tempera- 

 tures encountered in nature, namely, those at the surface of insolated soils, 

 are so intimately connected with the dryness of that layer and with the w.i ( or 

 supply and f r;i nspirational losses of young seedlings that evidence of direct 

 heat injury, apart from injury through water loss, may be very difficult to obtain. 



