30 Aug 2023
 | 30 Aug 2023
Status: this preprint is open for discussion.

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

Violeta Tolorza, Christian H. Mohr, Mauricio Zambrano-Bigiarini, Benjamín Sotomayor, Dagoberto Poblete-Caballero, Sebastien Carretier, Mauricio Galleguillos, and Oscar Seguel

Abstract. The Coastal Range in the Mediterranean segment of Chile is a soil mantled landscape with potential to store valuable supplies of fresh water and support a biodiverse native forest. Nevertheless, human intervention has been increasing soil erosion for ~200 yr, with intensive management of exotic tree plantations during the last ~45 yr. At the same time, this landscape has been affected by a prolonged megadrought, and is not yet well understood how the combined effect of anthropogenic disturbances and hydrometeorologic trends affect sediment transport at the catchment scale.

In this study we calculate a decadal-scale catchment erosion rate from suspended sediment loads and compare it with a 104-year-scale catchment denudation rate estimated from detritic 10Be. We then contrast these rates against the effects of discrete anthropogenic 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 two widespread wildfires (2015 and 2017) and one Mw 8.8 earthquake (2010).

We observe a decrease in suspended sediment load during the wet seasons for the period 1986–2018 coinciding with declines in streamflow, baseflow and rainfall. The low 104-year denudation rate agrees with a landscape dominated by slow diffusive soil creep. However, the low 10-year-scale erosion rate and the decrease in suspended sediments are not in agreement with the expected effect of intensive anthropogenic disturbances and increased structural (sediment) connectivity. These paradox 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 erosion, but both contribute to the landscape degradation.

Violeta Tolorza et al.

Status: open (until 11 Oct 2023)

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Violeta Tolorza et al.

Violeta Tolorza et al.


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Short summary
We calculated disturbances and landscape lowering rates at different time scales on a ~406 km2 catchment in the Chilean Coastal Range. There, intensive management of exotic tree plantations includes short rotational cycles (planting-harvest by clear cut-replanting) of 9–25 years long, dense networks of forestry roads increasing connectivity, and a recent increase in wildfires. At the same time, persistent drought conditions and high water demand of fast growing trees reduces water availability.