Influence of 20 years of anthropogenic disturbance, grazing, early fire, and selective cutting on aboveground biomass in savannah woodland of Burkina Faso, West Africa

Abstract

Savanna ecosystems support low biomass per unit area, but they are more widespread than humid forests and could contribute substantially to carbon storage. The objective of this study was to measure biomass accumulation in savannah woodland subjected to disturbances such as grazing, early fire, and selective logging. Biomass was estimated indirectly using allometric equations. To this end, dendrometric variables measured sequentially since 1992 were used. For each measurement period, biomass was calculated for each individual tree. Then, the individual biomass was added together to find the total biomass for each 2,500 m² (50 x 50 m) subplot for each 5-year period. A repeated measures analysis of variance was applied to the data, and a two-tailed test was used to compare the means of the different treatments. Aboveground biomass increased from 50 to 67 t ha-1, representing 35% increase over 20 years. This corresponds to an average annual increase of 0.8 t ha-1 year-1. The impact of early burning on root biomass varied by location: while it had no significant effect in one area, it led to a marked reduction in fine root biomass in the other. Aboveground, early burning consistently reduced wood density across both sites, though a negative impact on annual increment was localized to only one. Selective cutting negatively affected coarse roots in the more sensitive area and reduced aboveground biomass across both study sites. Similarly, grazing impacted both fine and coarse roots in only one location but showed no significant effect on aboveground biomass in either. Notably, the simultaneous application of early burning, grazing, and selective cutting resulted in a cumulative negative impact on aboveground biomass. The difference in the effects of disturbances between sites suggests that the biophysical conditions of the sites must be considered when using them as management tools.