SUMMARYSierra de Manantlán Biosphere Reserve (RBSM) in Jalisco is the most important reserve in western Mexico, where fires are one of the main forest disturbances. In order to reconstruct historical fire regimes, partial sections of Pinus douglasiana with fire scars were collected. Using dendrochronological techniques, the exact dating of 293 scars from 51 trees allowed the reconstruction of fire frequency for the period 1867-2010. We reconstructed mean fire interval of 5.5 years (MFI: all scars) and 3.6 years for the Weibull mean probability interval (WMPI). The MFI (≥ 25 % scarred) was 8.9 years and WMPI was 6.9 years. The seasonal patterns of fire occurrence showed that most fires (68.3 %) were formed in middle earlywood, 30 % in early earlywood and 1.7 % in late earlywood. Considering the phenology of the species, it was determined that 98.3 % of fires occurred in spring and 1.7 % at the beginning of summer. The fires were recorded in dry years, but the relationship was not statistically significant. A strong relationship between droughts and widespread fires was observed. Likewise, it was determined that climate variability was strongly related to ENSO; fires reconstructed from 1956 to 2010 correspond with both El Niño and La Niña events.Key words: Sierra de Manantlán, fire reconstruction, Pinus douglasiana, climate, El Niño Southern Oscillation. RESUMENLa Reserva de la Biosfera Sierra de Manantlán (RBSM) en Jalisco, representa la reserva más importante del Occidente de México, donde los incendios son uno de los principales elementos de la dinámica del bosque. Con el objetivo de reconstruir la historia del fuego, se colectaron secciones parciales de Pinus douglasiana con cicatrices de incendios. El fechado exacto de 293 cicatrices registradas en 51 árboles mediante técnicas dendrocronológicas, permitió reconstruir la frecuencia para el período 1867 a 2010. El intérvalo medio de frecuencia (MFI, por sus siglas en inglés) fue de 5,5 años y un intérvalo medio de probabilidad de incendios (WMPI, por sus siglas en inglés) de 3,6 años, para todas las cicatrices. Al considerar cicatrices que se encuentran en ≥ 25 % de las muestras, se determinó un MFI de 8,9 años y un WMPI de 6,9 años. La estacionalidad de ocurrencia indica, mayor cantidad de incendios (68,3 %) en la mitad de la madera temprana, 30 % al inicio y 1,7 % al final de la madera temprana. Considerando la fenología de la especie se determinó que el 98,3 % de los incendios corresponde a la estación de primavera y el 1,7 % al inicio del verano. Se observó relación entre la disminución de la lluvia (sequías) y la frecuencia de incendios severos, aunque no de manera significativa. Así mismo, se determinó que la variabilidad estuvo relacionada con El Niño Oscilación del Sur (ENSO), los incendios reconstruidos de 1956 a 2010 coinciden con eventos Niño y Niña.Palabras clave: Sierra de Manantlán, reconstrucción de incendios, Pinus douglasiana, clima, El Niño Oscilación del Sur.

Background: Fire scars are the primary source of physical evidence used to date past fires around the world, and to estimate parameters of historical fire regimes and fire-climate relationships. Despite an increase in studies about historical fire regimes and the relationship between fire and climate, these studies are still limited for Mexican forests. Our goal was to characterize the historical fire regime in two sites with different elevations, to assess changes in fire frequency, and to determine the relationship between fire regimes and climate patterns in a conifer-dominated forest in the Upper Nazas watershed of the Sierra Madre Occidental, Mexico. We were able to cross-date 50 fire-scarred samples of Pinus arizonica Engelm., P. strobiformis Engelm., P. teocote Schlecht. & Cham., P. durangensis Martínez, and Pseudotsuga menziesii (Mirb.) Franco and identify a total of 596 fire scars. Results: Evidences are compelling evidences on forest fires occurred frequently in the study area, starting in the mid eighteenth century and through the mid twentieth century. The season of fire occurrence was determined for 560 (94%) fire scars. Most fire scars at both of our sampling locations, found at low and high elevation sites, occurred in the spring (525, 93.8%) and only 6.2% occurred in the summer. The mean fire interval and Weibull median fire interval values were < 10 years for all fire-scar filter (all samples, ≥10% and ≥ 25%), and the average per-sample fire interval was < 25 years. Conclusions: Our results suggest fire frequency has changed over time at our study site, becoming less common and less extensive since the 1950s. In addition, extensive fires occurred in dry years as indicated by low precipitation, negative NIÑO 3 Sea Surface Temperature (SST) values, and negative Palmer Drought Severity Index (PDSI) values. Fire frequencies must be maintained within the amplitude of their historical regime, with the objective of reducing the risk of severe fires and preserving ecological and hydrological benefits in the watershed.

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SUMMARYThe studies on reconstruction of paleoclimatic series play an important role in analyzing regional climate variability when instrumental information is not available. Tree-ring samples from stands of Pinus durangensis, Pinus cooperi and Pinus lumholtzii were used to generate chronologies in order to reconstruct and analyze the historical hydroclimatic variability for the region of San Dimas, Durango, Mexico. A principal component analysis (PCA) indicates that those series showed a common variability; subsequently, a regional series was developed covering a period of 294 years. An analysis of response function suggests that the most important climate variable influencing tree growth was the seasonal winter-spring precipitation (January-June), which accounted for 52 % of the variability. Reconstruction of the variability of the winter-spring precipitation for the last three centuries was possible. Such a profile allowed us to identify important drought periods (1740-1746, 1766-1780, 1785-1786, 1819-1824, 1890-1900, 1935-1940, 1950-1957 and 2011-2015). Because of their extension, the periods 1766-1780, 1890-1900 and 1950-1957 were the longest. Based on historical records, four events of severe drought (1785-1786, 1935-1940, 1891-1902 and 1950) were associated with strong havoc in central Mexico and their effects were extended to northern Mexico. In addition, a wavelet coherence analysis suggests a strong relationship between the reconstructed precipitation (Winter-Spring) and NIÑO 3 SST (December to February) in phase from 1770 to 1978 with frequencies from 1 to 5 years, ENSO has significantly modulated climate variability in this region of the state of Durango.Key words: chronologies, reconstructed precipitation, drought, ENSO. RESUMENLos estudios de reconstrucción de series paleoclimáticas juegan un papel importante para analizar la variabilidad del clima regional cuando no se dispone de información instrumental. Muestras de anillos de árboles de rodales de Pinus durangensis, Pinus cooperi y Pinus lumholtzii fueron empleados para generar cronologías con el objetivo de reconstruir y analizar la variabilidad hidroclimática histórica para la región de San Dimas, Durango, México. Un análisis de componentes principales (PCA) determinó que las series presentan variabilidad común, se desarrolló una serie regional de 294 años. Un análisis de función de respuesta mostró que la variable climática que más ha influido en el crecimiento es la precipitación estacional invierno-primavera (enero-junio), que explicó el 52 % de la variabilidad. Se logró reconstruir la lluvia invierno-primavera para los últimos tres siglos. Esta reconstrucción permitió identificar importantes periodos de sequía (1740-1746, 1766-1780, 1785-1786, 1819-1824, 1890-1900, 1935-1940, 1950-1957 y 2011-2015), por su extensión destacaron los periodos 1766-1780, 1890-1900 y 1950-1957. Con base en archivos históricos, se corroboraron cuatro eventos de sequía severa (1785-1786, 1935-1940, 1891-1902 y la década de 1950) que propiciaron fuertes estrago...