Increasing sediment connectivity and decreasing water availability: the anthropogenic impacts of exotic tree plantations on a Mediterranean catchment in central Chile

The Coastal Range in the Mediterranean segment of the Chilean active margin is a soil mantled landscape of gentle hillslopes, able to store fresh water and potentially to support biodiverse native forests. In this landscape, anthropogenic intervention has been increasing soil erosion for ∼200 yr, with the last ∼45 yr experiencing intensive management on exotic tree plantations. Such intense forest management practices come along with rotational cycles as short as 9-25 yrs, depending on the tree species, dense forest road networks, and promoting wildfire susceptibility. 

Here we compare decadal-scale catchment erosion rates from suspended sediment loads with 10⁴-years-scale catchment erosion rate estimated from detritic ¹⁰Be in a ∼400 km² catchment. We relate these rates to land cover dynamics, sediment connectivity modified by forestry roads, and  hydro-meteorologic trends, because the catchment has been widely disturbed by forest management practices, wildfires, and  earthquakes, while an unprecedented drought started on 2010. 

Both, short- and long-term erosion show comparably low rates (0.018 ± 0.005 mm/yr and 0.024 ± 0.004 mm/yr). Recent human-made disturbances include logging operations every season and the building, the maintaining and the heavy machinery traffic on forestry road. Forestry roads often intersect streams, thus forming bypasses to route sediments between hillslopes and valleys. That is, increasing structural sediment connectivity. In addition, one Mw 8.8 earthquake and two widespread wildfires disturbed this catchment in 2010, 2015 and 2017, respectively. Mann-Kendall tests applied to decadal records of rainfall and streamflow resulted in decreasing trends. The suspended sediments fluxes of July also decreases in the same period, yet other subsets of that specific series were ruled out by autocorrelation or by completeness tests. 

The low 10⁴-years erosion rate agrees with a landscape dominated by slow soil creep. The low 10-years-scale erosion rate, however, conflicts with the observed disturbances and the increase in structural sediment connectivity.

The latter results suggest that, either the suspended sediment fluxes are underestimated, or the decennial sediment detachment and transport may be affected by the negative trends on rainfall and streamflows. Sediment mobilization depends mostly on specific thresholds of rainfall intensity on hillslopes and on water discharge in the streams, while the unprecedented drought starting in 2010 together with high water demands of fast-growing tree plantations mean a reduction in water availability. Ultimately, our findings indicate that human-made disturbances and hydrometeorologic trends may result in contrasting effects for the recent mobilization of sediments. However, both are negative for the resilience of ecosystems and then, for humans.