The magnitude and spatial extent of influence of Faidherbia albida trees on soil properties and primary productivity in drylands
2016
Gudeta Sileshi,
Faidherbia (Faidherbia albida) is being promoted widely in interventions for combating desertification, regreening of the Sahel and agroforestry projects in dry lands. Some scientists have questioned the wisdom of its wider promotion without clear evidence of its impacts. This review provides novel analyses of the magnitude and spatial extent of its influence on soil properties and primary productivity. A meta-analysis provided evidence for significant increases in soil organic carbon (SOC) (by 46%), total nitrogen (50%), phosphorus (21%), potassium (32%), and yields of maize (150%) and sorghum (73%) under the tree canopy compared to the open area. However, larger increases in SOC and nutrients occurred on inherently nutrient-poor sites than on nutrient-rich sites. Similarly, large increases in crop yields occurred in suboptimal conditions for crop productivity than in optimal conditions. The tree created predictable patterns in soil nutrients and crop yields consistent with distance–decay models of spatial interaction. Its growth and canopy development appear to explain the size dependence of the spatial extent of its influence, with a marked influence observed under large trees than small trees. The review also identified enormous variability in study design and statistical rigor, which appear to mask the expected patterns. Study designs and inferential statistics in current use neither address the intrinsic causality of patterns nor do they offer a mechanistic insight into the observed patterns. The major concerns and their implications are discussed and improvements for future research on single-tree influences on ecosystem properties in dry lands are suggested.
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