Chronology and pedogenic effects of mid- to late-Holocene conversion of forests to pastures in the French western Pyrenees

Abstract

Th is paper presents a place-based examination of the timing and long-term pedogenic eff ects of human-induced forest to pasture conversion in the French western Pyrenees Mountains, Basque commune of Larrau. We analyzed colluvial stratigraphic sections to derive the chronology of landscape change using radiocarbon dating, charcoal concentrations, magnetic susceptibility, and n-alkanes to reveal when forests were replaced by pastures (largely by intentional use of fi re). In addition, we compared properties of native forest soils to those of adjacent long-term pastures using a paired t-test approach. Results indicate that intense burning and clearing occurred in the late Holocene, starting at about 4,000 cal yr BP, but limited fi res occurred on the landscape during the early and middle Holocene. Aft er 4,000 cal yr BP the sedimentation rates signifi cantly increased, constituting “legacy” sediment, but post-4,000 cal yr BP sedimentation rates remain well within a range typical for forested hillsides (< 1 mm yr-1). Th us, erosional degradation is not apparent. Our paired analysis of old-growth forests compared to long-term pastures reveals that soils of millennial pastures are building up by additions of organic matter and phytoliths, as well as by decreases in bulk density of topsoils. Th e pastured A horizons are triple the thickness of those in forests, and pastures have signifi cantly lower bulk densities, resulting in much more rapid water infi ltration capacities. Although the concentrations of some inorganic nutrients in the pastured soils are signifi cantly lower than in forested soils (mg kg-1 basis), the overall result is that the soils in pastures are of higher quality than those in forests. Melanization of the pastured profi les is an indirect anthropogenic process that has built-up the A horizons through time, which testifi es to the importance of human agency in long-term soil evolution. Th e agropastoral uplands of Larrau stand in contrast to conventional degradation narratives of millennially grazed landscapes. Th e apparent sustainability of this landscape suggests that over the long term, agropastoral land use actually can result in changes to soils and landscapes that facilitate conservation.