part describes the energetic theory and methods which allow those functions
to be quantified. Our analytical results for Capitol Park, Sacramento,
California are presented, along with some implications of th.e results for city
planning. A final section discusses the energy quality and end use considera-
tions and the need for additional theoretical and applied work in this area.


 Natural Components in Urban Ecosystems: Their Function

 Natural components within or around urban systems provide a host of
 services of value to the city. Since the energy source for these natural
 work processes is the sun, no economic cost is associated with them. The
 exception is, of course, any fertilizing, watering or maintenance costs incurred
 to sustain nonnative species. Pollution abatement, hydrological control, and
 climate control have been recognized widely as functions which can be carried
 out, in part, by trees.

 Trees or greenbelts in general absorb noise, air pollutants, and water
 pollutants. Trees are widely used along highways and surrounding industrial
 parks as barriers to noise pollution. Along highways they also absorb
 pollutants from automobile exhaust systems. One study found that a 500-meter
 wide greenbelt surrounding a factory reduced nitric oxide concentrations in the
 air by 27%, sulfur dioxide by 26%, and hydrogen sulfide by 1% (Bach, 1971).
 Another compared particulate concentrations in the same part of town on tree-
 lined and non tree-lined streets (deAlbornoz, 1973). Along streets without
 trees, the air contained 10,000 to 12,000 particulates per cc.

 In addition, water pollutants in urban runoff waters are taken up by
 plant communities. In fact, little urban runoff from greenbelts occurs since
 the plants make use of both the water and pollutants. Municipal sewage
 effluent can be directed onto greenbelts (either natural or altered for
 recreational purposes such as a golf course), mitigating the need for advanced
 sewage treatment as well as for irrigation water normally drawn from the
 city's water supply system. In some cases, this water may also recharge an
 aquifer. Plant communities deter soil erosion which further reduces water
 pollution. As barriers along rivers and estuaries, they act as a buffer zone
 which protect the city from flood waters.

 Greenbelts also exert a major influence on microclimate within cities.
 In toto, they mitigate extremes, extremes in temperature, in wind, and in
 precipitation. It is widely recognized that trees and shrubs around buildings
 act as insulation. By absorbing wind energy, they reduce the air exchange
 between the inside and outside of a house. The American Physical Society
 (1975) found that wind dominates the dynamics of that air exchange; thus wind
 buffers can be an important energy conservation strategy. Farmhouses on the
 prairie grasslands are nearly always surrounded by cottonwoods and junipers.
 These farmers have known for centuries that trees mitigate the wind and
 temperature extremes in the farmhouse. Temperature measurements in forests
 substantiate this affect, indicating temperature differences of 10 to 20 degrees
between the forest floor and the top of the canopy.