Fine-scale spatial distribution of plants and resources on a sandy soil in the Sahel

Abstract

We studied fine-scale spatial plant distribution in relation to the spatial distribution of erodible soil particles, organic matter, nutrients and soil water on a sandy to sandy loam soil in the Sahel. We hypothesized that the distribution of annual plants would be highly spatially autocorrelated and that this would be positively linked with the spatial distribution of erodible soil particles, soil organic matter and nutrients. Further, we hypothesized that larger vegetated patches (a relatively high vegetation cover at coarser spatial scale) will be stronger linked with erodible soil particles, organicmatter and nutrients than smaller vegetated patches (a relatively low vegetation cover at coarser spatial scale). Generally, for ‘low cover’ and ‘high cover’ plots, spatially confined micro-elevations with a relatively high volume percentage of erodible very fine – medium sand (50–400 μm), were linked with a higher cover of annual plants. The robustness of vegetation patchiness (expressed as the proportion of the total variance accounted for by spatial dependent variance) was significantly higher for the ‘high cover’ plots. For the ‘low cover’ plots, higher vegetation cover was associated with higher elevations, soil moisture, and volume percentage of very fine to medium sand, but lower organic matter, total N and P. For the ‘high cover’ plots, micro-elevations also consisted of a relatively high volume percentage of very fine to medium sand, and this was associated with dryer conditions and higher total N. Additionally, dryer conditions were weakly correlated with higher organic matter. So, micro-elevations were indirectly associated with dryer and more fertile conditions, which was opposite to what we found for the ‘low cover’ plots. We propose that for the ‘low cover’ plots, micro-elevations or textural patches could become sparsely vegetated as seeds of annuals were not easily washed away from their surface. The micro-elevations became islands of humidity and infertility through their different textural origin and leaching by infiltrating rainwater. For the ‘high cover’ plots, surface water in micro-depressionswas not only working as an erosive agent, but also induced a higher soil water content and leaching of nutrients when the surface water is more stagnant. Additionally, we speculate that in the ‘high cover’ plots, annual plants on micro-elevations were more successful in holding the soil, and increasing organic matter and nutrients than in the ‘low cover’ plots. In conclusion, highly dynamical physical soil surface processes and soil surface characteristics are overriding factors explaining spatial plant distribution on a sandy soil in the Sahel.