ENVIRONMENT 109 



soil of this sort is full of clay which is formed by decomposition of the 

 rock. It is therefore water-tight compared to limestone soils. Hence, 

 moisture caught in the former stays in place and produces a uniform 

 tree-growth, while moisture entering the limestone soils readily passes 

 away from the roots. The growth over limestone has larger percent- 

 age variations with better climatic relationship. This confirms the 

 reference to this topic in Volume I, page 22. 



Soil-moisture gradient — It is possible that a criterion of this 

 difference could be found by the vertical soil-moisture gradient. 

 Certain species of pine can grow in very wet land. In such cases the 

 soil is wet at the surface, then soaked, and then full of water as one 

 goes down a few feet. Tree sections occasionally appear which show 

 an enormous increase in growth on draining such land. At an eastern 

 point (Cape Cod, Massachusetts) the surface soil near the pine trees 

 is sandy and below that are moist glacier gravels, down to water at 

 20 feet. In contrast with this, the trees around Flagstaff grow mostly 

 on a thin layer of soil, perhaps 2 to 10 feet, upon impervious, igneous 

 rocks, or upon porous and cracked limestone. Over the igneous rock 

 is often a layer of clay. During a large part of the year one may dig 

 about the tree, or near the tree, and find the ground apparently dry. 

 Clays and volcanic rocks hold layers of moisture for a considerable 

 time, but the soil over the limestone, as observed in some cases, gets 

 drier and drier as one goes down. The average soil-moisture gradient, 

 therefore, seems promising as a help in determining certain controlling 

 factors in tree-growth. 



Root conditions — Mr. G. A. Pearson, director of the Southwestern 

 Experiment Station, has very kindly supplied data regarding depth 

 of the root systems under certain trees near Flagstaff and per cent of 

 available soil-moisture, as follows: The greatest depth attained by 

 tree roots is usually around 4 feet, but only a few of them reach this 

 depth; the great masses of roots are found in the upper 2 feet. In the 

 case of spruce, very few roots are found below 1 foot in depth. These 

 measures cover the woodland (cedar), yellow pine, Douglas fir, and 

 Engelmann spruce. In his bulletin entitled "Natural Reproduction 

 of Western Yellow Pine," a series of graphs shows the available soil- 

 moisture in per cent of dry weight of soil, for the summer months, 

 including May to September. At 6 inches in depth the amount for 

 cedar and yellow pine varies from 1 to 9 per cent, and for the other 

 trees about twice as much. At 12 and 24 inches of depth the amount 

 for pines and cedars is between 5 and per cent, and for the other 

 trees about twice as much. The precipitation curves during the same 

 seasons, 1918 and 1919, show that rainfall in the preceding months is 

 felt by these trees at 6 inches, and by the high-level trees, fir and spruce, 

 at 12 inches of depth, for at such levels the rainfall is greater, but at 2 

 feet only the Douglas fir shows it. 



