66 BULLETIN 1105, U. S. DEPARTMENT OF AGRICULTURE. 



The results which are given in Table 18 corroborate in a rough 

 way those in Tables 16 and 17. They indicate that the influence 

 of tree roots upon soil moisture may extend at least 80 to 90 feet 

 from the trees. In other words, if a group of trees is cut, we may 

 expect an increase in soil moisture in an area extending from 80 

 to 90 feet or possibly more from the stumps, provided that the 

 roots of living trees are not encountered. If the stand is clean-cut, 

 we may normally expect an appreciable increase in soil moisture 

 over the entire area, save for openings which before the cutting were 

 so large as to be beyond the influence of tree roots. If only a part 

 of the stand is removed, the effect is of course less, though not 

 necessarily in direct proportion to the amount of timber left. It is 

 presumed that if areas from which trees are removed remain un- 

 occupied by trees or by herbaceous vegetation, roots from neighbor- 

 ing trees will reach into these vacant spaces and eventually take 

 possession of the soil. 



EFFECT OF CUTTING MODIFIED BY HERBACEOUS VEGETATION. 



In the preceding soil-moisture tests, complications due to herbace- 

 ous vegetation were to a considerable extent avoided by selecting 

 sites which were closely grazed. That a luxuriant growth of grass 

 and other herbs extracts large amounts of moisture from the soil 

 is obvious. It also checks evaporation from the surface layers, but 

 the amount conserved in this way is far less than the amount tran- 

 spired through the foliage. It is conceivable, therefore, that the 

 entrance of herbaceous vegetation after cutting may offset the sav- 

 ing of soil moisture brought about by removing the trees. This 

 actually happens in a great many instances, and there is always a 

 tendency in this direction. Cutting stimulates the growth of the 

 herbaceous cover which has been. held in check by the shade and 

 root competition of the trees. The total amount of vegetation of 

 all kinds which can exist on a given site in this region is deter- 

 mined by the amount of moisture available at critical times. When- 

 ever a surplus of soil moisture is created by removing a considerable 

 proportion of the trees, as in logging, the remaining plants, both 

 trees and herbs, respond by increased growth, and those which re- 

 produce most readily increase in numbers until the limit of moisture 

 supply is again reached. 



Changes in ground cover are most active on the areas released 

 from the domination of the trees which are cut, and particularly the 

 spots on which the herbaceous vegetation has been largely suppressed 

 by the trees. The conditions prevailing in these situations or stump 

 patches have been described under " Herbaceous vegetation." Im- 

 mediately after cutting the stump patches show a high excess of 

 soil moisture as compared with areas of established vegetation. But 



