Urban Ecology最新文献

The numbers and size of trees, saplings and shrubs were determined on 316 lots in an older Milwaukee suburb and recorded by lot location and land use. Vegetation pattern, structure, biomass and productivity were examined. Structurally, Shorewood vegetation resembles a savanna dominated by American elm but now shifting toward dominance by ash, maple and elm. Woody vegetation is predominant on back and front yards as opposed to side yards and parkways and on lots with one and two family residences as opposed to other land uses. Size class distribution for most species indicates high levels of reproduction and planting. Tree density is 32 stems per ha, sapling density is 64 stems per ha, and woody plants cover 42% of the surface. Biomass was estimated to be 36.48 t/ha, 97% of it in trees, while productivity is estimated at 0.49 kg/m² per year, to which trees contributed 45% and lawns 53%. The diversity of species and partitioning of biomass and productivity among vegetation strata make this urban ecosystem distinct from natural savannas.

Planted forest stands and tree groves in parks and open space areas in San Francisco are aging. These stands, dominated by Monterey pine (Pinus radiata), Monterey cypress (Cupressus macrocarpa) and eucalyptus (Eucalyptus globulus), were planted over a 30-year period beginning in the 1880's. Stand structure varies between park areas planted for amenity values and open space areas planted for wind protection. Park stands have larger diameter trees (92.7 vs. 48.6 cm), fewer trees per ha (147 vs. 265), and greater basal area (130.0 vs. 51.5 m²/ha) than open space stands. About half of the Monterey pine and eucalyptus trees and one-third of the Monterey cypress trees occurring in older stands were rated as being in fair or poor condition. Common defects of these trees included butt, bole and root rots, excessive lean, large dead branches, and small percent live crown. Regeneration was absent or judged inadequate in stands of Monterey pine and Monterey cypress in both the park and open space areas. Eucalyptus stands are regenerating through root sprouts.

Vegetation on extensive areas of land cleared of housing 5 years previously was surveyed. Parts had been disturbed again and there was a clear distinction by association analysis and in measured parameters between these and those carrying the more mature vegetation. The most recently disturbed sites, indicated by the presence of Atriplex hastata and/ or Polygonum aviculare were characterised by a higher number of species, a higher proportion of annuals, smaller ground cover and reduced canopy height. Undisturbed areas were dominated by a limited number of perennial herbs and grasses, although where Lolium perenne was present, species numbers and numbers of annuals were significantly greater, and canopy height smaller. Woody plants were scarce on extensive demolition sites remote from a seed source, but smaller sites adjacent to mature vegetation were readily invaded by trees and shrubs. The practical applications of this work are discussed.

Preliminary measurement of the characteristics of Dayton's vegetation configuration is provided in a context of the city's neighborhoods and land uses. Results show that 58% of the city's land is not covered with artificial surfaces, and that 37% (almost 2500 ha) of the non-surfaced land is covered with tree crowns. The city has nearly 2 km² of exposed soil. Only single and two-family residential land use has more area with tree cover than grass cover, but this land use category also has the greatest available space for increasing tree cover. Wide variations in land cover characteristics exist throughout the city and across its land uses. Results indicate a great potential for increasing tree-related benefits through broadened urban forestry management.

Patterns of distribution and change for Norway, sugar, and silver maple trees with scorch, chlorosis, small dead limb, and large dead limb symptoms were analyzed with statistical techniques, using data collected between 1975 and 1977 in Syracuse, NY. Increases in symptom severity and numbers of trees affected indicate a general increase in maple decline of the population over the 3-year-period. Nearest neighbor analysis procedure showed that symptomatic trees occur in non-random spatial patterns. Trees with scorch and dead branch symptoms were distributed in a linear pattern while trees with chlorosis were grouped in a concentrated circular pattern. There was little evidence for spread of decline from symptomatic to healthy trees. No change in the degree of aggregation of symptomatic trees was observed over the three years of observation. Scorch of Norway maple leaves was the only symptom that could be associated with an agent that spreads from tree to tree but the overall level of scorch in the population remained relatively constant.

This first of two special issues on urban forest ecology offers new empirical work describing forest structure and composition. Four avenues of inquiry are pursued in this introductory paper with the purpose of setting the contributions of the special issue in context of the existing literature. First, in temperate regions of the world, it is likely 60–80% of a city's area supports enough trees to meet conventional definitions of “forest”. Second, the geographical distribution of canopy cover is understood best as dependent upon the historical development of the city and its division into land-use sectors. Third, physiognomy is poorly understood and varies widely depending on the amount and kind of human intervention in the colonization and regeneration processes. Fourth, dominance and diversity are likely to be hotly debated questions of urban forest structure because of lack of agreement as to what constitutes a “good” composition. Finally, a typology of urban forest structure is needed to be followed by historical explanations of how biology and human agency combine to bring about these structures.

Four cities in the eastern United States were divided into ten land-use classes and measured for canopy cover with black-and-white, monoscopic aerial photographs. Mean citywide canopy cover is 24–37%, with a range of 5–60% for the mean canopy coverage of ten land uses. Available space for growing trees is 55–66% of the sample cities' area; the percentage of that space filled with canopy is 37–57%. The dominant land-use class, one- and two-family residential covering an average of 46% of the cities' area, shows little variation in both canopy cover and canopy stocking within the sample and, where available growing space increases, so does canopy stocking. Vacant land is second in areal coverage (14% of cities' area), and varies only moderately in canopy cover and stocking when the values in this class are divided into abandoned and undeveloped land. Regularities in the spatial distribution of canopy, among the sample cities, occur as a result of the location and extent of land use.

A survey of Los Angeles area urban vegetation was done in 1982 as an integral part of a larger project to determine an emmissions inventory for reactive organic compounds such as isoprene and monoterpenes. A random, multistage sampling procedure was used. The Los Angeles coastal plain was subdivided into 20 polygons based on distinctive reflective characteristics on color-infrared images obtained from a 1972 U-2 overflight. Ground sampling in randomly selected sub-areas of each polygon identified a total of 184 plant species distributed in six structural classes: broadleaf trees, conifers, palms, shrubs, grasses, and ground covers. The leafy crown volume and the area of ground surface occupied was determined for the most frequently encountered species, and the dry leafmass constant (gm^-3) was determined for 51 common species. A similarity index was used to compare the species composition of ground-sampled areas. All areas classed as residential had a mean index of 0.57 ± 0.07. Nonresidential compared with residential areas gave a mean index of 0.41 ± 0.16. Species with the highest importance values were California fan palm, Italian cypress, Monterey pine, Cocos palm and Canary Island pine. These data have implications for several aspects of landscape planning including the proper selection of species to provide maximum shading, air filtering capacity, noise suppression and a capacity for surviving polluted air and limited root space.

Greenspace and vegetation were examined on 487 residential lots in ten selected census tracts in Syracuse, NY to identify their variation in physical characteristics. Variation in residential lot size is about equally attributable to tracts, blocks, and individual lots; while areas of structures and paving have greater heterogeneity within tracts, and little relationship to lot areas. Among greenspace components, only lawn area shows substantial variation attributable to tracts and is highly correlated with the total greenspace area of lots. Shrub, other groundcover, and especially garden areas, are highly individual lot features and show little pattern among tracts. Most of the variation in tree number and canopy area on lots also cannot be attributed to factors identified in this study. Distribution and care of greenspace components in front, side, and back yards of lots were examined as evidence of resident orientations to the neighborhood, adjacent neighbors, and more personal interests. The greespace and trees, including tree species composition, of the residential lots were compared with the more limited resources of the adjacent public streetsides. This study documents a large, heterogeneous, and practically managed greenspace and vegetation resource on residential lots in Syracuse.