The effect of changing climate and vegetation on the topographic evolution of catchments in the Chilean Coastal Cordillera over millennial timescales

Abstract

The complex interplay between climate change, the composition and density of surface vegetation cover and physical surface processes has been a focus of scientific research in the field of geomorphology for the last few decades. The classical approach for explaining differences in topography only considers the influence of tectonic processes and lithological material constants as endogenic forcings and the effect of precipitation as an exogenic forcing, where the stabilizing effect of vegetation cover has mostly been neglected. To develop a more complete view of the complex topographic system and incorporate the dynamics between climate and vegetation, a numerical model framework consisting of a dynamic vegetation model (LPJ-GUESS) and a landscape evolution model (Landlab) was developed and tested against four different study areas situated in the Chilean Coastal Cordillera on the basis of‚ available paleoclimate data. These study areas were chosen based on homogeneous endogenic forcings, with a comparable tectonic uplift rate and the same granodioritic lithology, all of which allow for better parameterization of surface processes. Three main simulation experiments were conducted: 1. LPJ-GUESS simulations, which gave insight about large-scale dynamic adjustments of vegetation cover and composition for the respective climate zones. 2. Landlab simulations which were designed to reproduce steady-state topographic metrics observed in the focus areas and to determine the reaction of the topographic system to transient, external changes in precipitation or vegetation cover 3. Coupled simulations with direct feedback during model-runtime between LPJ-GUESS and Landlab for the timeperiod since the last glacial maximum (21ka before present) to present day. The experiments show a complex reaction of both vegetation and topography to climatic forcings, with absolute changes in vegetation cover not exceeding 10%, but large-magnitude adjustments of plant composition due to changes in climate. Simulations show magnitudes and time-scales of adjustment are highly dependent on initial catchment vegetation cover and amount of annual precipitation received. Coupled simulations show large short-term variations in catchment-mean erosion rates for very arid and very humid focus areas, while areas with mediterranean climate show lesser magnitude fluctuations but a more pronounced long-term decrease in mean erosion rates. In summary, this thesis helps in understanding the complex climate-erosion interactions by showing the nature of the threshold-controlled system which governs the reactions of topography to natural changes in climate or vegetation cover.