White sandy soils (Ustic Quartzipsamments), characterized by rapid drainage and low nutrient availability, are widely distributed across western Madagascar, including Ankarafantsika National Park. These sandy soils, derived from ancient marine deposits, shape unique vegetation mosaics but are highly vulnerable to degradation. Unlike stable Oxisols in the central highlands of Madagascar, these fragile sandy soils require a specific understanding of soil-root interactions for the effective conservation of dense forest ecosystems in Ankarafantsika.
Field surveys were conducted in natural forests and fire-affected areas to investigate soil properties, root distributions, and erosion processes. We measured soil penetration resistance, physicochemical properties, and root biomass distribution (0 to >100 cm depth). Additionally, silt fences were installed to measure soil erosion rates in forests under different fire histories (natural primary (unburned), once-burnt, and twice-burnt forests).
Soils in the natural primary forest consist of more than 95% coarse sand, with nutrients concentrated in the surface layer. Penetration resistance measurements indicated that forest soils remained loose down to 30–50 cm, whereas soils under savanna vegetation were characterized by a hard soil surface crust that impeded root penetration. Correspondingly, soil profile surveys revealed that natural forests develop a dense "root mat" of fine roots in the 0–20 cm depth, while coarse roots extend into the deeper sandy layers (20 to >100 cm). This dual structure likely allows dense forests to thrive on oligotrophic sandy soils: the shallow root mat ensures efficient nutrient and water uptake, while deep roots provide physical support in fragile sandy soils and access to groundwater during the dry season. However, the increasing frequency of wildfires has substantially disrupted these soil-root dynamics. Fire caused severe loss of underground biomass, specifically destroying the nutrient-rich surface root mat. Given the low nutrient- and water-holding capacity of these sandy soils, nutrients released from the ash of burned litter and roots were rapidly leached after wildfires. Furthermore, the loss of the protective above- and below-ground biomass led to significantly higher erosion rates in burned areas, highlighting the critical role of natural primary forest systems in stabilizing sandy soils in Ankarafantsika National Park. Therefore, the conservation of the seasonally dry forest ecosystem is tightly linked to the stability of the invisible underground soil and root systems.