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.

How to Extract Climate Variability from Tree-Rings.

Tree rings have been used to reconstruct climatological variables in many locations around the world. Moreover, tree-rings can provide valuable insights into climatic variability of the last few centuries and, in some areas, several millennia. Despite the important development, that dendrochronology has had in recent decades to study the dendroclimatic potential of a large number of species present in different ecosystems, much remains to be done and explored. In addition to this, in the last few years more people (students, teachers and researchers) around the world are interested in implementing this science to extend the timeline of climate information backwards and understand how climate has changed on scales of decades, centuries or millennia. Therefore, the objective of this work is to describe the general aspects and basic steps needed to conduct a tree-ring climate reconstruction, from site selection and field sampling to laboratory methods and data analysis. In this method's video and manuscript, the general basis in tree-ring climatic reconstructions is explained so newcomers and students can use it as an available guide into this field of research.

Using Tree-Rings to Reconstruct Fire History Information from Forested Areas.

Annual tree-ring patterns are a source of ecological and environmental information including the history of fires in forested areas. Tree-ring based fire histories include three fundamental phases: field collection, laboratory methods (preparation and dating), and data analysis. Here we provide step-by-step instructions and issues to consider, including the process for selecting the study area, sampling sites, plus how and which fire-scarred trees to sample. In addition, we describe fire-scar sample preparation and dating which are done in the laboratory. Finally, we describe basic analysis and relevant results, including examples from studies that have reconstructed fire history patterns. These studies allow us to understand the historical fire frequency, changes in those frequencies related to anthropogenic factors, and analyzes of how climate influences fire occurrence over time. The description of these methods and techniques should provide a greater understanding of fire history studies that will benefit researchers, educators, technicians, and students interested in this field. These detailed methods will allow new researchers to this field, a resource to start their own work and achieve greater success. This resource will provide a greater integration of tree-ring aspects within other studies and lead to a better understanding of natural processes with forested ecosystems.

Orbital pacing and ocean circulation-induced collapses of the Mesoamerican monsoon over the past 22,000 y.

The dominant controls on global paleomonsoon strength include summer insolation driven by precession cycles, ocean circulation through its influence on atmospheric circulation, and sea-surface temperatures. However, few records from the summer North American Monsoon system are available to test for a synchronous response with other global monsoons to shared forcings. In particular, the monsoon response to widespread atmospheric reorganizations associated with disruptions of the Atlantic Meridional Overturning Circulation (AMOC) during the deglacial period remains unconstrained. Here, we present a high-resolution and radiometrically dated monsoon rainfall reconstruction over the past 22,000 y from speleothems of tropical southwestern Mexico. The data document an active Last Glacial Maximum (18-24 cal ka B.P.) monsoon with similar δ(18)O values to the modern, and that the monsoon collapsed during periods of weakened AMOC during Heinrich stadial 1 (ca. 17 ka) and the Younger Dryas (12.9-11.5 ka). The Holocene was marked by a trend to a weaker monsoon that was paced by orbital insolation. We conclude that the Mesoamerican monsoon responded in concert with other global monsoon regions, and that monsoon strength was driven by variations in the strength and latitudinal position of the Intertropical Convergence Zone, which was forced by AMOC variations in the North Atlantic Ocean. The surprising observation of an active Last Glacial Maximum monsoon is attributed to an active but shallow AMOC and proximity to the Intertropical Convergence Zone. The emergence of agriculture in southwestern Mexico was likely only possible after monsoon strengthening in the Early Holocene at ca. 11 ka.

Modelling vegetation diversity types in Mexico based upon topographic features

Se evaluó el papel de la topografía como variable sustituta para explicar patrones de diversidad de tipos de vegetación (DTV). Se calculó el índice de diversidad de vegetación de Simpson para todo el territorio mexicano utilizando mapas de uso del suelo y vegetación por medio de SIG. Se determinó la correlación entre diversidad de vegetación y algunos atributos topográficos (altitud, rango de elevación, pendiente, rugosidad y exposición), encontrándose correlación significativa entre las variables topográficas y la diversidad de vegetación (Coeficiente de Spearman >0,4, p=0,01 con tres variables: rango y promedio de elevación, y pendiente) con una ventana óptima de 80×80km². Luego se modeló los DTV con base en los atributos topográficos usando un enfoque de redes neurales artificiales. La comparación entre los mapas de DTV modelados y reales mostró que el modelo es una buena estimación de la diversidad de vegetación. El análisis de errores sugiere que los DTV no pueden explicarse totalmente por atributos topográficos, aunque éstos juegan un papel fundamental en los DTV a escala regional y continental. Las coberturas del suelo y la vegetación reflejan los patrones de distribución de la biodiversidad, por lo que la modelación de los DTV es un enfoque prometedor para evaluar la biodiversidad. La conclusión principal es que las variables topográficas, disponibles en diferentes escalas de resolución para la mayor parte del mundo, pueden ser utilizadas para representar patrones regionales de biodiversidad. Este aspecto es crucial en países tropicales, los cuales presentan alta biodiversidad pero frecuentemente carecen de bases de datos sobre coberturas del suelo confiables y actualizadas.