Pinus plantations impact hillslope stability and decrease landscape resilience by changing biogeomorphic feedbacks in Chile

Abstract

Forest plantations with exotic species are planted extensively in the southern hemisphere for soil conservation and shallow landslide mitigation. The extent to which these are suitable for fulfilling their protection goals is the subject of debate. A biogeomorphic framework was applied to link land use, soil conservation and natural hazards. It consists of a feedback loop with the two effect pathways hillslope stability and vegetation fitness. The study site is located in Chilean Patagonia, where thixotropic Andosols are widespread and Pinus plantations were planted initially in conservation areas, and later on private land. We were testing the hypothesis whether soil stability differs between primary and secondary forests, Pinus plantations, wildfire sites (ex. plantations) and pastures. Shear strength, liquid limit, consolidation degree and available water capacity were used as soil stability indicators and set as dependent variables using non-metrical multidimensional scaling (NMDS), representing mechanical and hydrological biogeomorphic interactions. Soil texture, topographic and vegetation properties were post-hoc correlated as independent variables. Vegetation assemblage correlates most strongly with soil indicator variance. The soils under secondary native Nothofagus forests have significantly higher liquid limits than Pinus plantations (41 ± 4.9% vs. 31 ± 14%, p < 0.1, A Horizon). Consolidation degree is higher under secondary forests than in Plantations (A and B Horizon), due to a significantly higher root abundance. Primary forests provide landscape by maintaining the water cycle balance and biodiversity. Secondary forests establish the biogeomorphic feedback loop through mechanical effects and enhancing vegetation fitness. Pinus plantations cause a slight improvement in soil stability properties, but with trade-offs in water balance and vegetation fitness. Landscape resilience is thus impaired by the higher risk of wildfires, erosion and landslides. Pastures show good values in the soil stability parameters, but their biogeomorphic interactions are unlikely to rebuild landscape resilience.