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in urban landscapes by providing beauty, shade, and cooling ( McPherson et al., 1989 ), but they also can consume significant amounts of water ( Pataki et al., 2011 ). In Los Angeles, it is estimated that trees cover more than 13,000 ha, totaling more

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Oakland, California has been persuing an aggresive urban tree planting program since 1978. I have spent a year studying this program from the sociological perspective and believe that insights gained may be helpful to similar programs elsewhere.

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Shade acclimation response of Emerald Queen Norway maple street trees to variable urban irradiance levels was investigated. Specific leaf area, trunk growth, and crown density were measured from trees in 13 sites ranging from urban canyons in the business core to open exposures in residential areas of Seattle, Wash. Percentage of potential seasonal input of global shortwave radiation for each site was modeled based on the azimuth and elevation angles of the surrounding horizon topography. Building height in the business core reduced estimated irradiance to a range of 27% to 90% of that for an unobstructed horizon topography, while those outside the business core had 90% to 95% irradiance. As estimated potential irradiance decreased, growth of these maple street trees exhibited responses characteristic of shade acclimation in a dose-response pattern. Specific leaf area increased and trunk growth and crown density decreased to acclimated levels at -70% of potential irradiance. These acclimation responses did not degrade the function of the trees in their urban-canyon locations. Their foliage was healthy, and reduced crown density was not apparent since there were no full-sun-grown trees for comparison.

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In a survey, residents of the largest metropolitan areas in the continental United States rated the social, environmental, and practical benefits from trees in urban areas highly. They ranked the ability of trees to shade and cool surroundings highest. The potential of trees to help people feel calmer was ranked second highest. Survey respondents were not very concerned about potential problems with trees in cities, and felt that trees should be planted in cities regardless of any annoyance. Practical problems with trees, such as causing allergies, were bigger concerns than were financial issues. Responses varied slightly, based on childhood background and current demographic factors. For example, people who grew up with a garden near their home or actively worked with plants during childhood were more likely to appreciate the potential benefits of trees than were those who did not have such early experiences. People who strongly agreed that trees were important to their quality of life and those who did not strongly agree ranked the tree benefits and problems similarly, however. Those who strongly agreed that trees were important to their quality of life rated the benefits of trees more highly than people who did not strongly agree.

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More than 70 biogenic hydrocarbon (BHC) compounds are known to be emitted by plants, but only a few are emitted in relatively large quantities. The magnitude of BHC emissions from individual trees is affected by ambient light and temperature, species-specific emissions rates, and leafmass. Like other volatile organic compounds (VOC), BHC emissions react with oxides of nitrogen (NOx) to form ozone and, thus, can contribute to urban air pollution. On average, BHC emissions are as reactive or more reactive than the VOC emissions from automobiles and can have higher ozone-forming potential. An accurate estimate of the overall magnitude of BHC contributions is important in formulating strategies to reduce peak ozone concentrations because an effective strategy will take into account the relative strengths of NOx and VOC emissions. The choice between NOx and VOC controls is crucial since an incorrect emphasis may result in non-attainment of ozone-reduction goals and control measures for either NOx or VOC involve enormous costs. As part of a program to develop a reliable BHC emission inventory for the Central Valley of California, a quantitative investigation of the leafmass of urban trees was conducted. Twenty-one trees in Bakersfield, Calif., were harvested and leaves removed, dried, and weighed. Leaf masses per tree ranged from 1.5 to 89.6 kg. Leaf mass densities (dry leaf mass per area of crown projection) ranged from 150 to 3200 g·m-2, as much as eight times greater than leaf mass densities for deciduous forests and more than twice those for coniferous forests. These data suggest the BHC contributions of urban trees may be underestimated if their foliar masses are calculated using forest-based leaf mass density data.

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trees also affect the energy use for heating and cooling of buildings ( Akbari et al., 1997 ; McPherson et al., 1988 ; Simpson and McPherson, 1998 ; Stec et al., 2005 ). Although several studies on the cooling effect of shade trees in temperate urban

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Airborne pollen concentrations (grains/m3) within and near trees of 2 cultivars of Olea europaea L. were studied during the 30-day pollination period at 2 urban sites in Tucson, Ariz. ‘Manzanillo’, the dominant horticultural cultivar, was compared to the fruitless ‘Swan Hill’. Air sampling using a Burkard trap was undertaken from 2 Apr. until 1 May 1985; during this period, 95% of the 1985 Olea pollen was airborne. Peak atmospheric Olea pollen concentrations at both sites occurred on 14 Apr. 1985. Pollen concentrations around the ‘Manzanillo’ site ranged from 7 grains/m3 to 6196 grains/m3 per day. At the ‘Swan Hill’ site, daily totals were an order of magnitude less, from 5 to 309 grains/m3 per day. Hourly pollen concentrations for the ‘Manzanillo’ site on the peak day varied from 1000 to 18,133 grains/m3 per hr. Hourly values at the ‘Swan Hill’ site on the peak day varied from 7 to 896 grains/m3 per hr. Both sites exhibited rapidly increasing pollen concentrations at sunrise with a sharp increase for the ‘Manzanillo’ site between 1100 to 1300 hr. Both cultivars produced about 85,000 pollen grains per anther. An unknown anatomical or physiological factor in ‘Swan Hill’ inhibits stomial rupture, resulting in 85% inhibition of anther dehiscence and pollen-shedding.

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Growth responses during nursery production in 2.2- and 11.4-liter plastic containers to conventional and alternative media of four species of small trees of limited availability for potential use in urban sites in the southwest United States (Acacia wrightii, Chilopsis linearis, × Chitalpa tashkentensis, and Rhus lanceolata) were compared to that of a commercially available small tree (Fraxinus velutina). Four media combinations, at 3:1 (v/v) of bark: sand (conventional), bark: coconut coir pith, kenaf stalk core: peatmoss, and kenaf: coir, with three fertilizer concentrations (3.6, 7.2, and 10.7 kg·m–3 of 18N–2.6P–10K Osmocote) were tested with each species. All species exhibited commercially acceptable growth (80 to 167 cm mean heights in 11.4-liter containers in 240 days) with near 100% survival in most media and fertilizer combinations with the following exceptions: shoot extension of Rhus lanceolata was reduced by 20 to 30 cm and survival by 20% to 50% in kenaf media with high fertility rates; and Acacia wrightii had acceptable shoot extension but exhibited poor trunk diameter growth across media relative to the other species. Slightreductions in growth of some species were noted with kenaf media and slight increases with coconut coir, but differences were not likely of commercial significance. Kenaf media was significantly lighter (20% to 80%) than bark media, but had elevated initial electrical conductivity (EC) and shrank to 60% to 70% of its initial volume after 240 days. Kenaf: peatmoss media had a slightly lower mean pH (6.34) compared to the other media (pH 6.41–6.49).

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Two inch caliper Acer rubrum, Quercus phellos, and Platanus occidentalis were planted March 26, 1990, into 8' × 8' planting holes that were lined with either Typar Biobarrier, Dewitt Pro-5 Weed Barrier or left unlined as a control. There has been little or no root penetration beyond the Biobarrier for the 3 tree species during the first 3 years of this study. At the end of 1990, the control and the Dewitt Pro-5 had similar root penetration numbers. By the end of 1991, the Dewitt Pro-5 had greater root penetration than did the control for A. rubrun. Root penetration of Dewitt Pro-5 and the control treatment was similar for Q. phellos and P. occidentalis. There were no differences in root penetration for Dewitt Pro-5 and the control in 1992 for any species. There were no differences in height for any tree species following the 1990 or 1991 growing seasons and no difference following the 1992 growing season for A. rubrum and Q. phellos. The control treatment had the grearest height for P. occidentalis in 1992. There were no differences in caliper due to root control treatment for the 3 species during the first 3 years of this study.

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exam viewed plants, their positive feelings increased, while fear and anger decreased ( Ulrich, 1979 ). Even brief visual contact with plants, such as urban tree plantings or office parks, might be valuable in restoration from mild daily stress. Views

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