Isadora Schneider-Valenzuela

Increasing drought frequency and severity threaten Mediterranean forest ecosystems worldwide, yet mechanisms underlying their resistance and resilience remain insufficiently understood. Positioned at the dry margin of forest existence, these ecosystems are highly vulnerable to reorganization. Because most assessments assume gradual responses to drought, the point of recovery failure remains largely unknown. In central Chile, a multiyear megadrought followed by an exceptional 2019 hyperdrought, reducing precipitation to 30% of the long-term average, caused widespread browning across Mediterranean sclerophyllous forests. Using 161 forest plots surveyed along a 400 km gradient prior to the hyperdrought, we combined field inventory, spectral vegetation indices, and climatic data to disentangle how pre-existing forest attributes and climate jointly shape drought responses. Our results show that pre-drought forest structure and post-drought climatic conditions influence resistance and resilience, strongly mediated by environmental context. While resistance declines smoothly with drought exposure and topographic wetness, resilience drops sharply once the hyperdrought year deficit exceeds ~70%. Furthermore, forest composition modifies both responses along a productivity gradient, showing that homogeneous and heterogeneous forests react differently to drought intensity. Overall, Mediterranean forest responses to extreme drought are highly context-dependent and may shift abruptly beyond critical thresholds. Vulnerability assessments assuming gradual responses may therefore underestimate the speed of ecosystem change. As drought regimes intensify under climate change, accounting for nonlinear variability is essential for anticipating tipping points and designing effective, spatially targeted adaptation strategies.

During the late Nineteenth and Twentieth centuries there was an intense exploitation of the long-lived Nothofagus obliqua forests in the temperate region of South America due to the quality of their hardwood. This exploitation resulted in degradation of this Northern Patagonian ecosystem with severe biodiversity implications. This also has prevented the development of multicentury N. obliqua tree-ring width chronologies, with implications for regional environmental and paleoclimatological studies. The large-scale manufacturing of railroad ties distributed across Chile drove part of this exploitation. This study evaluated the use of this cultural material from abandoned tracks preserved in the southern Atacama Desert to strengthen the existing N. obliqua tree-ring network in Patagonia. We dated this historical wood using classical crossdating ring-width methods corroborated with wiggle matching of radiocarbon series from railroad tree-rings using the hemispheric ¹⁴C curve. Correlation analysis with climate data and paleoclimate field reconstructions shows a clear hydroclimate signal contained in the resulting regional tree-ring record with a consistent spatial pattern across northern Patagonia. The dendrochronological use of railroad ties preserved in the Atacama Desert opens a new avenue for dendroclimatic studies and highlights its heritage importance as a past environmental archive in southwestern South America.

Soil moisture (SM) is a crucial factor in the water cycle, sustaining ecosystems and influencing local climate patterns by regulating the energy balance between the soil and atmosphere. Due to the absence of long-term, in-situ measurements of SM, studies utilizing satellite-based data and tree-ring analysis have become valuable in assessing variations of SM at regional and multi-century scales, as well as determining its effects on tree growth. This information is particularly pertinent in biodiversity hotspots made up of semi-arid ecosystems currently threatened by climate change. In the Mediterranean Chile region (MC; 30°–37° S), an ongoing megadrought since 2010 has resulted in a significant decline in the forest throughout the area. However, the impact of SM on tree growth at a multi-species and regional level remains unexplored. We analyzed a new network of 22 tree-ring width chronologies across the MC to evaluate the main spatiotemporal tree-growth patterns of nine woody species and their correlation with SM, using PCA. We also reconstructed the SM variations over the past four centuries and assessed its connection with large-scale climate forcings. Our results indicate that the primary growth patterns (PC1) explained 27% of the total variance and displayed a significant relationship with SM between 1982–2015 (r = 0.91), accurately reflecting the current megadrought. The tree-ring SM reconstruction covers the period 1616–2018 and shows a strong decrease around the year 2007, revealing an unprecedented recent change in aridity with respect to the last four centuries. The intensity of the South Pacific subtropical anticyclone, which primarily owe their existence to the subsiding branch of the Hadley Cell, appears as the primary climatic mechanism correlated with the reconstruction and the present aridity conditions in MC. The current SM conditions align with anticipated aridity changes in MC, providing a bleak perspective of future regional climate.

Este trabajo presenta nuevo registro sedimentológico y palinológico que reconstruye la historia ambiental durante la transición del Holoceno medio a tardío en el centro sur de Chile (34°-38°S). El sitio de estudio corresponde a un pequeño humedal llamado Laguna Verde (36º47’S, 73º09’W; 22 m s.n.m.; ~4 ha), el cual se emplaza en una micro-cuenca (136 ha) rodeada de vegetación nativa (i.e. bosque esclerófilo). Testigos de sedimentos obtenidos desde el humedal, permitieron reconstruir la evolución de este durante el Holoceno, dando cuenta que la formación del sitio ocurre alrededor de los 6.800 años cal AP, como resultado de un proceso de represamiento por parte de dunas. Posteriormente, se registra un aumento importante en la concentración de materiales líticos movilizados por el viento y la precipitación, dando como resultado la formación de un cuerpo de agua permanente en torno a los 4000 años cal AP. El registro palinológico muestra un predominio de taxa arbóreos y acuáticos durante este periodo, indicando el desarrollo ecosistemas de bosque en la cuenca en respuesta al incremento en las precipitaciones durante este periodo. Finalmente, se observa un cambio abrupto en el registro sedimentológico y palinológico hacia el año 1850AD, relacionado con una transformación en el paisaje vegetal y el régimen sedimentario debido a la actividad antropogénica. En resumen, este estudio proporciona información nueva y relevante para comprender la historia paleoambiental de los ecosistemas costeros del centro sur de Chile.

Multiproxy reconstructions of fire regimes in forest ecosystems can provide a clearer understanding of past fire activity and circumvent some limitations of single proxy reconstructions. While inferring fire history from scars in trees is the most precise method to reconstruct temporal fire patterns, this method is limited in Araucaria araucana forests by rot after fire injuries, successive fires that destroy the evidence and the prohibition of sample extraction from living Araucaria trees. In this context, dendrochemical studies in Araucaria trees and charcoal analysis from sediment cores can complement and extend the time perspective of the fire history in the relictual Araucaria-Nothofagus forests of the coastal range. We used dendrochemical, fire scar and charcoal records from the Nahuelbuta Coastal Range (37.8° S; 73° W) spanning the last 1000 years to reconstruct the fire history. The results indicate that periods with higher fire activity occurred between 1400 and 1650 AD. Long-term changes in the fire regime are related to increased climate variability over the last 1000 years, and especially with the arrival of settlers to the area after 1860 CE. The most severe fire events in the Nothofagus and Araucaria forests occurred when suitable fire-prone conditions were superimposed with high human densities.

Las Soluciones de base Natural (SbN) permiten enfrentar desafíos y riesgos socioambientales mediante la protección, restauración y gestión sostenible de los ecosistemas, proporcionando bienestar humano e incrementando la biodiversidad. En la última década, la Ecorregión Mediterránea de Chile (EMC, 29-37ºS) ha experimentado una continua sequía y numerosos conflictos hídricos asociados al desarrollo de actividades industriales en zonas que proveen servicios ecosistémicos de aprovisionamiento y regulación hídrica. En este contexto se evaluó el uso de SbN en conflictos de escasez hídrica en la EMC como forma de sustentar el diseño de políticas públicas que incorporen esta aproximación. Para esto, se recopilaron conflictos asociados a la escasez hídrica en la EMC y se identificaron SbN para enfrentar conflictos similares en otras regiones del mundo. Se determinaron tres categorías de SbN: (i) restauración de cuencas e infraestructura verde, (ii) infraestructura ancestral y natural y (iii) prácticas agrícolas integradas. Este análisis indica que el uso simultáneo de estas tres estrategias puede contribuir a la solución de conflictos hídricos en la EMC. Las SbN se proyectan como una opción eficiente e integradora para enfrentar los problemas de escasez hídrica, toda vez que sean consideradas dentro de las políticas nacionales y diseñadas según las condiciones ambientales y contexto social de cada sector en conflicto.

Gray mold disease, which is caused by Botrytis cinerea Pers ex. Fr., results in serious economic losses to Lycopersicum esculentum (tomato) crop productivity. In this study, we explored the possibility that mixtures of essential oils (EOs) and their respective hydrolates (HYSs) could be used to control this disease. Thus, EOs and HYSs were obtained from Origanum vulgare, Thymus vulgaris, Citrus limon, and Citrus sinensis by hydrodistillation. In vitro antifungal activities were evaluated, and EC₅₀ values of 15.9 and 19.8 µg/mL were obtained for EOs of thyme and oregano, respectively. These activities are due mainly to volatile compounds, thymol and carvacrol. Results from in vivo assays show that although most tomatoes were infested five days after inoculation, the damage was considerably reduced by the application of an EO/HYS mixture of thyme. The disease incidence indexes of B. cinerea tomato rot, percentage and severity, measured four days after inoculation, were reduced by 70% and 76%, respectively, as compared with the inoculum control. These results suggest that a combination of HYSs and EOs enhances antifungal activity, and that optimization of relative concentrations, volumes, and the nature of the compounds, could design a formulation able to control B. cinerea inoculum on tomato fruits.