The role of lavaka in the landscapes of Madagascar: a process-based approach.

The role of human disturbance in driving the environmental changes experienced in Madagascar since human settlement remains poorly understood. At the centre of this debate are lavaka, large inverse teardrop-shaped gullies that scar the rolling hills of the central highlands of Madagascar. The physical processes and environmental drivers affecting lavaka formation remain unclear. Therefore, the overarching goal of this study was to shed new light on the long-standing debate surrounding lavaka erosion. We did so by first investigating the long term (1000 - 1 000  000) yr processes and rates at which the Malagasy landscape is shaped, and then assessed how these rates and processes have changed in the more recent past. We showed that long-term erosion rates in Madagascar are very low (2- 51 mm per kyr), where most variations in 10Be-derived erosion rates are explained by differences in river concavity, the frequency of seismic events and lavaka densities. We found that hillslope erosion increases from the hillslope top towards the bottom and that a change in land use from forest to grassland corresponds to an increase in erosion rates of two to three orders of magnitude. By combining floodplain sedimentation data with lavaka birth-, growth- and stabilization rates from historical aerial images and recent satellite imagery (1949-2010s) we inferred that the current lavaka population is strongly growing, where the timing of this increase in erosion and floodplain sedimentation rates corresponds to the timing of cattle introduction and a growing human population in the central highlands. High-resolution digital elevation models allowed to establish an area-volume relationship for lavaka, which enabled the derivation of current lavaka mobilization rates based on the observed growth over the period 1949-2010s. Lavaka currently mobilize between 7 and 129 tonne per hectare per year, or 500 to 9 000 mm per kyr. With this work we show that the lavaka erosion processes have recently increased and that this is likely related to human influences.