Site-specific controls on rockfall activity: A case of study from north and south faces on the Pyrenees

In mountain environments, rockfalls pose a significant natural hazard. Climate change is expected to influence their magnitude and frequency, yet understanding long-term dynamics and specific triggers remains challenging due to scarce observational data. Dendrogeomorphology offers a valuable tool to reconstruct past rockfall passage frequency. This study investigates contrasting rockfall dynamics and their climatic triggers on opposing north (Artouste forest, France) and south (Pineta valley, Spain) Pyrenean faces. We analyzed 261 increment cores from 142 impacted Abies alba Mill. and Fagus sylvatica L. trees across four plots with varying exposure and vegetation density. Rockfall Rate (RRt) was reconstructed from the early 20th century using the conditional impact probability method (CIP method). Terrestrial Laser Scanning (TLS) was used to characterize block sizes and accurately determine tree locations. RRt was statistically analyzed (Spearman, LMMs) against daily E-OBS climate data to identify key climate-rockfall relationships. Our results reveal distinct rockfall dynamics. The north-face exhibited higher activity, strongly linked to autumn freeze-thaw cycles. By contrast, the south-face showed lower activity influenced by both spring/autumn freeze-thaw cycles and intense precipitation events (> 20 mm/day), particularly in the less vegetated plot. Interestingly, our results provide evidence that local site conditions such as release zone lithology and vegetation density strongly modulate the sensitivity to these climatic triggers. Our findings highlight the spatial variability in Pyrenean rockfall dynamics, underscoring the need for site-specific hazard assessments and suggesting potential increases in rockfall activity under projected climate warming.