| Abstract |
Namibia’s dry forests and woodlands are vital for biodiversity conservation, rural livelihoods, and ecosystem services; however, they are increasingly threatened by climate change, deforestation, and anthropogenic disturbances. The regeneration of indigenous hardwood species is often constrained by poor seed germination, seed predation, environmental stress, and competition. Assisted regeneration (AR) has emerged as a cost-effective approach for restoring degraded ecosystems, although its successful implementation in Namibia remains limited by insufficient ecological and silvicultural knowledge.
This study was conducted at the Ongava Research Centre (ORC) in the Kunene Region and the Mashare Irrigation Training Centre (MITC) in the Kavango East Region. The study investigated the effects of soil media, seeding depth, seed age, and season on seed germination; the influence of soil media and seeding depth on seedling development and survival; and the effects of bio-inoculation treatments on seedling growth and survival. The indigenous species studied at MITC included Pterocarpus angolensis, Guibourtia coleosperma, Baikiaea plurijuga, Dialium engleranum, Schinziophyton rautanenii, Burkea africana, and Terminalia sericea, while species at ORC included Colophospermum mopane, Sterculia africana, Peltophorum africanum, Terminalia prunioides, and Sclerocarya birrea.
Results showed that soil media had no significant effect on the germination of ORC species but significantly affected germination at MITC for B. africana (59% in loam and 58% in sandy soil compared to 40% in clay) and D. engleranum (34% in sandy soil, 29% in loam, and 16% in clay). Seeding depth significantly influenced germination in T. prunioides (33% at one seed diameter compared to 8% at two seed diameters) and B. plurijuga (67% versus 56%). Seed age also influenced germination, with older seeds of C. mopane showing higher germination rates (72% versus 36%), whereas newer seeds of G. coleosperma and B. plurijuga performed better than older seeds. Seasonal variation significantly affected germination, with higher germination during the rainy season for species such as T. prunioides (33% rainy versus 2% dry) and during the dry season for P. angolensis (47% dry versus 0% rainy).
Soil media significantly influenced seedling development, with T. prunioides performing best in river sand mixed with compost (88%) and P. africanum showing high survival in red regonols (76%) and grey calcisols (74%). Seeding depth also affected seedling survival, with sowing at one seed diameter resulting in higher survival rates for C. mopane (74%) and P. africanum (74%) compared to sowing at two seed diameters (52% and 48%, respectively).
Bio-inoculation treatments alone did not significantly influence seedling growth across species at either
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site; however, they positively affected the survival of B. africana at MITC (85% survival) and T. prunioides at ORC (100% survival). Inoculants also enhanced interactions between soil media and seedling growth for P. africanum in grey calcisols, P. angolensis in loamy soil, and B. plurijuga in sandy soil. Seasonal conditions significantly influenced the interaction between inoculation and seedling growth in Sterculia africana, with inoculated seedlings exhibiting improved growth during the dry season compared to uninoculated seedlings.
The study recommends the use of nutrient-rich and well-aerated soil media, such as sandy or mixed substrates containing river sand and compost, particularly for tree species of the Kalahari and Mopane woodlands. The study further recommends optimising seeding depth according to seed size and soil texture, as well as applying bio-inoculants during early seedling development under favourable microbial conditions. Furthermore, adjusting watering regimes and shade-net management according to seasonal conditions, together with the use of larger polybags to support root development, can enhance nursery performance. These findings provide important insights for improving assisted regeneration and sustainable nursery practices for indigenous tree species in Namibia. |
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