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.

Moderate to large earthquakes can increase the amount of water feeding stream flows, raise groundwater levels, and thus grant plant roots more access to water in water-limited environments. We examine tree growth and photosynthetic responses to the Maule Mw 8.8 Earthquake in small headwater catchments of Chile’s Mediterranean Coastal Range. We combine high-resolution wood anatomic (lumen area) and biogeochemical ( of wood cellulose) proxies of daily to weekly tree growth on cores sampled from trees on floodplains and close to ridge lines. We find that, immediately after the earthquake, at least two out of six tree cores show changes in these proxies: lumen area increased and decreased in the valley trees, whereas the sign of change was reversed in trees on the hillslope. Our results indicate a control of soil water on this response, largely consistent with models that predict how enhanced post-seismic vertical soil permeability causes groundwater levels to rise on the valley floor, but fall along the ridges. Statistical analysis with boosted regression trees indicates that streamflow discharge gained predictive importance for photosynthetic activity on the ridges but lost importance on the valley floor after the earthquake. We infer that earthquakes may stimulate ecohydrological conditions favoring tree growth over days to weeks by triggering stomatal opening. The weak and short-lived signals that we identified, however, show that such responses are only valid under water-limited instead of energy-limited tree growth. Hence, dendrochronological studies targeted at annual resolution may overlook some earthquake effects on tree vitality.

A geomorphological key paradigm predicts that intact forests are erosional idle, however comprise an efficient weathering machine sustaining high soil production rates. Only during times of disturbance, e.g., by earthquakes, those forests are observed to jump up to high-erosion-state, then being capable of releasing some of Earth’s highest sediment yields involving massive pulses of organic carbon. Coastal temperate rainforests, in particular, do not only store unparalleled carbon stocks building up a globally important carbon sink, but are also home to high (endemic) biodiversity. Here we document extraordinarily high catchment-averaged denudation rates, across multiple disturbance cycles, under the dense vegetation of the Patagonian rainforests. There, 10 Be-derived denudation rates of >0.8 m kyr^-1 exceed any known value from the entire Chilean Andes orogen, a highly variable >3.000 km long natural laboratory involving steep climatic and topographic gradients. We argue that such high denudation rates are consistent with a first-order control of the rainforest itself. High biomass loads exert a soil surcharge that promotes landsliding already along a relatively low critical slope angle. In contrast, denudation rates from more arid, and less forested sectors of the Chilean Andes though going along with steeper critical slope angles remain below half of our new rates derived from the Patagonian rainforests. Taken together, our study provides indication that denudation, to a higher degree than hitherto agreed on, operates as a continuous process involving soil production, vegetation, physical erosion and ecohydrological processes. Such a holistic denudational continuum, finally, is different from prevailing views that vegetation generally stabilizes hillslopes, thus promoting steep slope gradients, however, limiting landsliding activity.

Dense tree stands and high wind speeds characterize the dense temperate rainforests of southern Chilean Patagonia, where landslides frequently strip hillslopes of soils, rock, and biomass. Assuming that wind loads on trees promote slope instability, we explore the role of forest cover and wind speed in predicting mapped landslides with a robust Bayesian logistic regression. We find that more crown openness and higher wind speeds credibly predict higher probabilities of detecting landslides moderately well regardless of topographic location, though much better in low-order channels and on midslope locations than on open slopes. Wind speed has less predictive power in areas that were smothered by tephra fall from recent volcanic eruptions, while the influence of forest cover remains.