Billions of people depend on trees for fuel, medicine, food, tools and containers, fodder for livestock, shade, and watershed maintenance, not to mention paper products and construction materials. Globally, thousands of tree species are useful, but in a particular area the number of heavily used species is much lower. The genetic diversity within these valuable tree species allows them to adapt locally so they can grow over a wide range of environmental conditions.
The quality and quantity of products from different individuals of the same species may vary greatly from tree to tree. This has allowed traditional societies and other users to select, protect and plant the best producers, gradually improving the quality and quantity of goods obtained. Currently, modern selection and breeding programmes are being applied to many useful tree species. These programmes make use of the available genetic diversity, selecting the individuals within the species with the most desirable characteristics (e.g. Dawson and others, 2012). Whether in agroforestry systems or natural forests, this increases the quality of useful trees allows them to adapt to changing conditions and continue to produce the goods and services that people need.