74 MISC. PUBLICATION 257, U. S. DEPT. OF AGRICULTURE 



respect to (3) than to either (2) or (1), the latter being the most unsat- 

 isfactory of the methods tried. 



Other workers have devised special apparatus for creating an 

 artificial drought in the laboratory in order to test the drought 

 resistance of potted plants (tree seedlings). Thus Shirley (191) 

 developed a desiccating chamber with a constant (or nearly so) satura- 

 tion deficit, in which plants are left until dead. The length of time 

 the plants survive and the soil-moisture content at death are used as 

 criteria of the drought resistance. With this apparatus, Shirley 7 

 determined that of four species of seedling conifers tested, jack pine 

 was the most drought resistant, followed by Norway pine, white pine, 

 and white spruce. A more elaborate apparatus, in which heated air 

 is forced over the plants, has been described by Aamodt (1). 



SUMMARY AND SUGGESTIONS FOR RESEARCH 



Trees growing under dry conditions seem to be able to adapt 

 themselves thereto (within certain narrow limits) either by checking 

 their transpiration water losses or by providing for an adequate water 

 supply. The transpiration losses are checked largely through changes 

 in the stomata, which serve (inadequately in most cases) to control 

 the transpiration , although if the water supply remains insufficient for 

 an extended period no amount of stomatal control can keep the plant 

 from ultimately dying. 



Under unfavorable conditions, however, trees may be able to survive 

 if they have a root system sufficiently extensive to reach moist soil, 

 if they have some water stored within them, or if they possess any 

 other means of conserving or adding to their water supply. Herbace- 

 ous forms and xerophytes have many such methods, such as the 

 production of mucilages (pentosans) and other water-holding colloids, 

 but the (mesophytic) trees of the Temperate Zone (i. e., the forms here 

 under consideration) seem to have few such additional means to insure 

 their survival. 



In view of the importance of planting drought-resistant species in 

 regions where the water supply is below the optimum for most tree 

 species, it is extremely urgent to know more about what qualities make 

 for drought resistance and what species possess these qualities to the 

 greatest degree. Also there is evidence that relative drought resistance 

 varies with age, soil composition, etc., so that an exhaustive study now 

 needs to be made of the relative drought resistance of our various 

 economic species and of the factors with which this drought resistance 

 may vary. There is thus evidence that seedlings have very special 

 problems and that the difficulties involved in starting plantations in 

 regions of suboptimum moisture are very different from those involved 

 in growing or maintaining middle-aged and old stands in these same 

 regions. In such regions the interrelations between site and species 

 become especially important. Not only may the tree change its 

 drought resistance properties as it grows older, but also as the root 

 system enlarges the available water supply may increase. The de- 

 velopment of our knowledge of this interrelation between the growing 

 tree and its site consequently opens up a very large field for future 

 research, where much work remains to be done. 



i Unpublished manuscript 



