43 



CITY TREES 



IRVING C. ROOT, CHARLES C. ROBINSON 



We rate a tree in a forest as poten- 

 tial lumber according to the texture 

 of its wood and clean bole, its rapidity 

 of growth, market value, and avail- 

 ability. City trees have price tags on 

 them, of course (perhaps as much as 

 $20 per inch of trunk diameter) , but 

 we appraise them largely on intangible 

 values of shade and beauty. We judge 

 the city tree by the shape of its canopy, 

 its habit of growing tall and slender or 

 small and spreading, its spring bloom 

 or fall color, the shape and size of its 

 leaves, and its evergreen or deciduous 

 nature. Important, too, is whether it 

 has any tendency to break in storms, 

 whether it is a clean tree or a dirty 

 one, its susceptibility to insect pests 

 and disease, and its ability to adapt it- 

 self to the artificial conditions of cities. 



No single item distinguishes a city 

 more than its green areas, and prob- 

 ably our first reaction to a community 

 is to its abundance or lack of trees. 

 The shade they give from the sun 

 makes the summer heat more tolerable, 

 and filters for grateful eyes the dazzling 

 reflection from masonry and concrete. 

 A city of monumental buildings, like 

 Washington, particularly needs trees 

 to interrupt the reflected light. The 

 landscape architect uses trees to soften 

 hard building lines and accentuate 

 vertical or horzontal details. 



The home owner plants trees to give 

 scale and proportion to desired features 

 and to delight his family and neighbors 

 with spring bloom and fragrance, green 

 coolness in summer, color in autumn, 

 and interesting branch-and-twig pat- 

 terns in winter. 



Trees form vistas, frame views, and 

 define park areas. They can screen out 

 undesirable sights, and separate active 

 from passive recreation. They border 

 our city lakes and streams and cast 

 their reflections in our pools. Groups 

 of trees are a back drop, a cyclorama. 



For all their beauty, city trees are no 



weaklings. It is sometimes surprising 

 how they can survive the artificial and 

 adverse conditions under which they 

 grow. Smoke and gases, physical in- 

 juries, the disrupted water table, hard- 

 packed soil, lack of humus renewal 

 and mulch, inadequate root space, re- 

 flected heat from pavements and build- 

 ings, and glacial blasts of air through 

 the wind tunnel formed by street and 

 buildings, all make their lot hard. 



Because trees manufacture their 

 food by the action of sunlight on ele- 

 ments in the leaf, any substantial ac- 

 cumulation of soot or residual oil from 

 the air will screen out sunlight and 

 retard this process of photosynthesis, 

 resulting in a weakening of the tree 

 from starvation. Trees such as ailan- 

 thus, korsechestnut, hackberry, Ameri- 

 can ash, ginkgo, poplars, sycamores, 

 willows, lindens, and elms are all toler- 

 ant of soot and smoke. Others, like the 

 sugar maple, sourgum, sweetgum, and 

 honeylocust, can grow well only if the 

 air is unpolluted. 



No one seems prepared to say why 

 one tree and not another can stand 

 smoke and soot. It may be related to 

 the effect of smoke and soot-impreg- 

 nated soil on the mycorrhiza and their 

 relation to root growth and feeding. 

 Why some trees are more affected than 

 others might be a fertile field for re- 

 search. Perhaps investigation would 

 show us how to grow sugar maples in 

 smoky, soot-laden air where now they 

 cannot survive. Perhaps some simple 

 treatment of the soil or tree may some 

 day make this possible. 



Another factor that seems to affect 

 the ability of a tree to withstand smoke 

 and gases is the nature of its leaf sur- 

 face. A rough, heavy, or sticky leaf 

 will accumulate more soot and residual 

 oil than a smooth or waxy one, and the 

 latter is more easily cleaned off by rain- 

 fall and wind. 



City trees are subject to physical in- 



