These functions are becoming widely recognized and widely discussed,
 There are many others that are more subtle and more difficult to define, but
 probably no less important. One of these more subtle functions is performed
 by the dimension of diversity that ecosystems add to the urban system as a
 whole. Social science studies as well as evidence from the biological
 sciences are beginning to show that a diversity of natural and technological
 systems in cities has value; for example, diversity may represent an element
 of crime control.

 Technology can also perform these functions, and the trend in our cities
 over the past 100 years has been to engineer some kind of a device to perform
 them. A wall constructed of special materials will insulate people from
 highway noise or industrial park noise; automobile emission control systems
 will remove air pollutants from car exhaust; stack gas control systems will
 remove pollutants from industrial stacks; larger sewage treatment plants and
 separate storm water sewerage systems will control water pollutants; recharge
 wells will recharge aquifers; specially engineered barriers will stop soil
 erosion and protect cities from floods; consumption of large amounts of fuel
 energy for heating and cooling will control temperatures within buildings; and
 even larger police forces will be invoked to regulate dense populations. All
 of these technological devices replace functions that can be performed entirely
 or in part by plant communities and all require enormous fuel investments.

 To the extent that "urban forests" can replace technology in cities
 providing equivalent functions, scarce fuels will be conserved and city budgets
 may be reduced. We only have to examine the budgets of our big cities to
 recognize that just the public service component is enormous and growing
 exponentially. Perhaps some of those services could better be accomplished by
 urban forests. Solar energy (as it is manifested in trees) can substitute for
 fuels (as they are manifested in technology) and consequently, can conserve
 fuels and save money.

 The extent to which urban forests can substitute for technology and perform
 the services described depends largely on the relative density of human activity
 (and associated industry), forest activity and on the manner in which the
 byproducts of technology are coupled to or interfaced with the forests. A
 great deal of research remains to be done on interface designs and on the
 choice of appropriate species of trees. Trees which are more or less isolated
 from each other by concrete and buildings do not represent much of an interface;
 much of the energy flow through the trees is uncoupled. For example, trees in
 isolation and surrounded by tall buildings have a limited exposure to the
 sun and cannot collect rain. The small volume of soil around the tree lacks
 humus; the salt load in urban runoff becomes highly concentrated and may reach
 levels which are toxic to the tree. The roots of the tree may cause the
 concrete to crack and falling limbs become a hazard. Similarly, some
 ecological communities and species types are more adapted than others to
 pollutant uptake and recycling. The byproducts of technology may be responsible
 for the diseases which have nearly eliminated the American elm (Ulmus americana)
 and chestnut (Castanea dentata). In contrast, sugar maples (Acer saccharum)
 seem to do quite well. A study of sugar maples in three sections of Montreal
where air pollution levels ranged from very high to very low indicated that leaf
hairs on the leaves may develop in response to the high pollution levels
(Elias and Irwin, 1976). It was hypothesized in the study that the leaf hairs
collected the particulate matter in the air, keeping it from entering the pores.