Annual biomass increment of Xerophytic thickets and sustainability of woody charcoal production in southwestern Madagascar

- Xerophytic thickets from southwestern Madagascar have high level of species endemism.
- Xerophytic thickets above ground biomass productivity is dry 0.38-0.99 t ha−1 year−1.
- Growth duration was evaluated by dendochronological methods to calculate thickets annual biomass increment.
- Xerophytic thickets annual biomass increment is less than the exported woody charcoal biomass.
- Woody charcoal production from xerophytic thickets is not sustainable and will stop within a period of 15 years.

The sustainability of woody charcoal production activity is analysed in xerophytic thickets in southwestern Madagascar. The above ground biomass productivity of xerophytic thickets and the biomass corresponding to woody charcoal production in the Soalara-Sud commune were estimated and compared. All individuals >3 cm diameter in 40 4 × 4 m2 plots were harvested for above ground biomass measurements. Four treatments, defined by soil type (lixisol and calcisol) and distance from villages (near ≤ 4 km; far > 4 km), were tested. The growth rings, assumed to be annual, of the shrub trunk with the largest diameter, presumed to be the oldest specimen on each 4 × 4 m2 plot, were counted to estimate the duration of biomass production on the plot. Above ground biomass productivity was estimated by the ratio between above ground biomass and growth rings number. The mean above ground biomass productivity varied between 0.38 and 0.99 t ha−1 year−1 of dry mass according to the four treatments. It did not vary significantly with soil type and increased with distance from villages on lixisol where woody charcoal is produced. The total above ground biomass of xerophytic thickets used for woody charcoal production on the current woody charcoal production site is around 862.55 t year−1 of fresh matter, equivalent to 107.82 t of woody charcoal. However, the effective woody charcoal production on the study site in 2013 was equal to 600.90 t, which is higher than the woody charcoal production allowed by the xerophytic thickets above ground biomass productivity. Consequently, woody charcoal production activity in the study site is unsustainable and will result in the disappearance of mature individuals belonging to species used for woody charcoal production in less than 15 years. Once this occurs, woody charcoal production will be moved to other xerophytic thickets on calcisol.