Managing genetic diversity in a tree improvement program
Southwest US
Maintenance of species and within-species diversity is a goal of the USDA Forest Service, Pacific Southwest Region, which is examining and monitoring the genetic effects of silvicultural activities using isozyme analysis in conjunction with traditional methods. An effective monitoring and evaluation program will help improve silvicultural practices. Regional policies on gene conservation focus mainly on artificial regeneration. Tree improvement, nursery, and reforestation activities in the PSW Region may significantly affect adaptability, genetic diversity, and economic timber traits. The Base-Level Tree Improvement Program (TIP) aims for: ( 1 ) high adaptability via use of native species and local wild seed; (2) high within-species genetic diversity via designs for sampling several local seedstands and via long term plans for improvement using simple recurrent selection and bulking among the seed-production-area plantations; and (3) moderate improvement in performance via mild selection for superior phenotypes. The ceiling on genetic diversity is established by the seed collected. Nursery and reforestation practices may result in some loss of original (seed collection) diversity. Nursery practices aim to maximize plantable seedlings per unit of seed and thereby preserve diversity through sound management. Reforestation practices aim to achieve high local diversity by mixing appropriate species at planting, allowing natural regeneration to established in plantations, encouraging high survival and using higher planting densities so that natural selection can operate effectively. The High-Level TIP involving selective breeding and orchards will supplant the wild collections for major species and productive sites. Orchard seed will have a base of 50-60 unrelated, interbreeding parents, each representing a separate stand. Cooperative programs will enhance the genetic base by providing a broad range of material for breeding. New genes may be infused into breeding populations from material selected in natural forest reserves to increase diversity. Production plantations will be genetically more diverse than naturally regenerated stands.
