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

Abstract

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. Such intense forest management practices come along with rotational cycles as short as 9-25 yrs, the construction of dense forest road networks, and the fostering of wildfire susceptibility due to the high amounts of fuel provided by dense plantation stands. Here we first compare decadalscale catchment erosion rates from suspended sediment loads with a 10ˆ4-years-scale catchment erosion rate estimated from detrital 10 Be. We then explore these erosion rates against the effects of discrete disturbances and hydroclimatic trends. Erosion 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 w 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 10ˆ4-years erosion rate agrees with a landscape dominated by slow soil creep. The low 10-years-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.