Some Urban Trees of California: Maintenance Problems and Genetic Improvement Possibilities

--Tree crew supervisors in 48 California cities were asked what problems they encounter with the five most common trees along their streets. The top five trees of the 20 most often named were sweetgum, Modesto ash, southern magnolia, planetree, and Chinese or lacebark elm. Major problems reported included root damage to sidewalks and curbs, growth of suckers from the root crown, exceptionally fast growth which necessitates frequent pruning, and litter of leaves and fruit. Genetic improvement possibilities to minimize these and other identified problems are discussed. A LOGICAL STARTING POINT in the genetic improvement of tree species for urban areas is to identify traits that create problems and possible remedies. City tree workers who routinely work on trees are quite perceptive about the problems of various trees and can help pinpoint the direction for needed genetic improvement efforts. Therefore I did a study to determine maintenance problems and other traits of different urban trees based on the experiences and suggestions of tree crew supervisors. To obtain this information, I visited at least one tree crew supervisor in 48 California cities during 1981 and 1982. Most of these supervisors led a tree crew and routinely worked with the crew on the city's trees. Some supervisors previously had been tree crew leaders and now supervised several crews. All supervisors had been in their present position for at least one year and generally had had several years' previous experience as tree crew members. Most of them had taken college courses in horticulture and other biological sciences. The meeting with each supervisor lasted for about two hours. Time did not permit covering all species of trees which a city maintains and consequently, our discussion was limited to the five most common species found on the city's streets. For each species, we discussed (1) pruning requirements, (2) problems encountered or unexpected maintenance, (3) unique assets, (4) biological improvements needed, and (5) whether the species was still being planted, and, if not, whether the city was keeping or removing existing trees of that species. This paper summarizes problems and genetic improvement possibilities of the 20 most prevalent tree species, including clones, along streets of the 48 cities (Table 1). Table 1. Twenty kinds of trees most frequently named as being among the five most prevalent along streets in 48 California cities. Common name Scientific name No. of Cities Sweetgum or liquidambar Modesto ash . . . . Southern magnolia . . Planetree . . . . . Chinese or lacebark elm Camphortree . . . . Carrotwood or cupania . Carob . . . . . . Crapemyrtle . . . . White mulberry, fruitless clones . . Chinese pistache. . . Holly oak . . . . . Glossy privet. . . . Jacaranda . . . . . Brazilian peppertree . Siberian elm . . . . Indian laurel fig . . Shame1 ash. . . . Honeylocust, thornless and fruitless clones. Flowering plum, cherry plum, or myrobalan plum, purpleleaf clones. . . . . . .