The point is that greenbelts probably need to be complete ecological
 communities of a size, spacing and composition as yet largely unknown. But,
 more serious is the adequacy of methods for measuring the contribution of
 urban forests (i.e., of solar energy) to the city's economy. The purpose of
 the Capitol Park study was to make such measurements, evaluating the economic
 and energetic benefits of the park ecosytem surrounding the Capitol complex.
 Both traditional economic methods and a relatively new energetic approach were
 used and the results of the two compared.


 Theory and Methods: Energy Flow as a Measure of Ecosystem Service


 The most common approach to the problem of valuing environmental services
 is one of evaluating the dollar cost of all the technologies and labor which,
 in combination, would have to be invoked to replace the services provided by
 the environment. This method is indirect at best. Because no money is
 exchanged when services are provided by trees and greenbelts, economics must
 consider those services as external to urban function and urban productivity.
 In contrast, the theory and methods for evaluating ecosystem functions which
 are summarized here may allow those services to be internalized and evaluated
 directly.


 Theory

 Recognizing that we do not pay a tree, greenbelts, or the sun for the
 services each provides, Odum (Odum, 1971; Odum and Odum, 1976) sought a different
 metric than dollars for evaluating their value. The one commodity which is
 stored and flows through any kind of system is energy. Energy flow occurs in
 both technological and natural systems and its role in both cases is similar.
 More specifically, a machine and living organism are the same in that they
 both depend on a flux of energy in order to function (Odum and Odum, 1972).
 The machine depends on concentrated fuels; energy is routed through the machine
 and is eventually dissipated as heat. In the interim, work is done by the
 machine; some commodity is manufactured or upgraded. Ecologists are quite
 familiar with the role of sun energy in ecosystems and energy flow among trophic
 levels. Less familiar is the concept that energy flow through an ecosystem
 measures the work done by nature, just as energy flow through a machine measures
 work done. For example, the gross primary productivity of an ecosystem
 (expressed as energy flow per unit of time) is a measure of the amount of
 sunlight captured and concentrated by the ecosystem. Portions of this energy
 are then used by the plants and animals to maintain metabolic processes, to
 reproduce and to grow. At the ecosystem level, these processes in turn enable
 "pollutants" to be absorbed, degraded and recycled. In the physics context,
 energy flow per unit of time is a measure of work performed.

 In an urban system, some of the work in support of the city is carried
out by technologies using fuels and some of the work is carried out by
technologies using fuels and some of the work is carried out by technologies
using fuels and some of the work is carried out by nature using the sun.
In Capitol Park, the five acres of buildings, air conditioners, elevators,
and lights are a marked contrast to the redwoods, pines and groundcover