Introduction

Microsymbionts encompass soil-living organisms that form symbiosis with plant roots. There are three types of organisms that are important for cultivated plants: mycorrhizas, rhizobia, and frankiae. Mycorrhiza (meaning 'fungus-root') is formed by virtually all forest trees. Many trees grow poorly, especially under infertile soil conditions, without a mycorrhizal symbiont. A large group of important forest and agroforestry trees of the legume family (Leguminosae) depends on the bacterial symbionts, rhizobia (largest genus Rhizobium), which cause the formation of nitrogen-fixing root nodules. Some trees like Alnus and Casuarina species form nitrogen-fixing symbiosis with the bacteria Frankia. The bacterial associations rhizobia and Frankia are exclusively linked to nitrogen fixation while mycorrhiza play multiple roles in nutrient uptake (mainly phosphorus) and in protecting roots from infection and stress. Many leguminous and actinorhizal (associated with Frankia) trees depend on an association with both mycorrhiza and rhizobia or Frankia and must be inoculated with both.

Introduction

Seed propagation is the principal mode of propagation for trees in temperate as well as in tropical regions. Seeds are unique in natural regeneration and propagation because:

1. Seeds constitute unique genetic compositions, resulting from mixing parental genetic material. The result is genetic variation of the offspring, which in turn enhances ecological adaptability.
2. Seeds are usually produced in large numbers and are readily available, each year or at longer intervals.
3. Seeds are (usually) small concentrated packages of plants-to-be, containing nutrients for the establishment of the plant and, except for recalcitrant (short-lived) seeds, usually resistant to damage and environmental stress.
4. Many seeds can be stored for long periods under cold and dry conditions.

Introduction

People, plants and animals thrive in sheltered environments. Creating a sheltered environment is an important farm management objective. The only practical way to provide shelter for broadscale agriculture and pasture is by establishing windbreaks. Farmers and other land managers increasingly recognize the value of shelter, and windbreaks are more and more a feature of rural areas throughout the world. Windbreaks have a place on almost every farm.

Key design criteria

A drive through the countryside often reveals many poorly designed and haphazardly established windbreaks. Faults are difficult to correct after planting. A little thought and care in the design of your windbreak will result in a valuable long term asset and improve a farm's productivity and profitability.

Introduction

Throughout the world, agricultural systems are potential sites for a great diversity of managed and collected plant and animal foods. Conventional agricultural research and extension, by focusing only on the main food crops, chiefly cereals, roots and domesticated livestock, have long ignored the range of other plants and animals that also make up agricultural systems.

Studies of this diversity and attendant complexity are demonstrating the importance of understanding the full range of products harvested. For instance, in the apparently maize-dominated agricultural system of Bungoma in Kenya, people consume at least 100 different species of vegetables and fruits drawn from 70 genera and belonging to 35 families (88; 729). Similarly, the agropastoral Tswana people in Botswana use 126 plant species and 100 animal species as sources of food (261). Similar patterns are shown in SE Asia (179; 132; 152; 188), Himalayan areas (160), Central America (682), Latin America (2; 3; 126), northern Europe (866; 867) and elsewhere in Africa (47-49; 82; 273; 400a; 493; 494; 770).

Introduction

"Food will last so long as forests do"...so runs an ancient Kashmiri adage (Ann poshi tele yeli poshi van -- Sheik Nur-ud-Din Wali)

Forestry has a large and indispensable role to play in improving present and future food security. Although a great deal remains to be understood about the specifics of this role, it is clear that foresters must make food security a basic consideration in policy formulation, as well as in programme planning, design and implementation.

Trees have been an integral part of the food security strategies of rural people for so long that it is curious and disturbing to note how this relationship has often been neglected in the planning of forestry activities. Even more disturbing, agriculture and forestry have often been, and sometimes still are, viewed as being in opposition. Project reports include such statements as "farmers may be too concerned over providing the daily food to become interested in planting trees". This false dichotomy is perhaps based on the outdated view that forestry is concerned only with raising timber trees on government lands and that agriculture only involves growing crops in open fields.

Introduction

Our forefathers planted American elms throughout the towns and cities of eastern United States. Rather than being an unconsidered idea, our early horticulturists were taking advantage of the beauty and adaptability of a native tree that Thomas Jefferson called "Nature's noblest vegetable". The accidental introduction of Dutch elm disease and the consequent destruction of millions of city trees served not only spur the search for replacements for American elm, but also to focus attention on urban forests.

We will not, and indeed should not, replace American elm with any single tree species in the quantities previously allotted to American elm. Instead, we need a diversity of trees in our urban forests, not only to guard against disasters like Dutch elm disease, but also to "put the right tree in the right place" as the evolution of our cities and suburbs creates new sites and settings for tree planting.