Using dendrogeomorphic and lichenometric approaches for rockfall analysis in the high mountains of Central Mexico

Rockfall represents one of the most destructive geomorphic processes for infrastructure and settlements located at the foot of mountain slopes. Furthermore, it poses a hazard for visitors and hikers. Despite the high anthropic activity in these environments, research on the reconstruction of rockfall in the high mountains of Mexico is still scarce. We used dendrochronological, dendrogeomorphological and lichenometric approaches to study the age and rockfall dynamics in a talus slope in central Mexico. Tree- ring chronologies were constructed from 140 samples of 50 Pinus hartwegii trees, 10 Juniperus monticola shrubs and 16 Ribes ciliatum shrubs to determine the age, frequency and rockfall stability at the upper limit of the forest (~4000 m a.s.l.). 52% of the tree samples showed impact scars, 39% callus tissue, 7% growth suppression and 2% corresponded to trees killed by rockfall. The frequency of rockfalls has increased since the second half of the 20th century, with the 1990s being the period of greatest activity. The years with the greatest disturbance were 1991, 1994 and 1998, possibly due to the intense rainfall that accumulated during the summer, as well as the earthquakes recorded in central and southern Mexico with magnitudes ⩾6. For the lichenometric analysis, 231 thalli of Rhizocarpon geographicum were measured in an active rockfall area. The results suggested three areas of rockfall activity. In the first area the ages were from 61 (±5 year) to 322 years (±41 year). In the second area, the ages were from 12 (±3 year) to 50 years (±12 year). The third area corresponds to an active zone with lichen-free blocks, located near the escarpment. The combination of dendrochronological and lichenometric methods allows a better determination of the minimum ages of rockfall, frequency, spatial distribution, and their possible factor triggers.