Understanding the Structure rban forests are complex Whitney and Adams 1980, ecosystems created by the Sanders 1984). Morphology, interaction of anthropo- natural factors, and management genic and natural processes. One systems interact to create diverse key to better management of forest structures across urban these systems is to understand landscapes. Despite this diver- urban forest structure and its re- sity, there are some broad simi- lationship to forest functions. larities in urban vegetation pat- Through sampling and invento- terns. A number of studies have ries, urban foresters often obtain examined urban forest structure structural information (e.g., from the gross structural charac- numbers, location, size, and con- terizations of tree canopy and dition) on street- and/or park- land use to detailed investiga- tree populations to aid manage- tions of individual trees. ment. However, these public Urban forest cover. There are trees compose only a small por- significant variations in urban tion of the urban forest. forest cover both within and Management of the entire ur- among cities. Past studies indi- ban forest ecosystem requires in- cate that urban tree cover ranges formation on all vegetation and from a high of 55 percent in Ba- other attributes of the system ton Rouge, Louisiana, to a low of across the urban landscape. This 1 percent in Lancaster, Califor- type of structural information es- nia, with a US average of 26 per- tablishes a basis for comprehen- cent (Nowak et al., in review). sive management that recognizes Tree cover in the United States linkages among the multiple tends to be highest in cities de- land uses and owners of the ur- veloped in naturally forested ar- ban forest. Forest structure also eas (32%),followed by grassland provides a means to estimate the cities (18%) and desert cities actual and potential physical, bi- (10%) (Nowak et al., in review). ological, social, and economic Within a city, the highest per- functions of the urban forest. Ur- centage of canopy cover is gener- ban foresters can then develop ally found on vacant, park, and plans and programs that provide residential land (table I). Tree for these functions across the ur- cover analyses across an entire ban landscape. city reveal tree distribution and interconnections among land use Urban Forest Structure types (Rowntree 1984b). How- Urban forest structure is the ever, more detailed investigations spatial arrangement of vegetation of structure are necessary for im- in relation to other objects, such proved understanding and man- as buildings, within urban areas agement of specific areas within (Rowntree 1984a). Three broad the city to enhance particular factors determine urban forest functions. structure: urban morphology, w .omposition and structure. A num- ates the spaces available for ve - :tation; ber of udies have evaluated street-tree natural factors, which influence the populations or limited portions of non- amount and type of vegetation likely to be street-tree populations, but there have found within cities; and human manage- been few ground-based urban forest struc- ment systems, which modify vegetation tural analyses of an entire urban area. Var- configurations across land-use types (e.g., By David J. Nowak ious typologies have been developed to 42 October 1994 Reprinted from the Journal of Forestry, Vol. 92, No. 10, October 1994. classify urban vegetation across a city 1974). In the Chicago area (Cook and trees constitute 1 of every 37 trees, and 1 (Brady et al. 1979, Sukopp and Werner DuPage Counties), the exotic and highly of every 77 trees in DuPage County. Al- 1983), but land-use type is probably the invasive buckthorn is the most common though only a minor component of the ur- most common classification used. tree species, composing a significant pro- ban forest as a whole, street trees are a sig- Overall, urban trees are relatively small portion of trees on institutional, vacant, nificant component of the street-corridor in stature, with the majority of them less and residential lands. Although this spe- and human environment (Schroeder and than 6 inches dbh (Dorney et al. 1984, cies will not likely dominate the urban for- Cannon 1987). Nowak 1993a, Nowak 1994a). The distri- est overstory canopy because of its rela- Individual tree attributes. Individual bution of tree sizes usually varies depend- tively small size, its abundance and inva- tree attributes, such as leaf surface area and ing on the history and intensity of vegeta- sive characteristics pose problems for vege- transpiration, are critical to many urban tion management. Less-managed or natu- tation managers. forest functions, such as reductions in air rally managed land tends to have the Four of the eight most common spe- temperature and air pollution removal. highest proportion of small trees-due in cieslgenera in the Chicago area are native Typical leaf area indices (LAI) are 10 to 11 part to natural regeneration in these areas. pioneer species: greenlwhite ash, boxelder, for tropical rain forests, 5 to 8 for decidu- Tree species composition varies by willow, and cottonwood, which account ous forests, and 9 to 11 for boreal conifer- land-use type, but overall, the urban forest for 25 percent of the population of trees ous forests (Barbour et al. 1980). The av- composition is generally dominated by a below 3 inches dbh. Canopy dominance erage LA1 (exclusive of grass) for tree-cov- few species. In Oakland, four species make in the urban forest of the Chicago area will ered areas in Chicago is 6.0, which is at the up 49 percent of the tree population likely shift more toward these native pio- low end of the normal range of LAIs ex- (Nowak 1993a); in Chicago, six species or neer species. Although silver maple, white hibited for deciduous forests. This rela- genera constitute more than half of the oak, and bur oak account for one-third of tively low LA1 is attributable to the lack of population (Nowak 1994a); and in Ath- the trees above 18 inches dbh, they ac- understory trees in some urban areas, ens, Greece, five species groups compose count for only 3 percent of small trees and where the understory is occupied only by much more than half of the tree popula- are not likely to increase in significance in grass or impervious materials. tion (Profous et al. 1988). the future. Another successful pioneer spe- Other individual tree attributes, such Exotic species can also play an impor- cies regenerating in many urban areas as in-leaf and out-of-leaf shading densities tant role in the urban forest ecosystem. In across the United States is the exotic ailan- (McPherson 1984) and growth rates Oakland, 69 percent of the existing trees thus. (Fleming 1988), are important for esti- are not native, and these exotic species Street trees. Street trees account for 1 of mating current and potential urban forest contribute to high species diversity. Ur- every 10 trees in Chicago, and 1 of every 4 functions such as energy conservation and banization has increased Oakland's tree trees in Chicago's 1- to 3-family residential carbon dioxide sequestration. species diversity index from 1.9 (Shannon- areas. Due to their relatively large size, Weiner diversity index) to 5.1 (Nowak Chicago's street trees contribute 24 per- Urban Forest Functions and 1993b). Although citywide diversity can cent of the total city leaf-surface area, and Management be increased by the introduction of exot- 44 percent of total leaf area on 1- to 3-fam- Urban forest structure is a key determi- ics, diversity in natural forest stands in ur- ily residential land. Street trees play an im- nant of function. To enhance forest func- ban areas can decrease as a result of human portant role in heavily urbanized areas tions across the urban landscape, managers activity. For example, air pollution has in- where artificial surfaces and land-use activ- and planners strive for appropriate forest creased tree mortality (Iizumi 1983). ities limit their space, but are a less signifi- designs and linkages, healthy trees, and Data from Chicago and Oakland reveal cant component of the forest in suburban long-term tree survival. By understanding that most urban trees are located on insti- areas. In suburban Cook County, street urban forest structure, how it relates to tutional and vacant land. Such land is often the most natural and exhibits Opposite: Urban ecosys- typical forest stand struc- tems are a combination tures. But in some in- of interactive anthropo- stances, human activities genic and natural sys- that lead to changes such tems. Understanding the as reduction in fire fre- structure of these systems quency and introduction can lead to better plan- of exotic species have al- ning andmanagement of tered stand structure. urban vegetation. In Oakland, Califor- Left: Benefits of urban nia, bay is dominating re- forests such as New York production in many City2 Central Park in- stand types. In the ab- clude not only beauty sence of fire, bay wood- and recreational oppor- lands will likely replace tunities but improved air oak/bay woodlands in the quality and reduced en- Oakland area (McBride ergconsumption. Journal of Forestry 43 maximum potential an- However, increased tree cover is not appro- nual effect of trees on re- priate in all urban areas. Some manage- ducing conventional ment objectives, such as prairie restoration house energy use is or active recreation fields, call for minimiz- about 20 to 25 percent, ing trees. Other areas, such as low-income but energy costs can in- multifamily residential areas, commercial- crease with improper industrial areas, freeways, schools, and tree configurations (He- passive recreational parks, could be greatly isler 1986). Esthetic im- enhanced with more trees. provement plans need to consider people's prefer- Conclusion 0-3 3-6 6-12 12-18 18-24 24-30 30+ ences for tree density Urban vegetation has many different Inches and arrangement functions that provide a wide range of ser- Figure I.