Editor's Note

This is the second in a three part series by special guest author Dr. Alex L. Shigo, retired chief scientist of the U.S. Forest Service and author of numerous books including Modern Arboriculture. Understanding trees and how to plant them, and planting the right tree in the right place are essential for tree planting success. Here Dr. Shigo introduces some key "tree basics."

Some Tree Basics

Trees are plants that are:

• perennial--live for several to many years
• woody--have tough cell walls of wood
• shedding--use and shed woody and non-woody parts
• compartmented--made up of many compartments.

Trees usually have a single stem over three yards (meters) tall.
Shrubs usually have many stems less than three yards (meters) tall.

Editor's Note

In this edition of The Overstory special guest author Dr. Alex L. Shigo, retired chief scientist of the U.S. Forest Service and author of numerous books including _Modern Arboriculture_, clarifies twelve common misconceptions about trees. As Dr. Shigo says, "If we can understand more completely how trees function, then our work will be more enjoyable and profitable and our efforts to maintain healthy trees will be more effective."

Twelve Tree Myths

"Myth" is used here to include misconceptions, misunderstandings, and, mostly, half-truths.

Many of the corrections to the myths listed here are known by many people. I believe the myths persist because many of the myths taken alone appear trivial or a matter of semantics. However, each myth is like a thread. When a hundred or more weak threads are used to make a fabric called a profession, the profession will only be as strong as the threads that form it.

Editor's Note

In this edition of The Overstory, special guest author Richard Finlay-Jones introduces some of the methods landholders can use to optimize commercial timber potential. While written primarily for production of hardwoods in plantations on the North Coast of New South Wales (NSW), Australia, the principles are also applicable to timber production in many kinds of agroforestry systems.

Introduction

There are many ways growers can increase the commercial potential of timber stands on their land. These methods can be divided into separate areas: planning, establishment, management, and harvesting. Following harvest, marketing the product according to specific customer requirements may dictate the level of processing required and the potential for increased returns.

Planning

Plantation planning (Reid, Abel) provides a good method of working out a balance between environmental and economic benefits using commercial tree species. This requires that the landholder to develop a property management plan, which includes having an understanding of the existing problems of the property as well as some ideas about the local timber marketplace and its potential.

Editor's Note

Agroforestry systems can have a beneficial influence on the global climate. This edition of The Overstory introduces how conserving soils and planting trees can slow or reverse the release of carbon into the atmosphere.

What is the Greenhouse Effect?

Greenhouse gases such as carbon dioxide are a natural and essential component of the Earth's atmosphere. Atmospheric gases such as water vapor, carbon dioxide, carbon monoxide, methane, and ozone, absorb heat, and keep heat from radiating away from earth into outer space. This effect is much like the way glass traps heat in a greenhouse, therefore the natural warming of Earth by its atmosphere is called the "greenhouse effect."

Carbon dioxide is the most important greenhouse gas, accounting for about half of the greenhouse effect. The natural concentration of greenhouse gases (GHG) has been essential to life as we know it on earth, creating the average temperature of 15 degrees Celsius (59 degrees Fahrenheit). Without the naturally occurring greenhouse effect, the average temperature would be minus 18 degrees Celsius (0 degrees Fahrenheit)!

Editor's Note

Biological nitrogen fixation is an important part of many agroforestry, sustainable agriculture, and land rehabilitation practices. Although the terms "nitrogen fixing plants" and "nitrogen fixing trees (NFTs)" are widely used, the plants themselves do not have the ability to make use of the nitrogen gas in the air-- it is only through the symbiotic association with rhizobia bacteria that the process takes place. Simply planting leguminous "nitrogen fixing" plants or trees will not ensure that nitrogen will be accumulated; the process of biological nitrogen fixation depends on the presence of the correct rhizobia bacteria. This edition of The Overstory introduces how biological nitrogen fixation works, why inoculation is advantageous, and how to use rhizobia inoculants.

Biological Nitrogen Fixation

Nitrogen is commonly the most limiting element in agricultural production, and one of the most expensive to purchase as fertilizer (NifTAL 1984). There is an abundant supply of nitrogen in the air (the air is 80% nitrogen gas, amounting to about 8000 pounds of nitrogen in the air over every acre of land, or 6400 kilograms above every hectare). However, the nitrogen in the air is a stable gas, normally unavailable to plants. Many leguminous plants are able to utilize this atmospheric nitrogen through an association with rhizobia, bacteria which are hosted by the root system of certain nitrogen fixing plants.

Editor's Note

Tropical homegardens are gaining recognition as highly productive, low input systems. In this edition of The Overstory special guest author Dr. P.K.R. Nair offers an introduction to tropical homegardens.

Homegardens

Home gardening has a long tradition in many tropical countries. Tropical homegardens consist of an assemblage of plants, which may include trees, shrubs, vines, and herbaceous plants, growing in or adjacent to a homestead or home compound. These gardens are planted and maintained by members of the household and their products are intended primarily for household consumption; the gardens also have considerable ornamental value, and they provide shade to people and animals. The word "homegarden" has been used rather loosely to describe diverse practices, from growing vegetables behind houses to complex multistoried systems. It is used here to refer to intimate association of multipurpose trees and shrubs with annual and perennial crops and, invariably livestock within the compounds of individual houses, with the whole crop-tree-animal unit being managed by family labor (Fernandes and Nair, 1986).