Coastal temperate rainforests are global players in Earth’s carbon cycle (Keith et al., 2009). At the same time, these forests are frequently disturbed by, for example volcanic eruptions, rain-, and windstorms. All these distinct disturbances may cause and/or trigger shallow landslides, thus unleashing massive pulses of carbon contained in biomass and soils. Cycles of disturbances can either promote carbon-storage (net sink) or -depletion (net source) - depending on the relative reduction in carbon during the distinct disturbance event, the mean frequency of such events, and the recovery time of the disturbed ecosystem. Areas of high and low disturbance frequencies maintain in a relatively lower carbon status due to limited inter-event periods restricting vegetation regrowth or lowered productivity due to depleted resources, respectively. In RETROGRESS, we hypothesize that areas of moderate disturbance (exposure) form spots of relative maximum carbon abundance stored in biomass and soil.

RETROGRESS explores the hillslopes of Douglas Tompkins Pumalín National Park (NP) in Northern Patagonia. This jewel protects pristine Patagonian coastal rainforests that is marked by a highly moderating, cool Pacific maritime climate providing continuously wet conditions that favor growth of dense, contiguous evergreen broadleaf forests. Prominent tree species include Nothofagus nitida, Podocarpus nibigens, Drimys winterii, Amomyrtus meli, Luma apiculata and the long-lived Fitzroya cupressoides (Lara et al., 1993) . Our estimates point to a minimum of 370 tC/ha contained in biomass and up to twice as much in soil organic carbon (Mohr et al., 2017).  The topography is largely a function of glacial erosion, which carved deep fjords between eroded islands and peninsulas, leaving behind a landscape of steep slopes connected to the fjords via flat-bottomed valleys. The regional disturbance regime is mostly conditioned by topography-controlled process agents (Sommerfeld et al., 2018). Recent examples comprise post-eruptive landsliding on steep hillslopes along Chaiten volcano (Korup et al., 2019) and the prime role of wind on triggering landslides in this specific biome (Parra et al., 2021).

Biota is recognized as an important control for Earth surface processes, though their causes and effects are not necessarily unidirectional. For example, forest biomass surcharge may drive landsliding, thus promoting erosion (see preprint Mohr et al., 2022). At the same time, resulting landslide scars open new gaps for vegetation succession which in turn reinforce the hillslope strenght by root cohesion but also provide growing 'sails' for strong Patagonian winds. In Retrogress we test the biomass-wind-landsliding-carbon nexus. To this end, we aim to develop process-based landslide models (Landlab) including time-varying biomass loading, root cohesion, and wind forcing. We feed our models with airborne LiDAR data, field data, hydroclimatic monitoring and environmental seismology. The latter enables us to measure how trees transfer kinetic energy via swaying into the shallow subsurface. 

Finally, temperate coastal rainforests are globally subject to shifts in the climate and disturbance regimes. Globally, such changes are proceeding in similar directions but at different rates. By quantifying disturbance responses in Patagonian forests, we may form useful ‘blueprints’ informing research on coastal temperate rainforests of the Pacific Northwest and New Zealand eventually in cooperation with the Coastal Rainforest Margins Research Network.