24 MANUAL OF TREE DISEASES 



Trees vary greatly in their susceptibility to the poisonous 

 substances in smoke. Conifers are more susceptible than decid- 

 uous trees. The following order, from most susceptible to less 

 susceptible, is reported by one writer: Alpine fir, Douglas 

 fir, lodge-pole pine, western yellow pine, limber pine. Rocky 

 Mountain juniper and dwarf juniper. In like manner the same 

 writer reports the deciduous trees in order of least to greater 

 resistance, thus : white elm, sycamore, locust, yellow poplar, 

 Cottonwood, black gum, dogwood, red maple and white oak. 

 In Des Moines, Iowa, the willows and cottonwoods were found 

 to be the most resistant trees. Also in that city the following 

 deciduous trees were more resistant than pines : locust, white 

 elm, ash, sycamore, silver maple, bur oak, white oak, red oak 

 and. box-elder. 



Symptoms. 



The symptoms of smoke- and gas-injury are variable ac- 

 cording to whether the injiu-y is acute or chronic and accord- 

 ing to the species and age of the tree. When smoke is dense, 

 more damage may apparently come from the coating of soot 

 formed on the leaves than from the gases accompanying the 

 smoke. In such cases the foliage is covered with the tar-like 

 coating of soot and the leaves appear sickly and dwarfed. 

 The tree as a whole is usually scrawny and makes but little 

 growth each year. When the injury is more acute, and 

 apparently due to the poisonous gases emitted along with the 

 smoke, the leaves ttirn brown in spots and die. In the case 

 of coniferous trees, the needles turn brown from the tip. 

 Deciduous leaves usually appear as if sun-scorched, the portion 

 of the leaf between the main veins turning brown first. In 

 general, trees exposed to smoke have much smaller leaves than 

 normal trees and often the leaves are crumpled. The killing 

 of part of the foliage and the dwarfing and twisting of other 

 leaves cause a greatly reduced leaf-surface for the manufacture 



