The influence of forests on the environment forms part of a vast and complex relationship between the environment and forest vegetation. Wherever forest vegetation exists, it is in some stage of natural evolution, in the case of undisturbed primary forest, or succession or regression along a time line determined by human interventions. The impact made by trees and forests at each stage of their growth on the environment, both locally and globally, is what is meant by forest influences.
Forest influences can be divided into three practical scientific groups: the influence on climate; the influence on soil; and the influence on water. This issue of Unasylva presents a collection of articles offering new information or perspectives on each of these groups. A fourth category relating to psychological or spiritual influences, although potentially significant and worthy of investigation, is not considered here.
In the past, consideration of forests' influence on climate was focused at the microclimatic or local level. More recently, increasing attention has been directed at the potential impact of forests on global conditions. One of the most debated topics with regard to the environment today is that of climate change whether the earth is, in fact, entering a period of increased temperature, what the causes and effects may be and what could be done to mitigate them. The potential role of forests requires further investigation and a better understanding in a broader context. S. Brown of the United States Environmental Protection Agency sets out the most recent thinking on the present and potential future roles of forests in the global climate change debate.
In the various phases of forest dynamics, the corresponding plant associations affect the soil. The use of windbreaks and shelterbelts to protect agricultural land is an important example of forest influences, both in terms of climate and soil amelioration. An article drawing on the work of three Chinese scientists - Zhang Feng, Fu Jing and Hou Qing Jiang - associated with the Forestry Research Institute in Chifeng, Inner Mongolia, briefly describes the evolution of efforts in farmland shelterbelt construction in this area, including specific efforts and initial results in two townships.
The actual impact of trees on farmland is an important element of the overall discussion on forest influences. But perhaps even more important than the actual benefits may be the perception of influences, i.e. what farmers - the crucial decision-makers - consider the relative advantages and disadvantages of on-farm trees, whether their reasons are scientifically demonstrable or not. A thought-provoking article adapted from a 1995 study undertaken by the Australian Bureau of Agricultural and Resource Economics provides an analysis of farmers' perceptions of the influences that trees have on farms in Australia, with an emphasis on planted trees.
One of the main tenets of agroforestry is that trees can enhance soil fertility, the capacity of soil to provide essential nutrients for plant growth. The article by P.A. Sanchez (Director-General) and C.A. Palm (Senior Scientist) of the International Centre for Research in Agroforestry (ICRAF) examines the role of agroforestry in nutrient cycling in different ecosystems and then focuses on research into smallholder maize-based agroforestry systems in Africa.
Forests are commonly acknowledged as playing a major role in many components of the water cycle but hard scientific data are often lacking, leading to the risk of decision-making based on intuitive or even emotional inputs. The article by M. Meunier discusses the influence of forest cover on runoff and flooding. Based on the results of carefully controlled studies in watersheds of Draix, France, the article provides hard evidence to help answer crucial practical questions such as: What level of plant cover is required for the forest to have a significant effect on flooding? What is the rainfall threshold at which forests no longer curb runoff? Is wholesale deforestation in some parts of the world responsible for the widespread devastation caused by major floods?
Forestry has traditionally been associated almost exclusively with rural areas but, growing rising urbanization, the role of trees and forests in high population-density, industrialized settings has taken on increasing importance. The article by S. Murray considers the opportunities and challenges facing tree and forest management for positive environmental impacts in urban and pert-urban areas of developing countries.
The steady increase in the amount of water used and waste water produced by urban communities and industries has led to a search for safe, environmentally sound and cost-efficient ways to treat and dispose of waste water. One opportunity is the use of municipal waste water (both sewage and industrial effluents) to irrigate forests, forest plantations, greenbelts and amenity trees. The article by S. Braatz and A. Kandiah discusses some of the experiences to date and various issues related to the use of waste water for forest and tree irrigation.
This issue of Unasylva is intended to stimulate further reflection on the current and potential role of forests and trees in moderating and ameliorating the environment at local, regional and global levels - an important topic within the wider debate on the conservation and sustainable development of forests. Unasylva would welcome proposals for additional articles on this subject for future issues.
