Tree planting on farms in East Africa: how to ensure genetic diversity? A case study and teachers notes

Abstract

Tree planting, whether it is on farms or in commercial plantations, for forest restoration or species conservation, depends on a ready supply of germplasm (seed or vegetative material). Germplasm collection methods may vary, depending on the particular species or context. The need to use genetically diverse germplasm is, however, universal if plantings are to be productive, viable in the long term and resilient. Many tree species are outbreeding and generally carry a heavy genetic load of deleterious recessive alleles. Any inbreeding, in particular selfing, may thus have negative impacts such as reduced seed set and survival, leading to poorer regeneration, slower growth rates and productivity in progeny, limited environmental tolerance and increased susceptibility to pests and diseases. Consequently, maintenance of a wide genetic base will not only assist tree future adaptive capacity but also help to ensure that people continue to obtain the various benefits they derive from tree resources. Intra-specific genetic diversity may, however, be limited by a number of factors. Farmers, nursery managers or commercial collectors may collect germplasm (seed or cuttings) from a small number of trees. Variability between trees in fertility can contribute to a rapid accumulation of relatedness and inbreeding in subsequent generations. Furthermore, after initial introductions, germplasm for subsequent generations of planting may be harvested from the same introduced trees, limiting subsequent inflows of new diversity. Genetic issues can also be of particular concern for nursery material where inbred seed may survive benign nursery conditions, leading to planting stock being genetically compromised