Exotic tree plantations in the Chilean Coastal Range: Balancing effects of discrete disturbances, connectivity and a persistent drought on catchment erosion

The Coastal Range in the Mediterranean segment of the Chilean active margin is a soil mantled landscape able to store fresh water and potentially support a biodiverse native forest. In this landscape, human intervention has been increasing soil erosion for ~200 yr, with the last ~45 yr experiencing intensive management of exotic tree plantations. At the same time, this landscape has been affected by a prolonged megadrought, and how the anthropogenic disturbances and hydrometeorologic trends affect sediment transport is not yet well understood. In this study we calculate a decadal-scale catchment erosion rate from suspended sediment loads and compare it with a 10-year-scale catchment denudation rate estimated from detritic 10Be. We then contrast these rates against the effects of discrete disturbances and hydroclimatic trends. Erosion/denudation rates are similar on both time scales, i.e. 0.018 ± 0.005 mm/yr and 0.024 ± 0.004 mm/yr, respectively. Recent human-made disturbances include logging operations during each season and a dense network of forestry roads, which increase structural sediment connectivity. Other disturbances include the 2010 M 8.8 Maule earthquake, and two widespread wildfires in 2015 and 2017. A decrease in suspended sediment load is observed during the wet seasons for the period 1986-2018 coinciding with a decline in several hydroclimatic parameters. The low 10-year denudation rate agrees with a landscape dominated by slow soil creep. The low 10-year-scale erosion rate and the decrease in suspended sediments, however, conflicts with both the observed disturbances and increased structural (sediment) connectivity. These observations suggest that, either suspended sediment loads and, thus, catchment erosion, are underestimated, and/or that decennial sediment detachment and transport were smeared by decreasing rainfall and streamflow. Our findings indicate that human-made disturbances and hydrometeorologic trends may result in opposite, partially offsetting effects on recent sediment transport, but both contribute to the degradation of the landscape.