Artificial and assisted natural regeneration of socio-economically important southern African tree species

Abstract

Several socio-economically important indigenous tree species of southern Africa show limited natural regenera-tion while also being threatened by land conversion and overharvesting. Assisted tree regeneration — both artifi cial regen-eration in nurseries and assisted natural regeneration in forests — is needed to allow the sustainable use of forest resources. Five studies of artifi cial and assisted natural regeneration of indigenous timber and fruit trees were performed independently from one another in Botswana, Namibia, and South Africa. They aimed to investigate and improve cultivation of seedlings, especially by testing the eff ects of temperature and diff erent seed pretreatments on germination in incubators and the eff ects of soil inoculation with plant growth–promoting bacteria in nursery conditions. One study compared direct seeding with enrichment planting in the forest. The germination tests showed that the seeds of most indigenous species tested should be given at least six weeks to germinate, with the exception ofBauhinia petersiana. Seed pretreatment, especially nicking and/or soaking, can improve germination for Strychnos cocculoides (+17%), Dialium englerianum (+68%), Erythrophleum africanum (+22%), and P. angolensis (+24%). Guibourtia coleosperma seed germinates well without any treatment. The ger-mination rate of P. angolensis seedimproved (+21%) after soil inoculation with plant growth–promoting and nitrogen-fi xing bacteria. The important fruit tree Schinziophyton rautanenii showed poor seed germination (7% to 30%), but the use of cut-tings for S. rautanenii gave a 100% survival rate after about six months. Broadcasting seeds in the forest and covering them with soil resulted in more surviving seedlings than planting because of damage caused by rodents. Further nursery studies should focus on vegetative propagation and the optimal conditions for seedling growth and establishment of potential agro-forestry tree species, while a range of assisted natural regeneration techniques remain to be tested for forests in the region.