126 THE ECOLOGICAL RELATIONS OF ROOTS. 



In the Rocky Mountains, adjoining the plains at Colorado Springs, 

 the root systems of plants were studied in the gravel-slide, half-gravel- 

 slide, and forest communities respectively. These communities form 

 a developmental series. All plants of the gravel-slide conmiunity are 

 similar in possessing root systems adapted to secure moisture and 

 nutrients from the surface foot, and few extend beyond a depth of 18 

 to 24 inches. They have a remarkably wide lateral spread, combined 

 with a most profuse system of branching. 



The surface soil consists of coarse, angular rock fragments, which 

 are from over an inch to only a few milhmeters in diB,meter. Below 

 this he a few inches of coarse gravel and sand which gradually give 

 way, at a depth of 1 to 2.5 feet, to poorly disintegrated granite. The 

 precipitation falls largely as frequent summer showers which seldom 

 wet the soil below 18 to 24 inches. The surface gravel serves efficiently, 

 both in preventing run-off and in retarding evaporation. Moisture 

 determinations show that the soil moisture, while at no time high, is 

 rather equally distributed throughout the surface 18 inches of soil. 

 Plants are so sparsely spaced that relatively little water is removed by 

 transpiration. Wind movement averaged 103 miles daily. The aver- 

 age daily evaporation throughout the sunamer was 34 c.c. The com- 

 munity root habit is clearly a response to the environment. The 

 development of strong laterals, characteristically up the slope, seems 

 to be due in part to a response to the soil shpping down the mountains. 

 Root competition on the gravel-shde is not at all severe. 



An examination of roots in the half-gravel-slide revealed quite a dif- 

 ferent type. In addition to the shallow, widely spreading root habit, 

 most species had supplementary deep roots which penetrated from 

 2 to 4 feet. The soil of the former gravel-slide has now become deeper 

 and richer in humus and much more densely populated. More than 

 half of the surface is occupied by plants, dominant among which are 

 certain grasses, while all of the soil to a depth of 3 feet is occupied by 

 roots. The surface gravel has become more disintegrated and com- 

 pacted and is less efficient in preventing evaporation. Because of 

 competition for light, and especially for water, most gravel-slide 

 plants have disappeared. Although wind movement is somewhat less 

 and evaporation considerably lower than in the former barer habitat, 

 the water-loss by transpiration from the denser plant jwpulation is 

 higher. During periods of drought plants must rely largely upon 

 the water in the deeper soil. More favorable deep-soil conditions, 

 together with intense shallow-root competition, are factors largely 

 determining root depth. 



Through a transition chaparral stage, the half-gravel-slide gives 

 way to forest. Pinus ponderosa, Pseudotsuga mucronata, or Picea engel- 

 manni is the dominant, depending upon the amount of humus and 

 moisture. The 19 herbs and shrubs examined on the forest floor were 



