Combining exposed tree roots and UAV imagery to quantify land denudation in central Mexico.

Approximately 42 % of Mexico is affected by soil denudation resulting from moderate to severe sheet erosion and gullying processes. At Huasca de Ocampo (central Mexico), soil degradation has been linked to intense land use dating back to pre-Hispanic times as well as to unfavorable geological, geomorphic, and climatic conditions. Here, we quantify erosion rates with high precision at annual to multi-decadal timescales by combining, for the first time, dendrogeomorphic reconstructions and UAV-based remote sensing. To assess rates of sheet erosion and gullying processes over the longer-term erosion rates (10-60 yrs), we assessed the age and first exposure of 159 roots to determine sheet erosion rates and gullying processes. At shorter timescales (<3 yrs), we employed an Unmanned Aerial Vehicle (UAV) to develop digital surface models (DSMs) for February 2020 and September 2022. Exposed roots provided evidence of sheet erosion ranging between 2.8 and 43.6 mm yr-1 and channel widening ranging between 11 and 270 mm yr-1, with highest erosion rates found along gully slopes. The UAV-based approach pointed to intense gully headcut retreat with rates between 164.8 and 870.4 mm yr-1; within gullies, channel widening rates ranged between 88.7 and 213.6 mm yr-1 and gully incision rates were between 11.8 and 109.8 mm yr-1. The two approaches yielded very comparable results regarding gully erosion and channel widening; this underlines the potential of using exposed roots to quantifying soil degradation processes retrospectively and considerably beyond the period covered by UAV imagery.