Ecosystem processes at the watershed scale: Mapping and modeling ecohydrological controls of landslides

Mountain watersheds are sources of a set of valuable ecosystem services as well as potential hazards. The former include high quality freshwater, carbon sequestration, nutrient retention, and biodiversity, whereas the latter include flash floods, landslides and forest fires. Each of these ecosystem services and hazards represents different elements of the integrated and co-evolved ecological, hydrological and geomorphic subsystems of the watershed and should be approached analytically as a coupled land system. Forest structure and species are important influences on the partitioning of precipitation, the lateral redistribution of water, runoff and sediment production, weathering and soil development. Forest regulation of hydrologic dynamics contributes to the development of patterns of soil pore pressure and slope instability during storms or snowmelt. The spatial patterns of root depth, structure and strength, developed by the below ground allocation of carbon in the forest canopy in response to limiting resources of water and nutrients, contributes to slope stability and drainage, and the maintenance of stomatal conductance linking water and carbon cycling. This in turn provides the photosynthate required to build leaf area, stem and root biomass. The linked ecological, hydrologic and geomorphic systems are characterized by specific catenary patterns that should be captured in any coupled modeling approach. In this paper we extend an ecohydrological modeling approach to include hydrologic and canopy structural pattern impacts on slope stability, with explicit feedbacks between ecosystem water, carbon and nutrient cycling, and the transient development of landslide potential in steep forested catchments. Using measured distributions of canopy leaf area index, and empirically modeled soil depth and root cohesion, the integrated model is able to generate localized areas of past instability without specific calibration or training with mapped landslides. As the model has previously been shown to simulate space/time patterns of coupled water, carbon and nutrient cycling, the integration of slope stability as a function of hydrologic, ecosystem and geomorphic processes provides the ability to closely link multiple ecosystem services with a unified approach.