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.

Potencial dendroclimático del encino rojo, Quercus sideroxyla (Fagaceae) en México / Dendroclimatic potential of red oak, Quercus sideroxyla (Fagaceae) in Mexico

Resumen Introducción: Los estudios dendrocronológicos en México se han basado principalmente en las coníferas, mientras que las especies de madera dura han sido poco estudiadas. Este ha sido el caso del género Quercus, con una alta diversidad taxonómica en el país pero que no ha sido estudiado con fines dendrocronológicos, a pesar de los valores ecológicos y económicos de sus especies. Objetivo: En la presente investigación se determinó el potencial dendroclimático de Quercus sideroxyla en el noroeste de México, y su relación con variables climáticas como precipitación y temperatura. Métodos: La investigación se desarrolló en el estado de Durango en el ejido Chavarría Viejo en las coordenadas (23º43' N & 105º33' W), se recolectaron muestras de 5 a 7 cm en dos sitios bajo aprovechamiento forestal y se procesaron mediante técnicas dendrocronológicas convencionales. Resultados: Se desarrolló una cronología de ancho de anillo total, la cual se compuso por 30 muestras de 16 árboles para el período de 1917 a 2018 (101 años). Se obtuvieron valores de intercorrelación entre series de 0.43, de sensibilidad media de 0.36, relación señal-ruido de 3.53 y autocorrelación de primer orden (0.58). En cuanto a la relación clima-crecimiento, los valores de índice de ancho de anillo se correlacionaron con datos de la estación climática más cercana al sitio de estudio; donde la precipitación invierno-primavera (enero-mayo) fue la variable de mayor influencia en el crecimiento de la especie. Conclusiones: Con base en los parámetros dendrocronológicos se demuestra el alto potencial de la especie para ser empleada en estudios dendroclimáticos en la región, la respuesta de la especie a la precipitación es similar al de las coníferas con las que cohabita.

Abstract Introduction: Dendrochronological studies in Mexico have been mainly based on conifers, while hardwood species have been little studied. This has been the case of the genus Quercus, which has a high taxonomic diversity in the country but has not been previously studied for dendrochronological issues, despite the ecological and economic values of oak species. Objective: In the present investigation, the dendroclimatic potential of Quercus sideroxyla in Northwestern Mexico was determined, as well as its relationship with climatic variables such as precipitation and temperature. Methods: The research was carried out in the state of Durango, Chavarría Viejo with coordinates (23º43' N & -105º33' W). Samples of 5 cm to 7cm were collected in two sites under forest exploitation and processed by conventional dendrochronological techniques. Results: A chronology of total ring width was developed, which was composed of 30 samples from 16 trees for the period from 1917 to 2018 (101 years). Inter-series intercorrelation values of 0.43, mean sensitivity of 0.36, signal-to-noise ratio of 3.53 and first-order autocorrelation (0.58) were obtained. Regarding the climate-growth relationship, the ring width index values were correlated with data from the climatic station nearest to the study site, where winter-spring precipitation (January-May) was the variable with the greatest influence on the growth of the species. Conclusions: Based on the dendrochronological parameters, the high potential of the species that were used in dendroclimatic studies in the region has demonstrated that the response of the species to precipitation is similar to that of the conifers with which the Quercus sideroxyla shares its habitat with.

Reconstrucción de precipitación y temperatura con anillos de crecimiento anual del ciprés Taxodium mucronatum (Taxodiaceae) en Coahuila, México / Reconstruction of precipitation and temperature with annual growth rings of the cypress Taxodium mucronatum (Taxodiaceae) in Coahuila, Mexico

Introducción: La determinación del clima en cuencas productoras de agua, como la del río Sabinas, Coahuila, donde no existe información de su variabilidad, se puede estimar mediante anillos de crecimiento de ahuehuete (Taxodium mucronatum Ten.), especie longeva con anillos bien definidos, presente en las zonas ribereñas, constituye un "proxy" o método indirecto de la variabilidad climática interanual y multianual. Objetivo: Desarrollar una reconstrucción estacional de precipitación y temperatura máxima y analizar la influencia que ejercen fenómenos de circulación global en el crecimiento de la especie. Métodos: Los núcleos de crecimiento de T. mucronatum fueron datados a través de técnicas dendrocronológicas para producir una serie de ancho de anillo. Se utilizaron datos de mallas del clima de dos bases de datos para desarrollar el análisis de la función de respuesta con fines de reconstrucción climática. Resultados: Una cronología de anillo total de 218 años (1808-2018) se desarrolló con especímenes de T. mucronatum en parajes del Río Sabinas, con la que se generó una reconstrucción estacional de precipitación abril-junio y de temperatura máxima junio-julio. El periodo detectado más seco se presentó de 1815 a 1818 con una precipitación de 27.39 mm y una temperatura máxima de 41.16 °C; mientras que el año más húmedo fue 1828 con 393.72 mm. La cronología de anillo total se correlacionó con los índices de sequía SPEI y PDSI, y con índices de fenómenos atmosféricos como El Niño Oscilación del Sur, a través del Índice de Oscilación del Sur (SOI) y el índice Multivariado (MEI); Oscilación Decadal del Pacífico (PDO) y Oscilación Multidecadal del Atlántico (AMO). La relación entre el índice de anillo total y el del Índice Estandarizado de Precipitación Evaporación (SPEI) mostró significancia en el mes de junio (r = 0.52, P < 0.01), al igual que el Índice de Severidad de Sequía del Palmer (PDSI), PDSI anual (r = 0.38, P < 0.05). El SOI reconstruido noviembre-febrero, se asoció significativamente con la serie dendrocronológica (r = -0.41, P < 0.01). La Oscilación Decadal del Pacífico y la Oscilación Multidecadal del Atlántico, no mostraron significancia. Conclusiones: En este estudio, desarrollamos una función de respuesta climática y reconstruimos variables climáticas estacionales (precipitación, temperatura máxima) de importancia para desarrollar estrategias de manejo para la conservación de T. mucronatum en esta cuenca, e implementar acciones de mitigación para la presencia de eventos climáticos extremos que se pueden presentar en los próximos años.

Introduction: Dendroclimatic reconstructions in water-yield basins lacking hydroclimatic data, such as the Rio Sabinas is important to analyze its interannual and multiannual climatic variability. One of the species useful for this purpose is the Montezuma baldcypress (Taxodium mucronatum Ten.), a long-lived species with well-defined annual rings, present along the riparian zone of the Rio Sabinas that constitutes a "proxy" of interannual and multiannual climate variability. Objective: Develop a seasonal precipitation and maximum temperature reconstructions, and to analyze the influence of global circulatory modes on the species annual radial increase. Methods: Increment cores of the Montezuma baldcypress specimens were dated through dendrochronological techniques to produce a ring-width series. Climate gridded data from two databases were used to develop a response function analysis for climate reconstruction purposes. Results: A ring-width chronology extending from 1808 to 2018 (211 years) was developed and used to develop a seasonal April-June precipitation and a mean June-July maximum temperature reconstruction. The driest period detected on the rainfall reconstruction occurred from 1815 to 1818 with 27.4 mm and a maximum temperature of 41.2 °C; while the wettest year was 1828 with 393.72 mm. The ring-width chronology was correlated with the Standardized Precipitation Evaporation Index (SPEI) and the Palmer Drought Severity Index (PDSI), and with indices of atmospheric phenomena such as El Niño Southern Oscillation (ENSO), through the Southern Oscillation Index (SOI) and the Multivariate Index (MEI); Pacific Decadal Oscillation (PDO), and the Atlantic Multidecadal Oscillation (AMO). The relationship between the ring-width series and drought indices (SPEI, PDSI) was significant in June (r = 0.52, P < 0.01), and June-August (r = 0.38, P < 0.05) for the SPEI and reconstructed PDSI, respectively. It was found a significant association between the ring-width chronology and the reconstructed November-February SOI (r = -0.41, P < 0.01). The Pacific Decadal Oscillation and the Atlantic Multidecadal Oscillation were not significant. Conclusions: On this study, we developed a climatic response function and reconstructed seasonal climatic variables (precipitation, maximum temperature) of importance to develop management strategies for conservation of the Montezuma bald cypress on this basin, and to implement mitigation actions for the presence of extreme climatic events that may occur in coming years.

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.

Reconstrucción histórica de la precipitación en la Reserva de la Biosfera El Cielo, México, mediante anillos de crecimiento en Taxodium mucronatum (Cupressaceae) / Historical precipitation reconstruction of El Cielo Biosphere Reserve, Mexico, using Taxodium mucronatum (Cupressaceae) annual growth rings

Introducción: El conocimiento histórico del clima es fundamental para analizar su variabilidad en el tiempo y su impacto en los ecosistemas y poblaciones humanas. Objetivo: Analizar el crecimiento anual de los árboles de sabino, también conocido como ahuehuete (Taxodium mucronatum) del río Sabinas para reconstruir la variabilidad de precipitación histórica en la Reserva de la Biosfera El Cielo, Tamaulipas, México. Métodos: Se fecharon los crecimientos anuales de 116 muestras de sabino, a partir de las cuales se desarrolló una cronología de 544 años (1474-2017). El periodo que sustentó un número de muestras adecuado para el análisis climático se extiende de 1550 a 2017 (468 años). Resultados: Con base en el análisis de función respuesta se determinó que la precipitación de invierno-primavera (noviembre-mayo) influyó de manera significativa en el crecimiento de los sabinos en el área de estudio (r= 0.77, P < 0.001). La reconstrucción registró una variabilidad climática alta a nivel interanual y entre décadas, en la cual, se identificó la presencia de sequías a principios y finales de cada siglo, así como la presencia de sequías extremas cíclicas cada 50 años. Estas sequías destacaron por su impacto social y económico a nivel nacional y regional. Las sequías más importantes son el "Año Uno Conejo" de acuerdo con el calendario Azteca, en 1558; la sequía de1696 con un impacto fuerte en el noreste de México; "El Año del Hambre" en 1785-1786, en la Reserva de la Biosfera El Cielo desde 1784; la sequía de 1801 que trajo consigo la escasez de alimentos; la de 1951 dentro del periodo de "Migración masiva del sector rural"; y la sequía más reciente, registrada en 2011. Las lluvias extremas más representativas sucedieron en 1756 y 1816. Conclusiones: De seguir el patrón de precipitación registrado en la Reserva de la Biosfera El Cielo, es posible esperar la presencia de sequías extremas a mediados y finales del siglo XXI.

Introduction: Historical knowledge of climate is essential to analyze its variability over time, as well as its impact on natural ecosystems and human populations. Objective: To analyze the annual growth of the sabino trees, also known as ahuehuete trees (Taxodium mucronatum) from the Sabinas River to reconstruct the historical variability of precipitation in El Cielo Biosphere Reserve, Tamaulipas, Mexico. Methods: The annual growth of 116 sabino samples was dated to develop a chronology of 544 years (1474-2017). The period that admitted a series of samples suitable for climate analysis extends from 1550 to 2017 (468 years). Results: Based on the analysis of the response function, it was determined that the winter-spring precipitation (November-May) significantly influenced the radial growth of sabinos in the study area (R= 0.77, P < 0.001). The reconstruction recorded high climatic variability at interannual and interdecadal levels, in which the presence of droughts was identified at the beginning and end of each century, as well as the presence of extreme cyclical droughts every 50 years. These droughts stood out for their social and economic impact at the regional and national level. The most important droughts are the "Año Uno Conejo" ("Year One Rabbit") according to the Aztec calendar in 1558. In 1696 with a strong impact in Northern Mexico. "El Año del Hambre" ("The Year of Hunger") in 1785-1786. Also, the drought of 1801 that brought alongside food shortages. 1951 within the period of "Mass migration of the rural sector"; and the most recent drought, recorded in 2011. The most representative extreme rainfalls occurred in 1756 and 1816. Conclusions: If the precipitation pattern registered at El Cielo Biosphere Reserve continues, it is possible to expect the presence of extreme droughts in the mid and late 21st century.

Continental-scale tree-ring-based projection of Douglas-fir growth: Testing the limits of space-for-time substitution.

A central challenge in global change research is the projection of the future behavior of a system based upon past observations. Tree-ring data have been used increasingly over the last decade to project tree growth and forest ecosystem vulnerability under future climate conditions. But how can the response of tree growth to past climate variation predict the future, when the future does not look like the past? Space-for-time substitution (SFTS) is one way to overcome the problem of extrapolation: the response at a given location in a warmer future is assumed to follow the response at a warmer location today. Here we evaluated an SFTS approach to projecting future growth of Douglas-fir (Pseudotsuga menziesii), a species that occupies an exceptionally large environmental space in North America. We fit a hierarchical mixed-effects model to capture ring-width variability in response to spatial and temporal variation in climate. We found opposing gradients for productivity and climate sensitivity with highest growth rates and weakest response to interannual climate variation in the mesic coastal part of Douglas-fir's range; narrower rings and stronger climate sensitivity occurred across the semi-arid interior. Ring-width response to spatial versus temporal temperature variation was opposite in sign, suggesting that spatial variation in productivity, caused by local adaptation and other slow processes, cannot be used to anticipate changes in productivity caused by rapid climate change. We thus substituted only climate sensitivities when projecting future tree growth. Growth declines were projected across much of Douglas-fir's distribution, with largest relative decreases in the semiarid U.S. Interior West and smallest in the mesic Pacific Northwest. We further highlight the strengths of mixed-effects modeling for reviving a conceptual cornerstone of dendroecology, Cook's 1987 aggregate growth model, and the great potential to use tree-ring networks and results as a calibration target for next-generation vegetation models.

Respuesta del crecimiento de Pinus oocarpa a variables climáticas en Chiapas, México / Growth response of Pinus oocarpa to climatic variables in Chiapas, Mexico

Resumen Los estudios dendrocronológicos se utilizan para reconstruir algunas variables climáticas; en México, estos estudios se han centrado en los bosques templados del centro y del norte, donde los árboles presentan anillos anuales bien definidos. Pocos estudios se han llevado a cabo en la parte sur del país, donde el crecimiento anual del anillo no se identifica fácilmente y, por lo tanto, esto hace que la datación se dificulte. Se analizó el potencial dendrocronológico de Pinus oocarpa para la reconstrucción de variables climáticas en la porción noroeste de Lagunas de Montebello, Chiapas. Empleando un muestreo selectivo se recolectaron 65 núcleos de incremento de 34 árboles. Aunque las muestras presentaron una alta frecuencia de anillos falsos (8 a 60 %), se logró fechar 30 muestras de 22 árboles (46 %) mediante técnicas dendrocronológicas estándar y el desarrollo de cronologías de anillo total, madera temprana y madera tardía para un período de 91 años (1925-2015). Se encontró una influencia significativa de la precipitación media y de la temperatura media máxima y mínima del período 1961-2004 sobre el crecimiento anual de P. oocarpa. Los resultados muestran que la precipitación inviernoprimavera (enero-mayo) fue la más importante para el crecimiento del anillo anual de la especie. Sin embargo, la correlación más alta se observó entre la precipitación de primavera (marzo-mayo) y la cronología de la madera temprana (r = 0.719, P < 0.05). La cronología de la madera temprana también mostró potencial para reconstruir la temperatura mínima (marzo a mayo) (r = 0.732, P < 0.05), mientras que la cronología de madera tardía registra potencial para reconstruir la temperatura máxima (septiembreenero) (r = 0.714, P < 0.05). Estos resultados muestran que P. oocarpa puede emplearse para reconstruir variables climáticas en los trópicos mexicanos. Se recomienda explorar nuevas áreas con árboles más viejos a fin de aumentar la extensión de las cronologías y reconstruir los registros climáticos varios siglos en el pasado.(AU)

Abstract Dendrochronological studies are used to reconstruct some climatic variables; in México these studies have focused on central and Northern temperate forests where trees present well defined annual rings. Few studies have been carried out in the Southern part of the country where annual ring growth is not easily identified and thus makes cross-dating problematic. We analyzed the dendrochronological potential of Pinus oocarpa Schiede for reconstructing climatic variables in the Northwest portion of Lagunas de Montebello, Chiapas. We used a selective sampling approach and collected 65 increment cores from 34 trees. While our samples showed a high frequency of false rings (8 to 60 %), we were able to date 30 samples from 22 trees (46 %) using standard dendrochronological techniques and developed total chronologies for total ring width, earlywood, and latewood for a period of 91 years (1925-2015). We found a significant influence of mean precipitation and mean maximum and minimum temperature over the annual ring growth of P. oocarpa in the period 1961-2004. Our results show that winter-spring precipitation (January-May) was the most important for the species' annual ring growth. However, we found the highest correlation between spring (March-May) precipitation and the earlywood chronology (r = 0.719, P < 0.05). The earlywood chronology also showed potential for reconstructing minimum temperatures (March to May) (r = 0.732, P < 0.05), while the latewood chronology had the potential for reconstructing the maximum temperature (September to January) (r = 0.714, P < 0.05). These results showed that P. oocarpa can be used to reconstruct climatic variables in the Mexican tropics. We recommend that new areas with older trees should be explored in order to increase the depth of chronologies and reconstruct climate records several centuries into the past.(AU)

Testing a pyroclimatic hypothesis on the Mexico-United States border.

The "pyroclimatic hypothesis" proposed by F. Biondi and colleagues provides a basis for testable expectations about climatic and other controls of fire regimes. This hypothesis asserts an a priori relationship between the occurrence of widespread fire and values of a relevant climatic index. Such a hypothesis provides the basis for predicting spatial and temporal patterns of fire occurrence based on climatic control. Forests near the Mexico-United States border offer a place to test the relative influence of climatic and other controls in mountain ranges that are ecologically similar and subject to broadly similar top-down climatic influence, but with differing cultural influences. We tested the pyroclimatic hypothesis by comparing fire history information from the Mesa de las Guacamayas, a mountain range in northwestern Chihuahua, with previously published fire data from the Chiricahua Mountains, in southeastern Arizona, approximately 150 km away. We developed a priori hypothetical models of fire occurrence and compared their performance to empirical climate-based models. Fires were frequent at all Mesa de las Guacamayas study sites through the mid-20th century and continued uninterrupted to the present at one site, in contrast to nearly complete fire exclusion after 1892 at sites in the Chiricahua Mountains. The empirical regression models explained a higher proportion of the variability in fire regime associated with climate than did the a priori models. Actual climate-fire relationships diverged in each country after 1892. The a priori models predicted continuing fires at the same rate per century as prior to 1892; fires did in fact continue in Mexico, albeit with some alteration of fire regimes, but ceased in the United States, most likely due to changes in land use. The cross-border comparison confirms that a frequent-fire regime could cease without a climatic cause, supporting previous arguments that bottom-up factors such as livestock grazing can rapidly and drastically alter surface fire regimes. Understanding the historical patterns of climate controls on fire could inform the use of historical data as ecological reference conditions and for future sustainability.

El Niño-southern oscillation effect on a fire regime in northeastern Mexico has changed over time.

The El Niño Southern Oscillation (ENSO) is a climate-forcing mechanism that has been shown to affect precipitation and the occurrence of wildfires in many parts of the world. In the southern United States and northern Mexico, warm events (El Niño) are associated with moist winter conditions and fewer fires, while cool events (La Niñia) tend to favor dry winters and more fires. We tested this relationship in a region of northeastern Mexico by characterizing the historical fire regime and climatic influences: Fire regimes were reconstructed from fire-scar samples collected from 100 trees in three high-elevation sites on Peña Nevada in southern Nuevo Le6n. The sites were approximately 25 ha each, and the site centers were approximately 1 km apart. The earliest recorded fire occurred in 1521 and the time period we used for analysis was 1645-1929. The sites were characterized by frequent surface fires before the 1920s. In the three sites, mean fire intervals ranged from 8.6 to 9.6 years (all fires) and 11.9 to 18.6 years (fires that scarred > or = 25% of recording trees). The per-tree mean fire return interval was 17 years, and all three sites burned in the same year seven times between 1774 and 1929. After 1929, fires were nearly eliminated in all sites, likely due to human causes. We found a temporal change in the association between ENSO events and fires; before the 1830s La Niña events were significantly associated with fire years, while after the 1830s this association was not significant. In 1998, when the most severe El Niño event of the past century occurred, the three sites experienced severe, stand-replacing fires that killed many trees that had survived multiple surface fires in the past. Prior to the 1830s, fires tended to occur during dry La Niña years, but since then both La Niña and El Niño have been associated with dry years in this region, especially during the last three decades. This result suggests that ENSO effects have changed over time in this location and that phases of ENSO are not consistent indicators of precipitation, fire occurrence, or fire behavior in this area of northeastern Mexico.