Original paper

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

Leigh, David S.; Gragson, Theodore L.; Coughlan, Michael R.


This paper presents a place-based examination of the timing and long-term pedogenic effects 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 fire). 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 fires occurred on the landscape during the early and middle Holocene. After 4,000 cal yr BP the sedimentation rates significantly 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). Thus, 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. The pastured A horizons are triple the thickness of those in forests, and pastures have significantly lower bulk densities, resulting in much more rapid water infiltration capacities. Although the concentrations of some inorganic nutrients in the pastured soils are significantly 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 profiles is an indirect anthropogenic process that has built-up the A horizons through time, which testifies to the importance of human agency in long-term soil evolution. The agropastoral uplands of Larrau stand in contrast to conventional degradation narratives of millennially grazed landscapes. The 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.