Pinus hartwegii is a high elevation species forming the upper treeline in Mexico- from volcanoes of the Transmexican Volcanic Belt in central Mexico and from high elevation peaks of the Sierra Madre Oriental in northeastern Mexico. Pure stands of hartwegii pine have been severely logged in the past affecting the proper functioning of the ecosystem, impacting water yield, biodiversity, and other ecosystem services in detriment of the wellbeing of dense human settlements in the Valley of Mexico. In addition to land-use changes, climate warming may threat this ecosystem by altering their health, favoring the recruitment over the treeline where is not adapted, and affecting its dynamics, growth rates, and ecological relationships with associated species. Given the dendrochronological potential of hartwegii pine to produce centuries-long time series useful to analyze high and low frequency climate variability, and influence of atmospheric circulatory patterns, the objective of this study was to develop a network of treering chronologies for central and northeastern Mexico, analyze its potential for dendroclimatic reconstructions and to determine the potential impact and teleconnections of atmospheric circulatory patterns. Nine ring-width series of hartwegii pine were developed for the upper conifer forest of some of the main volcanic peaks in the Valley of Mexico and two more series were produced from trees located at the highest peaks in the Sierra Madre Oriental. A Principal Component Analysis of the chronologies defined two eigenvectors with the highest climate variability, the first component integrated several chronologies from central Mexico, e.g., Nevado de Colima, Pico de Orizaba, Iztaccihuatl, and la Malinche, and the second component from northeastern Mexico included Cerro Potosí, N.L. and Cerro La Viga, Coahuila. The chronologies with a common climate response were integrated into representative regional ring-width series, one of them was the chronology for the Transmexican Volcanic Belt with 320 years length (1690- 2009), whereas the one for the northeastern region covered the last 590 years (1420-2009). The regional chronology for central Mexico indicated below normal growth (<1.0) as related to droughts for the periods 1698-1702, 1716- 1720, 1746-1760, 1806-1813, 1841-1859, 1890-1894, 1935-1940, 1970-1979, and 2000-2004. Similarly, below normal growth for the northeastern ring-width series took place in periods 1450-1460, 1508-1538, 1565-1576, 1661- 1673, 1696-1705, 1757-1765, 1784-1790, 1804-1808, 1844-1849, 1866-1894, 1915-1933, and 1998-2003. Wet episodes (>1.0) were common in both representative series for the periods 1726 to 1728, 1753 to 1755, 1791 to 1797, 1831 to 1837, 1895 to 1914, 1923 to 1926, 1940 to 1941, 1957 to 1958, 1965 to 1969, 1985 to 1987, and 2001 to 2005. Verification of some of the detected droughts was done by comparisons with historical records of grain yields, colonial rogation ceremony records, dendroclimatic reconstructions, instrumental climate data, and climatic indices (Palmer Drought Severity Index, Southern Oscillation Index, Tropical Rainfall Index). Contrasting the regional ring-width indices for the common period (1690-2009) did not show a significant association (r= 0.087, p<0.05), however, both regional dendrochronological series responded to dry and wet episodes for particular years and showed opposite conditions (dipole) for some other years (i.e., wet in the center, dry in the north, and vice versa). This finding indicates that the chronologies responded mostly to local or regional climatic conditions but in certain periods may have been impacted by circulatory patterns affecting larger areas in northern and central Mexico. ENSO is the most important phenomena determining climatic differences between regions. In this study, the Niña conditions (cold phase) was characterized by producing dry conditions in the north and wet conditions in central Mexico (periods 1869 to 1874, 1886 to 1887, 1915 to 1917, 1955 to 1956, 1970 to 1975, and 1988 to 1999); however, other Niña events produced droughts in both regions as in years 1892, 1893, 1922, 1942, and 1998; but others did not have any significant impact such, i.e., 1903, 1908-1910, 1924, 1949, 1954, 1964, and 2007. On the other hand, the El Niño (warm phase) produced dry conditions in both regions, but only during intense ENSO events, that is 1880, 1888, 1899, 1905, 1913, 1918, 1929-1930, 1940, 1963, 1970, 1972, 1976, 1983, 1991, and 1997. A comparison between the ring-with indices of the regional chronology representative of the Transmexican Volcanic Belt and a seasonal (January- September) mean precipitation for the region indicated a significant association (r= 0.8, p<0.000) between both variables. It was not found a significant association with temperature. Even though temperature at high elevations may limit physiological processes for the species, precipitation seems to be the most limiting factor for growth. The wide distribution range of hartwegii pine from Mexico to Guatemala provides an excellent opportunity to expand the current network of treeline chronologies to better understand climate variability and impacts of climate change.
El bosque de Pinus hartwegii constituye el límite superior arbóreo en las montañas de México. En este estudio se desarrolló una red de cronologías de esta especie, localizadas en volcanes del Eje Neovolcánico Transversal, en el centro del país, y picos elevados de la Sierra Madre Oriental, en el noreste. El Análisis de Componentes Principales integró las cronologías en dos grupos, uno para el centro y otro para el noreste, con los que se desarrollaron dos cronologías regionales de 320 años (1690-2009) y 590 años (1420- 2009), respectivamente. El fenómeno de El Niño Oscilación del Sur (ENSO) en su fase cálida (El Niño) y fría (La Niña), mostró un impacto significativo en el comportamiento de la variabilidad hidroclimática descrita por ambas series. La Niña produjo condiciones climáticas contrastantes, es decir, secas en el noreste y húmedas en el centro, mientras que la fase de El Niño originó sequías en ambas regiones, pero solo durante eventos intensos de ENSO.