Mat 31, 1907] 



SCIENCE. 



871 



SPECIAL A.RTI0LE3 



SOME MUTUAL EFFECTS OF TREE-ROOTS AND 



GRASSES ON SOILS' 



It is coTnTnonly noticed that plants of the 

 Graminese do not grow readily under certain 

 species of trees, and while many reasons hava 

 been assigned, there does not seem to have 

 been much systematic experimentation to de- 

 termine the principal causes. 



Shade thrown by the trees themselves has 

 been commonly mentioned as an important 

 factor. Considerable shade in itself will re- 

 tard the growth of many grasses, but it is 

 hardly probable this is important in the case 

 of single trees; moreover, the shaded area 

 varies with the time of day, and the grass re- 

 ceives light much of the time. Also, if shade 

 is an important factor, why does not the grass 

 live on the sunny side of the tree? Near the 

 base of the trunk of low-branched trees this 

 might become a controlling factor. 



Another reason commonly assigned is the 

 removal of 'plant food' by the tree, thus 

 starving the grass. As the parts of the tree 

 roots most active in removing soluble salts 

 from the soil water are the newer root tips 

 and branches, it hardly seems that these could 

 be held responsible for the almost entire re- 

 moval of plant food over the entire affected 

 grass space. They would, at any rate, not 

 usually be active for some distance from the 

 tree trunk, and though roots may here be near 

 the surface or even exposed, they play no 

 active part in the removal of mineral consti- 

 tuents from the soil. As the soil solution is 

 practically a constant as regards the amount 

 of salts in solution, it would seem that were 

 the removal of plant food by the tree very 

 excessive, there would still be sufficient avail- 

 able for the grass owing to the nature of the 

 solubility of soil minerals. As, however, it 

 would be impossible to say in just what 

 quantity and in what combination the plant 

 food should be present for the best develop- 

 ment of tree and grass, respectively, this factor 

 of plant food removal is difficult definitely to 

 rule out. 



Another reason assigned, and perhaps the 



'Published by permission of the Secretary of 

 Agriculture. 



more logical, is the removal of water from the 

 soil by the tree. The average sized tree dur- 

 ing active growth transpires an enormous 

 amount of water, especially if the season be 

 hot and dry; but so does the grass, and it 

 would be about as logical to blame the grass 

 for removing so much water from the soil as 

 to cut short the available supply for the tree. 

 Here again, the root system for some distance 

 from the tree does not play an important role 

 in absorption of water. These possibilities of 

 plant food and water removal would seem to 

 be negatived by the experiments at Woburn 

 to be mentioned later. 



While all these factors working in con- 

 junction may produce an effect on the growth 

 of grass, there seems to be a much deeper 

 underlying principle involved. 



During some experiments carried out on a 

 lawn in Takoma Park, Md., where a few 

 scattered oak, chestnut and pine trees are 

 growing, it was found almost impossible to 

 obtain a stand of grass or clover. Beds were 

 spaded up, stable manure applied and late» 

 artificial fertilizers added and the best of care 

 given the plots. The grass and clover (the 

 latter also inoculated with nitrogen bacteria) 

 came up very well and for a time gave promise 

 of a good stand, but in a month or two all 

 died in spite of good care. When the plots 

 were originally spaded up, many tree roots 

 were encountered which were removed; the 

 soil being shallow, these naturally live near 

 the surface. The plots on which the grass had 

 died were later spaded up again and found 

 to be almost entirely filled up vnth young ac- 

 tively growing roots, the special preparation 

 of the soil having been very favorable ioi 

 their growth. 



As the lawn is everywhere permeated with 

 roots (though the trees are not close enough 

 to form a dense shade) it was thought that 

 these might have some malignant influence on 

 the grass. It has been shown that washings 

 from tree trunks and tree leaves are injurious' 

 to plant growth, which might account for 

 some of the trouble experienced in trying to 

 obtain a stand of grass, but as the trees do 



' Bull. 28, U. S. Department of Agriculture, 

 Bureau of Soils. 



