EFFECTS OF PRUNING THE ROOTS OF GAS‐INJURED TREES

Abstract

TRE.ES IN the vicinity of illuminating gas leaks in the soil may die rapidly, as indicated by a sudden wilting and drying of all the foliage, or they may succumb more or less gradually over one or more years. This latter type of injury may first involve uppermost twigs, the branches on one side, or a large branch or two, while -the remainder of the tree appears to be unaffected. Such unequal injury to the top of a tree frequently results in a tree of little value. In other cases, the damage may be minimized by judicious pruning. When one side or a few branches are killed, questions arise as to the manner in which this localization of injury occurs and whether a practice such as root pruning would be a benefit. Stone in the bulletin by Start, Stone, and Fernald (1908) stated that if symptoms of gas poisoning were discovered in only one root, and the poisoning has not extended to the tree trunk, amputation of the root is the best remedy. He also claimed that the cutting of tree roots under a roadbed when regrading was done or curbing was placed saved many trees from iiijury by gas. Fernow (1910) also recommended root pruning for trees detected in the early stages of gas poisoning. To secure evidence of the effect of root pruning as a means of aiding gas-injured trees, small oak trees were subjected to illuminating gas in water until definite symptoms of injury were produced. The most affected distal portions of the tap and fibrous roots were then pruned off, the trees were repotted in soil, and the appearance of the tops and roots was observed during the following two years. EXPERIMENTAL. -A fairly uniform group of thirty, twoand three-year-old red and black oak trees that had been grown in pots was divided into three lots of ten trees each. The roots were washed free of soil and placed in gassing chambers consisting of clean, one-gallon cans, such as are used for varnish. Holes were cut in one side to admit the roots of ten trees. The foliaged tops were sealed off from the roots by plastic clay molded around the stems. The roots stood in' 3 cm. of water into which measured volumes of illuminating gas were passed. Coke oven gas of the New Haven Gas Light Company was used. A complete analysis of this gas was given in a previous paper (Deuber, 1936). The content of illhminants was 3.00 per cent, carbon monoxide 8.70 per cent, and hydrocyanic acid was absent, according to the factory analyses. One cubic foot of this gas was passed into tlle water of the gassing chamber containing the first lot of trees; 17.2 cubic feet of gas during four hours into the water of the second lot; and 24.5 cubic feet in seven hours to the third lot together with a flow of 0.58 cubic feet of gas per hour for the next four