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Home , Urban forestry

Understanding the Structure

rban 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 structures across urban these systems is to understand landscapes. Despite this diver- 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 often obtain examined urban forest structure structural information (e.g., from the gross structural charac- numbers, location, size, and con- terizations of and dition) on street- and/or park- land use to detailed investiga- tree to aid manage- tions of individual . 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 . 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 , 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 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 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 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 - 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. Estimated pollution removal of individual urban (Schroeder and Green vices (Dwyer et al. 1992). Urban forest trees, by diameter chs, in Chicago, 1991 (Nowak 1994). 1985). These and other managers should decide which functions improvements in the ur- are important-for example, local air specific functions, and how structure and ban landscape can generate- millions of quality improvement, reduced energy us- functions link not only within the urban dollars in enhanced real estate values, local age, or improved esthetics-and develop area but between the urban area and sur- economic development, and other benefits plans and programs that will provide a veg- rounding forest environments, managers (Dwyer et al. 1992). etative structure for these functions. How- can better direct forest structure to en- All urban forest designs and plans need ever, efforts to enhance one function may hance particular functions. to consider the specific morphology,-. natu- impair others. Designs to improve air Enhancement of various functions may ral factors, and human influences. By un- quality, for example, may not be the most require different urban forest structures. derstanding land-use vegetation structure esthetic or conducive to recreational activ- For example, improving air quality calls in context with local conditions, managers ities. In many instances, the forest re- for a forest structure that includes maxi- can identify potential opportunities and sources over widespread areas within and mum leaf-surface area of pollution-toler- limitations to enhancing urban forest around cities interact to provide particular ant species in the most ~ollutedareas. functions. functions such as wildlife habitat. Thus, Trees in Chicago were estimated to remove Many urban areas offer potential space urban forestry plans and programs need to 65 1 tons of air pollution in 1991, with for trees, but the natural environment consider the wide spectrum of forest func- large trees removing 60 to 70 times more (such as lack of water), current land uses tions and how various management units air pollution than small trees (,.- - 1). To re- (such as baseball fields), management in the urban forest can be linked to en- duce building energy use, trees need to be practices (such as mowing), and/or limited hance overall functions. selected and located around buildings to managementtmaintenance budgets may By understanding forest structure maximize building shade in summer, min- preclude and maintenance. across the entire urban area, managers can imize winter shade, block winter winds, Some of these limitations can be overcome better evaluate the differences among the and provide transpirational cooling. The with education and/or increased funding. multiple owners of the urban forest and

44 October 1994

HEISLER,G.M. 1986. Energy savings with trees. J. Arboric. 12(5):113-25. IIZUMI,S. 1983. The urban vegetation of Tokyo and Sendai, Japan. In Man's impact on vegeta- tion, W. Holzner, J.A. Wenger, and I. Ikusima, eds., p. 335-40. W. Junk Publ., Boston, MA. MCBRIDE,J.R. 1974. succession in the Berke- ley Hills, California. Madtono. 22(7):317-29. MCPHERSON,E.G. 1984. Planting design for solar control. In Energy-conserving site design, E.G. McPherson, ed., p. 141-64. Am. Soc. of Landsc. Archit., Washington, DC. NOWAK,D.J. 1993a. Compensatory value of an ur- ban forest: an application of the tree-value for- mula. J. Arboric. 19(3):173-77. . 1993b. Historical vegetation change in Oakland and its implications for urban . J. Arboric. 19(5):313-19. . 1994a. Urban forest structure: the state of Chicago's urban forest. In Chicago's urban for- est ecosystem: results of the Chicago Urban Forest Climate Project, E.G. McPherson, D.J. Nowak, and R.A. Rowntree, eds., p. 3-18. USDA For. Serv. Gen. Tech. Rep. NE-186. . 1994b. Air pollution removal by Chica- go's urban forest. In Chicago's urban forest ecosystem: results of the Chicago Urban Forest Climate Project, E.G. McPherson, D.J. Nowak, and R.A. Rowntree, eds., p. 63-81. USDA For. Serv. Gen. Tech. Rep. NE-186. NOWAK,D.J., R.A. ROWNTREE,E.G. MCPHERSON, S.M. SISINNI,E. KERKMANN,and J.C. STEVENS. Urban tree cover analysis. Syracuse, NY. (In re- view.) PROFOUS,G.V., R.A. ROWNTREE,and R.E. LOEB. 1988. The urban forest landscape of Athens, Greece: aspects of structure, planning and management. Arboric. J. 12:83-107. ROWNTREE,R.A. 1984a. Ecology of the urban for- est-introduction to Part I. Urban Ecol. 8:l- 11. . 1984b. Forest canopy cover and land use in four eastern United States cities. Urban Ecol. 8:55-67. SANDERS,R.A. 1984. Some determinants of urban forest structure. Urban Ecol. 8: 13-27. SCHROEDER,H.W., and T.L. GREEN.1985. Public preference for tree density in municipal parks. J. Arboric. 1 1(9):272-77. SCHROEDER,H.W., and W.N. CANNON.1987. Vi- sual quality of residential streets: both street and yard trees make a difference. J. Arboric. 13(10):236-39. SUKOPP,H., and l? WERNER.1983. Urban environ- ments and vegetation. In Man's impact on veg- etation, W. Holzner, J.A. Wenger, and I. Ikusima, eds., p. 247-60. W. Junk Publ., Bos- ton, MA. WHITNEY,G.G., and S.D. ADAMS.1980. Man as a maker of new plant communities. J. Appl. Ecol. 17:43 1-48.

ABOUT THE AUTHOR David J. Nowak is research , USDA Forest Service, Northeastern Forest Experi- ment Station, 5 Moon Library, SUNK CESE Syracuse, NY 13210.

46 October 1994