EFFECT OF TREES ON SURROUNDING PLANTS. 28 I 



If the washings from the trees be collected before the/ 

 come In contact with the soil, they will often be found more 

 or less dark in color. This discoloration is mainly due to 

 soluble material which the rain water has removed from the 

 bark of the tree and would contain tannin, phenols, terpenes, 

 etc. 



The toxicity of water which has been in contact with the 

 bark and leaves of trees was shown by Livingston in a recent 

 bulletin of the Bureau of Soils.* 



It was found that water which had been sprayed upon 

 the bark of oak, chestnut, and pine trees and then carefully 

 collected was very harmful to plants. In the light of this fact 

 It is easy to see how the soil about the base of a tree may be- 

 come charged with substances deleterious to plant growth. 

 The writer has frequently observed that on sloping lawns the 

 grass was only lacking on the soil below the tree over which 

 the run-off from the tree flowed during rains. It seems to 

 make little difterence whether or not this area lies within 

 the region shaded most densely by the tree; well lighted areas 

 are often entirely destitute of grass. 



The roots of the trees may also exert a harmful effect 

 upon surrounding vegetation by substances excreted during 

 the process of growth. Jensen** has shown by a series of 

 experiments in pots that even small trees may exert quite 

 a harmful effect upon wheat growth in the pots during the 

 seasons of active tree growth, but that the harmful effects 

 of the tree roots largely vanished when the trees entered 

 upon their dormant condition in autumn. 



Summing up the foregoing remarks, it may be said 

 that the effect of the tree on the common conditions of plant 

 growth, such as water, plant nutrients and light, is not suf- 

 ficient wholly to account for the harmful effects often noted 

 upon surrounding vegetation. The more malignant effects 



"Bulletin 3(5, Bureau of Soils, U. S. Department of Agriculture, 1907. 



**Science 25: 871, 1907. See also Bulletin 40, Bureau of Soils, U. S. De- 

 partnu^nl of Agriculture; 1907. 



