Radiocarbon analysis of Pinus lagunae tree rings: Implications for tropical dendrochronology

A promising species for tropical dendrochronology is Pinus lagunae, a pine tree found in Baja California Sur (Mexico) around lat 23.5 degrees N. In 19 95, we sampled a total of 27 wood cores from 13 Pinus lagunae trees in Sier ra La Victoria (23 degrees 36'N, 109 degrees 56'W),just north of Sierra La Laguna, at an elevation of 1500-1600 m. Selected trees were locally dominan t, but their ring-width patterns could not be crossdated. To test the hypot hesis that visible growth layers in Pinus lagunae are formed annually, we m easured radiocarbon amounts in individual rings by means of accelerator mas s spectrometry (AMS). Twenty-three C-14 measurements were used to trace the location of the 1963-64 "bomb spike" in 3 wood increment cores. By compari ng the location of that Delta(14)C extreme with the number of visible radia l wood increments, it was possible to conclude that 2 cores had a number of locally absent rings, while the 3rd one included a few years with more tha n one growth layer. Therefore, ring-width patterns of sampled Pinus lagunae were not consistent from one tree to another, most likely because of clima tic regime in combination with microsite features. While the possibility of generating Pinus lagunae tree-ring chronologies cannot entirely be ruled o ut, the development of dendrochronological proxy records of climate from co niferous species in tropical North America should focus on species and site s that experience a more pronounced seasonality.