History of "olivillo" (Aextoxicon punctatum) and Myrtaceae relict forests of Isla Mocha, Chile, during the late Holocene

Here we present fossil pollen and charcoal concentrations results from two stratigraphic columns from Isla Mocha (38 degrees 19'-38 degrees 25'S; 73 d egrees 57'-73 degrees 52'W), Laguna Hermosa (350 m.a.s.l.) and Laguna Huair avos (125 m.a.s.l.). These results suggest that the forest of Isla Mocha wa s maintained at least during the late Holocene,judging from the continuous forest cover during the last 2000 years B.P. The Laguna Hermosa record indi cates that the development of the "olivillo" and mirtaceous forest was inte rrupted by several wet phases, indicated by the dominance of "canelo" (Drim ys winteri) forests most likely associated with flooded or saturated terrai nes, with the presence of paludal herbs (Cyperaceae-Juncaceae), Gunnera tin ctoria and ferns. We discuss the probable effects of tectonic events on the topography and drainage evolution of the island and thus on the vegetation , mainly the emergence of the island beginning 6000 years B.P. and the pron ounced uplift during the last 2000 years B.P. The palynological results are discussed in connection with size structure of monoespecific forests of ol ivillo in Chile, as these exhibit breaks in the lower to intermediate strat a, linked to reduced rates of self-placement. We propose that the maintenan ce of olivillo forests over long period of time is due to the coexistence o f auto- and allogenic processes during the late Holocene. Autogenic process es would be linked to the stability of the community and would be expressed through the gap-phase dynamics. Allogenic processes would be linked to the local changes in soil drainage and landslides, manifestations of tectonic stability. The Laguna Huairavos records shows an abrupt change, from olivil lo forests at the base to a canelo forest associated with herbs, paludal ta xa and ferns after 1760 years B.P. The presence of abundant microscopic and macroscopic carbon associated with these changes strongly suggest that man -made fires could have been the principal factor that caused the observed s uccession. This hypothesis is reinforced by two archeological records, both very close to the pollen sites and coevals with the deposition of sediment at Laguna Huairavos. The archeological sites have abundant lithological ma terials used to cut and wear wood, indicating the importance that the fores ts had for these cultures, and their clearing for agricultural and animal u se.