Burn severity, which can be reliably estimated by validated spectral indices, is a key element for understanding ecosystem dynamics and informing management strategies. However, in North Patagonian forests, where wildfires are a major disturbance agent, studies aimed at the field validation of spectral indices of burn severity are scarce. The aim of this work was to develop a field validated methodology for burn-severity mapping by studying two large fires that burned in the summer of 2013–2014 in forests of Araucaria araucana and other tree species. We explored the relation between widely used spectral indices and a field burn-severity index, and we evaluated index performance by examining index sensitivity in discriminating burn-severity classes in different vegetation types. For those indices that proved to be suitable, we adjusted the class thresholds and constructed confusion matrices to assess their accuracy. Burn severity maps of the studied fires were generated using the two most accurate methods and were compared to evaluate their level of agreement. Our results confirm that reliable burn severity estimates can be derived from spectral indices for these forests. Two severity indices, the delta normalized burn ratio (dNBR) and delta normalized difference vegetation index (dNDVI), were highly related to the fire-induced changes observed in the field, but the strength of these associations varied across the five different vegetation types defined by tree heights and tree and tall shrub species regeneration strategies. The thresholds proposed in this study for these indices generated classifications with global accuracies of 82% and Kappa indices of 70%. Both the dNBR and dNDVI classification approaches were more accurate in detecting high severity, but to a lesser degree for detecting low severity burns. Moderate severity was poorly classified, with producer and user errors reaching 50%. These constraints, along with detected differences in separability, need to be considered when interpreting burn severity maps generated using these methods.

RESUMEN. La presencia de nuevas especies arbóreas en ecosistemas boscosos desencadena modificaciones en la dinámica que se reflejan en su estructura. Estos procesos son especialmente relevantes desde el punto de vista de la conservación de bosques nativos. Los bosques de tala (Celtis ehrenbergiana var. ehrenbergiana) y coronillo (Scutia buxifolia) se consideran como una de las principales comunidades boscosas de la provincia de Buenos Aires y se encuentran protegidos en el marco de diversas reservas. En los talares de la Reserva El Destino, reconocidos entre los mejor conservados, se desarrolló un proceso de establecimiento de Ligustrum lucidum, especie que demuestra gran potencial invasor en otros bosques de la Argentina y del mundo. El objetivo de este trabajo fue caracterizar la estructura actual de estos talares e inferir los cambios en la dinámica vinculados con la presencia de esta especie. En la actualidad, estos bosques están dominados por ligustro, tanto en términos de densidad (93%) como de área basal (80%), y la estructura del componente nativo está lejos de los valores característicos. Cerca de 80% de los árboles nativos están muertos, valor muy superior al 20% de mortalidad natural, y se asocian de forma positiva con la densidad de ligustro. La exótica se instaló inicialmente en forma no selectiva y espacialmente aleatoria, y hoy llega a dominar el dosel al superar rápidamente en altura a las especies nativas. El estado actual se alcanzó en aproximadamente 20-25 años, lo cual implica un proceso de cambio estructural muy veloz. Estos resultados indican que estos talares cambiaron profundamente y, dado el grado de modificación, podrían concebirse como un sistema ecológico nuevo, sean ecosistemas noveles o híbridos. Cualquiera sea el enfoque, el control del ligustro y la restauración activa del componente nativo, así como la erradicación en áreas donde el establecimiento es reciente, resultan prioritarios para asegurar la perpetuidad de estos bosques.[Palabras clave: bosques nativos, especies invasoras, cambios estructurales, Ecorregión Pampeana] ABSTRACT. Talares from northeastern Buenos Aires in the presence of Ligustrum lucidum W. T. (Aiton):Changes in forest structure and dynamics. The presence of new tree species in forest ecosystems promotes changes in stand dynamics that are reflected by forest structure. These processes are particularly relevant in native forests conservation. Tala (Celtis ehrenbergiana var. ehrenbergiana) and coronillo (Scutia buxifolia) forests are considered among the main forest communities of Buenos Aires province, in Argentina, and are included in several protected areas. In the El Destino Reserve, a very well conserved native forest is currently in the presence of Ligustrum lucidum, which has demonstrated a high invasive potential in other forest of Argentina and the world. The aim of this work was to characterize present structure of these stands and to infer changes in the dynamics promoted by the presence of L. lucidum. We found that these talares are currently dominat...

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