Edition

Tree recruitment in West African dry woodlands: The interactive effects of climate, soil, fire and grazing

Biaou, S.S.H. (2009). PhD thesis, Wageningen University, Wageningen, The Netherlands. ISBN: 978-90-8585-318-3

Woodlands are among the most widespread seasonally dry forests, particularly in Africa. In dry regions, woodland is one of the few vegetation types having significant tree cover, thus supplying most forest products. Unfortunately, woodlands are disappearing rapidly due to the combined effects of unrestrained forest exploitation, increasing and frequent droughts, decreasing soil productivity, and disturbances by fire and herbivores. The scope of this study was to analyze how climate, soil, fire and grazing, and their interactions determine woodland dynamics: i.e. the competitive versus facilitative interactions between plants, the regeneration success of seedlings, and the species composition, richness and structure of woodlands. A broad-scale study was conducted in Benin where extensive examples of woodlands are still found. The results from this study confirmed that woodland dynamics is determined not only by the macro-climate (i.e. water stress), but also by soil-driven abiotic stress (i.e. salinity), disturbances by fire and grazing, facilitative plant-plant interactions, as well as interactions between these drivers. Facilitative plant-plant interactions contributed to successful recruitment in woodlands, and thus to high species richness and diversity at intermediate water stress levels. Disturbances by grazing and fire reduced competitive exclusion at intermediate water stress levels and improved the facilitative interactions among woodland tree species. Climate and soil conditions appeared to switch the direction of fire and grazing effects on tree recruitment and diversity, from positive at low abiotic stress levels to negative at high abiotic stress levels. Based on these results, it is suggested that forest managers should tune woodland harvesting in such a way that trees are left to create favourable conditions for new regeneration and plant nurse trees in degraded and open areas to create better conditions for seedling establishment.

Key-words: Fire, Grazing, Water stress, Woodland, Savanna, Species richness, Tree density, Facilitation, Tree clustering, Soil fertility, Soil salinity, Regeneration, West-Africa, Benin.

Honoré Biaou will defend his thesis on March 31, 2009

More information: