HOLOCENE AND PRESENT-DAY EVIDENCES OF RECURRENT POST-FIRE LANDSLIDES: geomorphological responses to climatic and environmental changes.

Given the accelerated pace of climate change (increased droughts and the frequency of intense rainstorms) and human-induced land-use changes, this work assesses the magnitude of their effects on slope evolution in the Serra do Mar mountainous domain of Rio de Janeiro, SE Brazil. Morphological, historical, and functional approaches were integrated to evaluate the conditions controlling landslides in response to Holocene bioclimatic changes and present-day environmental dynamics, with emphasis on the role of fire in intensifying these phenomena. Past processes dynamics were inferred through geomorphological, chronostratigraphic, and palynological evidence. Current studies include geological-geotechnical, hydro-geomorphological, and vegetation analysis; classification of landslide susceptibility and comparison with the January 2011 landslide inventory; monitoring rainfall and soil suction in fire-affected vegetation (degraded forest and herbaceous-shrubby vegetation); in situ tests of Ksat and fire-controlled field experiments. Regionally, colluvial deposits mark distinct landslide episodes throughout the Holocene, with local recurrence intervals of about 300 years. Variations in δ¹³C and palynological analyses suggest significant transformations in vegetation cover during the Mid-Holocene, with a predominance of herbaceous-shrubby post-fire vegetation and pioneer species; spores and pollen grains with mechanical damage, indicative of a high-energy transport environment, attest to landslide transport. Charcoal fragments in colluvial deposits suggest frequent paleofires during the Holocene. Nowadays, recurrent short-term fires (<10 years) replace forests with herbaceous-shrubby vegetation, where most of the landslides (70%, N=382) from the 2011 catastrophic event are concentrated. At some slopes, fires create a hydrophobic layer in herbaceous vegetation, and short roots (≤30 cm deep) reduce evapotranspiration, keeping soil conditions near saturation at 1,5 m depth, even during prolonged droughts. Post-fire, soil suction increases in the upper soil meter in both vegetation types, within a five to six-month delay. During the following rainy season or extreme rainfall, soils tend to saturate completely, leading to rapid suction loss and excess pore pressure that could trigger landslides. In degraded secondary rainforest, dominated by pioneer and early-succession species that sustain rapid hydrological responses to rainfall, the absence of functional anchoring roots would increase the likelihood of landslides.