Daily Weather and Tree Growth at the Tropical Treeline of North America

We present here the 2001–2004 results of observational field studies aimed at quantifying tropical timberline climate and radial increment of Pinus hartwegii Lindl. trees on Nevado de Colima, in the middle of the North American Monsoon region. An automated weather station was installed at 3760 m a.s.l., 19834.7789N latitude, 103837.1809W longitude, within a forest where multi-century tree-ring records had been previously developed. At the same time, automated electronic sensors for recording tree growth at 30-min intervals were set up at two sites within a 1-km radius from the weather station. Meteorological observations recorded every 30 min were summarized on a daily basis. Time-series patterns are reported for atmospheric pressure, precipitation, incoming solar radiation, air and soil temperature, relative humidity, soil moisture, and wind speed and direction. Of particular interest is the sudden decrease in air temperature after the onset of the monsoon season, which determines very high relative humidity over the summer and results in air temperature having a spring maximum. Despite sub-freezing air temperatures in most months, soil temperatures never drop below 08C. Dendrometer data show that the timberline growing season begins in March–April as temperature increases, then radial growth continues throughout the cool-wet summer monsoon, and ends in October– November. As an unexpected result, it was also possible to measure the progressive decline of Pinus hartwegii stem increment in response to an outbreak of roundheaded pine beetle (Dendroctonus adjunctus Blandford), which ultimately killed most trees at one of our two experimental sites.