Proximity to natural forests failed to rescue a declining agroforestry tree species

Abstract

The conservation of species in human-modified landscapes such as agroforestry systems is central to global conservation and sustainable use of biodiversity. Understanding the dynamics of key species in agroforestry systems is important for predicting the sustainability of these systems. However, the population dynamics of agroforestry tree species are often unknown, and planning for the management of these systems is generally grounded on the untested assumption that the tree component will persist over time. Here, we developed an integral projection model for Vitellaria paradoxa (Sapotaceae), one of the most important tree species in traditional agroforestry systems in West Africa. We collected three years of demographic data from six populations around the Pendjari Biosphere Reserve in Benin to parameterize this model and test the effect of populations’ proximity to natural habitats on the short- and long-term dynamics of V. paradoxa. However, models suggest that V. paradoxa populations are projected to decline at a yearly rate of 10.6% and within the next two decades, this tree will be locally extinct. We found a significant positive effect of proximity to natural forest on the long-term population dynamics, but this failed to save populations from decline. Elasticity analyses show that the best strategy to slow down such a decline includes preserving the largest mature trees. However, traditional management systems that protect shea butter trees in agroforestry systems are now replaced by increasing tree cutting to produce firewood, charcoal and carve out mortars and pestles. The decline of this key agroforestry tree poses a broader threat to long-term sustainability of agroforestry systems.