42 BASES AND CRITERIA. 



Measurement of habitats The importance of correlating indicator plant 

 or community with the controlling factors of the habitat has already been 

 emphasized. While the standard method of doing this has been by means 

 of physical instnmients, a number of attempts have been made to utilize 

 plants themselves for this purpose. While the work of Bonnier (1890 : 514), 

 in which he made reciprocal plantings of alpine and lowland plants, was essen- 

 tially of this nature, he seems to have had no thought of using plants as instru- 

 ments. The first conscious endeavor to do this was perhaps in 1906, when 

 potometers of several different species were used with recording instruments 

 to determine the effect of pressure on transpiration at different altitudes on 

 Pike's Peak (Clements, 1907 : 287; 1916 : 439). Sampson and Allen (1909: 

 45) employed sun and shade forms in different habitats at the Alpine Labora- 

 tory to determine transpiration in various light intensities, while standard- 

 ized plants of Helianthus annuus were utilized in habitat measurements con- 

 ducted by the Botanical Survey in Minnesota in 1909. During 1912-1913, 

 Pearson (1914 : 249) grew seedlings of Pseudotsuga beneath aspen and in open- 

 ings to determine the better habitat for planting operations, and the method 

 has since had a limited application by foresters. The most comprehensive 

 use of the planting method has been made by Hole and Singh (1916 : 48; cf. 

 Chapter III), who established experimental quadrats in the sal forests of 

 India to measure the role of shade and aeration in reproduction. 



McLean (1917 : 129; cf. Livingston and McLean, 1916) employed soy beans 

 to measure general climatic conditions by means of growth at two stations in 

 Maryland. The three main criteria used in determining growth were leaf 

 area, stem height, and dry weight of tops, all of which showed the Easton region 

 to be nearly 2.5 times as efficient as the Oakland one. A definite correlation 

 was established for temperature, but not for water, owing to auto-irrigation 

 of the plants. Weaver and Thiel (1917 : 46) measured the transpiration rela- 

 tion by means of bur-oak seedlings in three habitats, prairie, hazel-scrub, and 

 oak forest, near Minneapolis. Similar measurements were made with maple 

 and elm seedlings in scrub and prairie at Lincoln. Further experiments were 

 made with sun and shade forms of the same species, and with sun and shade 

 branches of the same plant. The species employed were Acer saccharinum, 

 UlmtiS americana, Fraxinus lanceolata, Rosa arkansana, Prunus serotina, and 

 Acer negundo. The general results showed a transpiration 2 to 3 times greater 

 in prairie than in scrub and 6 to 10 times greater than in the Typha swamp. 

 Evap>oration was regularly greater than transpiration, and no constant rela- 

 tion was found between the two, as would be expected. Sampson (1919 :4) 

 has recently made a comprehensive use of Pisum arvense, Triticum durum, 

 and Bromv^ marginatum as standard plants in measuring the differences of 

 the climax zones of the Wasatch Mountains in central Utah (cf. Chapter VII). 



The use of plants to measure light intensities has as yet received almost no 

 attention in spite of its great promise. This correlation has been made from 

 the standpoint of adaptation by E. S. Clements (1908 : 83); when combined 

 with growth and gross form, as in later studies, this method is simple 

 and of great value. Even more significant is the use of standard plants for 

 measuring light intensity and quahty by means of the photosynthate produced 

 in imit areas. Preliminary work of this nature has been carried out by Clem- 



