Patch Configuration Non-linearly Affects Sediment Loss across Scales in a Grazed Catchment in North-east Australia

Abstract

A principle of the cross-scale interaction (CSI) framework is that disturbance-induced landscape changes resulting in coarser-grained spatial structure may non-linearly amplify transfer processes across scales. We studied suspended sediment losses at two spatial scales (0.24 m2 plots and ca. 0.25 ha hillslopes of about 140 m in length) in a semiarid savanna landscape to determine whether the spatial structure of grassy and bare soil areas introduced a non-linear amplification of sediment loss. Sediment loss rates from 0.24 m2 bare plots averaged 1.527 t ha−1 y−1, which was 23 times the loss rate from nearby grassy plots (0.066 t ha−1 y−1). These rates were then extrapolated linearly to two hillslopes separated by only 200 m and having similar total grass cover, slope and soil type but differing in the spatial structure of bare soil patches. The coarse-grained hillslope had a large bare patch on its lower slope, whereas the fine-grained hillslope had no bare soil patches when quantified at a 4 m grid-cell resolution. Measured sediment loss from the fine-grained hillslope averaged 0.050 t ha−1 y−1, whereas the average sediment loss from the coarse-grained hillslope was 2.133 t ha−1 y−1. By linearly extrapolating from the plot scale, the expected sediment loss for the fine-grained hillslope was 0.066 t ha−1 y−1, which is similar to that observed. The expected sediment loss for the coarse-grained hillslope was 0.855 t ha−1 y−1, where linear extrapolation assumed a 46:54 ratio of bare to grassy plots and that the spatial arrangement of plots does not affect sediment loss processes. For the coarse-grained hillslope observed sediment loss is 2.5 times greater than that expected by linear extrapolation from the plot scale. This result indicates a cross-scale interaction related to spatial configuration of patches. We suggest that there were non-linearities in hillslope ecohydrological transfer processes (runoff, erosion) across scales due to a specific patch configuration that greatly amplified sediment loss because the pattern failed to slow runoff and retain sediment before it entered a creek. This example supports the CSI framework and indicates the importance of considering the effect of spatial structure when predicting system dynamics at different scales.