Abstract

Global sedimentation rates increased during the Quaternary due to frequent landscape adjustment to climatic oscillations. Although high rates of sediment transport are commonly associated with glacial conditions in mountainous terrain, the influence of climate and vegetation on geomorphic response is poorly constrained outside of glaciated settings. Along a low-gradient (<30%) hillslope-valley transect on a moderately dissected, loess-mantled fluvial terrace in the Charwell Basin, New Zealand, we coupled records of colluvial infilling and vegetation change (via phytoliths) to show that late Pleistocene sediment flux was ~0.0012 m3 m−1 a−1 under a shrubland/grassland mosaic and Holocene sediment flux was ~0.0022 m3 m−1 a−1 under forest. This near doubling through the last glacial-interglacial transition appears to reflect increased bioturbation and downslope soil transport associated with a forest ecosystem. Such an increase in sediment transport contrasts with a contemporaneous decrease inferred for adjacent steep, landslide-prone catchments, suggesting that geomorphic response to climate change is heavily modulated by biology, topography, and geological substrate. Our findings appear to contradict the commonly cited notion that forest colonization universally stabilizes soils and suppresses erosion. Instead, over long time scales, bioturbation associated with forests may increase transport along gentle portions of the landscape not subject to slope instability or erosion by overland flow.

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