By the year 2000 more than half of the planet's inhabitants are expected to be living in urban areas. Historically, the trend towards urbanization has been a gradual one but, with the exponential population increases of the twentieth century, the trickle has become a flood. The increase in urbanization is most dramatic in the developing world, where the number of urban dwellers has increased more than fourfold since 1950. Two-thirds of the population increase in developing countries is being absorbed by cities. Of the 66 cities expected to have a population greater than four million by the year 2000, 50 are in developing countries. This unprecedented increase in the pace of urban development has significantly affected man's relationship with trees and forests. Cities, by definition, are prominent or large concentrations of population, lacking self-sufficiency in the production of food and energy and usually depending on manufacturing and commerce to satisfy the needs of their inhabitants (cf. Webster's Third New International Dictionary). When the growth of cities and of the total population was slow, food and especially energy needs could be satisfied by trade with the surrounding areas. In practice, although there were generally few trees in the urban centre itself, the forest was not far removed.
As urban development expanded - still relatively slowly newer surrounding suburbs were planned with trees and wooded areas as a basic part of the overall environment (providing amenity values and supplementing purchased food and fuel supplies). Well-to-do residents were willing to pay premium prices for houses in these areas. But with the dramatic increase in urbanization, there has been a corresponding decrease in the ability of municipal planning authorities to keep pace. The result has been sprawling belts of uncontrolled development, often shanty towns, in which virtually all amenities and services are lacking.
Therefore, those assigned the responsibility of managing trees and forests in urban areas are confronted with three distinct situations in which the options and potential for success differ dramatically. In the historic centres of older cities, the main task is usually one of maintaining or replacing trees planted long ago. In planned suburban areas there is great scope for the incorporation of trees as an integral part of overall development, thereby maximizing their potential, particularly in terms of environmental benefits. In the urban fringe areas, where the planned city gives way to spontaneously growing settlements inhabited by rural migrants, population density is often so high that there is no undeveloped land and no trees or woody vegetation. Moreover, the prevailing economic situation in these new developments is such that many of the inhabitants are not able to generate the economic power to purchase food and, particularly, energy supplies. It is in these poor, unplanned urban areas that the need for the potential benefits from trees is highest, but also where their realization is most difficult. This issue of Unasylva analyses the urban forestry situation and considers its potential for increasing living standards in different urban settings.
In the lead article, G. Kuchelmeister and S. Braatz consider changing perceptions of urban forestry, the role of trees and other vegetation in and around densely populated areas as well as the opportunities and challenges related to their planting, conservation and use. Other articles examine the structure, conditions and management of the urban forests of Beijing, China; Mexico City; sub-Saharan Africa; and Prague, the Czech Republic. As with forestry in rural areas, it is increasingly recognized that the active participation of local people is essential for the success of urban forestry efforts, particularly in economically depressed cities. W.R. Burch, Jr and J.M. Grove discuss a project designed to apply social forestry principles to link urban revitalization and environmental restoration in the United States city of Baltimore. Many of the guiding principles of the Baltimore urban forestry project would be applicable in developing countries as well.
Although there has been much discussion of the beneficial impacts of trees on the environment of urban areas, there has been little hard data to support these claims. D.J. Nowak and E.G. McPherson report on the methodologies and the initial results of a research project, which is based in Chicago, Illinois and aimed at quantifying the impact of vegetation within urban areas on local climate, energy use and air quality.
From the articles in this issue of Unasylva it is clear that, although more is known about urban forestry today than a decade ago, it must still be considered a fledgling discipline. Many basic needs are yet to be met, including but not limited to: further quantifying the benefits trees can bring to urban dwellers in different situations, improving the incorporation of forestry into urban planning; developing specialized knowledge about the selection and management of trees specifically suited to urban ecosystems; improving institutional structures and legal frameworks for urban forestry; and fostering people's participation in the management of urban trees. A particularly important and as yet unanswered question is "who is or what should be the characteristics of the urban forester?". It is hoped that this issue of Unasylva will stimulate further analysis and discussion of the challenges and possible solutions related to the management of trees and forests in urban areas.
