Abstract
An inventory of the most common street tree species in 21 southern California cities was created. The inventory included information on tree species and diameter at breast height. The inventory indicates differences in existing species as well as differences in planting trends between coastal and inland regions. Links between the data and the potential for determining the age of the urban forest are discussed.
Street trees have long been an integral part of city and town life in the United States (1). The task of planning and maintaining these trees has been entrusted to a variety of professional municipal tree managers. These managers need to be aware of the status of the trees they care for—which trees live, which don’t, when the last pruning cycle occurred, and so on. Tree inventories are often performed to clarify these issues.
Recently, the desire for an accurate picture of municipal urban forests has become more apparent. In 1992, Bernhardt and Swiecki (2) reported that at least 275 of California’s municipalities had tree inventories, the majority of which were computerized. Outside consultants, often commissioned for the initial inventory, use a variety of database programs; one such program is Tree Keeper® (a tree management software package published by the Davey Resource Group, a division of the Davey Tree Expert Company). TreeKeeper records trees’ vital statistics (such as height, spread, dbh, condition, and growing space) as an aid to proper management of the urban forest. While many southern California municipalities have made use of this program, the data from several individual municipalities have not been analyzed to determine regional trends.
The purpose of this study was to determine overall street tree distribution and trends in southern California, using combined data from TreeKeeper inventories of different cities.
Methods
The results of municipal inventories from 21 southern California cities (inland, U.S.D.A. zones 8 and 9; coastal, U.S.D.A. zone 10) were combined into 1 database. Only trees within a city’s rights-of-way, usually 10 ft from the face of the curb, were inventoried. As a result, inventories include seedlings from native/naturalized species plus those planted by homeowners, in addition to municipally planted street trees.
Data were separated into either “inland cities” or “coastal cities” categories. Inland cities included data from Claremont, Corona, Monrovia, Pasadena, Rancho Cucamonga, Sunnyvale, Temple City, and Visalia. Coastal cities included data from Agoura Hills, Bell, Beverly Hills, Carpenteria, Huntington Park, Irvine, Laguna Niguel, Newport Beach, Orange, Redondo Beach, San Luis Obispo, Santa Ana, and Vista. Each inventory was performed by Davey personnel and took less than a year to complete. All were performed between 1988 and 1994.
The inventories were examined on a case-by-case basis for 2 categories of information: tree species and diameter at breast height (dbh). Data regarding street tree species were available from all 21 cities; data regarding street tree dbh, from 17 cities.
Results and Analysis
Species
Over 370,000 trees, representing 257 species spread across 123 genera, were ultimately recorded. Eucalyptus is the most widely represented genus, with 23 species present, followed by Pinus, 12 species; Quercus, 11 species; and Prunus, 10 species. In addition, 15 species of palm and 5 species of large shrubs (such as Photinia and Xylosma) were recorded as street trees. Approximately 150 more species, each with an insignificant number of specimens present in each city, were not included in the analysis.
The top tree species recorded in this study are ranked by region in Table 1. Only those species that comprise more than 1 % of the total number of recorded trees have been ranked (the complete list, which is too lengthy for this paper, is available upon request from the author). Approximately half of all trees planted are composed of only the top 15 species. In other words, fewer than 6% of the species make up 50% of the street trees in these 21 southern California communities.
Comparison of the most common existing street tree species in southern California. (Percent column indicates percentage of trees in the category. Small-statured trees are in italics; medium-statured trees are in regular typeface; large-statured trees are in bold.)
There is a marked difference in population size of certain species between cities with a strong coastal influence (characterized by warm summers and very mild winters with an average of 10 to 14 inches of rain), versus those in inland valleys and foothills (which experience occasional winter frosts, similar annual precipitation, and hot, dry summers). Orchid tree (Bauhinia), which ranked number 11 overall, is one notable example of species differences; the total inland population of this species amounts to about 1 % of the total coastal population. Coastal areas also seem to have a slightly greater species diversity than inland areas (223 species in coastal regions versus 192 species in inland regions), probably because the milder coastal climate is favorable to frost-sensitive species such as Erythrina and several tropical palms.
American sweet gum (Liquidambar styraciflua) is the most common street tree in southern California, as well as throughout the state as a whole (2). It accounts for about 1 of every 12 trees. It is in almost every community’s top 5 most common species. Communities with high degrees of new development tend to show a higher than average percentage of Liquidambar than other communities—as much as 14% of all street trees (compared to the average of 8.6%).
Table 1 also reveals that only 1 species of street tree found in the top 10, Mexican fan palm (Washingtonia robusta), is native to California. Overall, only 28 species of street tree in southern California are endemic to the western United States, and fewer than that to California.
Size
Tree dbh was divided into the following nine classes (by inches): 0-3, 3-6, 6-12,12-18, 18-24, 24-30, 30-36, 36-42, and over 42. In this study, such dbh classes will be referred to by the greatest size in the class: 3-inch, 6-inch, 12-inch, and so on. Data concerning street tree dbh were available from 17 cities, for a total of 162,560 trees.
A link between dbh and tree age within the urban forest involves the size of nursery-grown stock. Bernhardt and Swiecki (2) found that over 70% of new street trees throughout the state are planted as 15-gal container size, with an average dbh of .75 inches; an additional 10% to 15% are planted from 24-inch box size, which average 1.5 inches dbh. The balance of street trees are generally planted from 1 - or 5-gal containers, and only rarely larger than the 24-inch box size. Therefore, given the dbh size of many newly planted municipal trees, a reasonable indicator of the proportion of young and recently planted trees might be the abundance of trees in the 0 to 3 inch dbh class.
It is difficult to determine from the data exactly how palms fit into the overall scheme of new tree planting, since they do not increase in dbh as do most shade trees. Therefore, palms have not been included in the diameter analyses.
Analysis of the distribution of tree dbh reveals the following:
Many smaller trees, fewer larger trees. In this study, “small” trees attain a maximum height of 30 ft or less, “medium” trees, 30 to 60 ft, and “large” trees, 60 ft or more. Table 2, which lists the dbh of trees by region and tree stature, indicates a large percentage of small and/or young trees (12-inch dbh class and smaller) and relatively few large trees.
There are striking similarities in the dbh distributions of both medium and large trees. Even though the majority of species being planted are “medium” in stature (Table 3), as are most existing trees in this study, the data indicate that a relatively small percentage of municipal trees have reached sizes over 12 to 18 inch dbh. Small-statured trees may also show a proportionally similar pattern at lower dbh’s, but the dbh breakdown used may not be sufficient to determine whether or not this is the case.
Species diversity seems to be declining slightly. A pattern similar to that of existing species populations has emerged. A small number of species seems to make up the bulk of the recently planted population. The 10 most common recently planted species comprise about half of all trees being planted (Table 3). Furthermore, Liquidam-bar is in no danger of losing its place as “all-around most common species”; in fact, its rate of increase indicates that it may soon become even more prevalent as a municipal tree. Some other species seem to be growing in popularity, as evidenced by large numbers of trees in the smallest diameter classes. For example, white alder (Alnus rhombifolia), which ranked number 30 in overall existing populations, is currently among the 4 species most commonly planted by inland communities.
More large-statured trees are being planted in inland areas; more small-statured trees are being planted in coastal areas. Table 3 compares the 10 most common recently planted species among 17 communities in coastal and inland areas. From this table, it appears that communities in inland areas are planting species that will eventually reach much larger sizes than trees being planted in coastal areas. For instance, crape myrtle (Lagerstroemia indica) is the only species of small stature among inland communities’ most common recently planted species; 4 other species have the potential to exceed 60 ft in height. Coastal communities, however, have no species of large stature in their top 10; instead, 4 are of small stature, while the rest are of moderate size. Specific reasons for this difference are unclear from the data collected for this study.
Diameter breakdown of non-palm trees by region and tree stature. (Small trees are less than 30 ft; medium trees are 30 to 60 ft; large trees are more than 60 ft.)
Comparison of the 10 most common recently planted street trees (0 to 3 inch dbh). Percent column indicates the percentage of species in the 0- to 3-inch dbh class. (Small-statured trees are in italics; medium-statured trees are in regular typeface; large-statured trees are in bold.)
Bernhardt and Swiecki (2), both in 1988 and 1992, found that more than 50% of the street trees being planted throughout the state of California were medium-statured trees, about 35% were small-statured, and about 15% were large-statured. The present study, however, indicates that southern California’s proportions of small-, medium-, and large-statured trees vary somewhat from results obtained statewide. Out of the 17 communities’ recently planted non-palm trees (0- to 3-inch dbh class), about 44% of the trees planted are of medium stature, 33% are of large stature, and 22% are of small stature (Table 4). These proportions vary even more within specific inland or coastal regions.
Proportions of recently planted non-palm trees (0 to 3 inch dbh), by potential stature.
Discussion
Species Diversity and Local Selection
Urban forestry managers and urban landscape designers tend to look for species that pose as few maintenance and liability problems as possible. These problems include excessive water use, surface rooting, damage to sidewalks and curbs, and heavy debris production (13). However, in a contradictory note, the top 2 existing species of street tree in this study, Liquidambar styraciflua and Magnolia grandiflora, are both notorious for producing heavy amounts of debris and causing sidewalk/ curb damage through heavy surface rooting, as are at least 4 of the top 10 most commonly planted species (Table 2). Many of the most commonly planted species (such as Liriodendron tulipifera and Alnus rhombifolia) are also known for their lack of drought tolerance—a serious problem in a land for which the average annual rainfall is between 10 and 14 inches. It would seem, therefore, that to avoid future problems, many design professionals and municipal decision-makers need to become more aware of the dichotomy between what they say they want in a tree and what they actually plant.
Tree Lifespan
Some trees in the wild can live quite long. For example, the stately specimens of California’s old-growth forests, such as coast redwoods and giant sequoias, are known to live 1,000 years or more. Many common species of native California oak regularly exceed 250 years in age and have an average dbh of between 24 and 48 inches, depending on the species (11). Some oaks have even attained diameters of well over 9 ft, and probably exceed 500 years in age (3,17). Municipal trees, on the other hand, have a notoriously short lifespan—often no more than 35 or 40 years (1). And while no published studies have examined long-term municipal tree growth patterns in southern California, Nowak (10), in a study in New York, indicated that medium- to large-statured maples may add an average of 0.25 to 0.50 inches dbh for every year of mature life. Urban (14) obtained similar results for a variety of species at 13 different sites in the northeastern United States.
Based upon this information, it is likely that a tree with an 18 inch dbh may be between approximately 35 and 70 years old. Using dbh as an indicator of municipal tree age, Tables 2 and 3, along with the Nowak and Urban studies, indicate that existing medium-to large-statured municipal trees do not seem to reach their full potential in size. For example, only 1 in 10 of all inland coast live oaks (Quercus agrifolia) in this study, and only 1 in 50 in coastal areas, have even reached a 30 to 36 inch dbh (approximately 70 to 150 years old), which is the average size of oaks in their native southern California habitats (11).
Without detailed long-term recordkeeping, such as that provided by a tree inventory database, it is ultimately difficult to determine the age and survival rate of southern California’s municipal trees. Obviously, many factors influence a tree’s growth rate and survival, including extremes of watering, lack of soil oxygen (12), extremes in soil pH (15), high soil temperatures (7), lack of rooting space (6,9), and compaction due to heavy equipment (4,5,8). All of these stresses have negative long-term effects that impact a tree’s growth and ultimately shorten its lifespan. With this information in mind, it is not completely surprising that few trees in this study have reached appreciable dbh sizes.
It is also important to point out, especially in the context of this study, that southern California’s heaviest urbanization and tree planting have taken place in the last 50 to 75 years (16). Many trees have simply not had the time to grow to maturity; other species naturally have a relatively short lifespan. It is therefore difficult to accurately determine the degree to which the lack of mature trees in southern California is due to early tree mortality, or simply to an overwhelming abundance of “recently planted” trees. Finally, southern California’s mild climate provides a much longer growing season than do other areas of the United States. Many trees in southern California may grow faster than indicated by the Nowak and Urban studies. Further research on this topic is in order.
Conclusion
Urban forestry managers in coastal and inland regions seem to follow somewhat different patterns of species planting, with coastal areas focused on small-statured species and inland areas focused on medium- to large-statured species. Forestry managers and design professionals may also need to reexamine current species selection practices to avoid future problems, such as sidewalk damage and excessive maintenance, that can give an otherwise deserving species a bad reputation.
It also appears from this study that many municipal trees have not grown to their full potential. The exact causes for this are numerous and varied, but are most likely a combination of trees that have been planted relatively recently and the municipal environments into which street trees are placed.
- © 1996, International Society of Arboriculture. All rights reserved.