Paleovegetation simulations of lowland Amazonia and implications for neotropical allopatry and speciation

Paleovegetation modeling simulations of the lowland Amazon basin were made to assess the relative importance of glacial climate and atmospheric CO2 fo r altering vegetation type and structure, as well as to explore the potenti al physiological mechanisms underlying these ecosystem-level responses. Mod eling results support the view that widespread invasion of grasslands into the Amazon lowlands during the last glaciation was not Likely. Glacial cool ing was probably responsible for maintaining glacial forest cover via its e ffects in reducing photorespiration and decreasing evapotranspiration, whic h collectively improve plant carbon and water relations. Modeling results c onfirm that leaf area index (LAI), a common proxy for canopy density, is hi ghly sensitive to independent and interactive changes in climate and low co ncentration of atmospheric CO2, and the results show considerable region-to -region variation during the last glaciation. Heterogeneous variations in g lacial vegetation LAI may have promoted allopatric speciation by geographic ally isolating species (called vicariance) in the forest (sub)canopy. The p roposed vicariance hypothesis incorporating spatial variations in canopy de nsity conforms to many of the essential tenets addressed by previous neotro pical speciation models, but also helps to overcome some of their inconsist encies. (C) 2001 University of Washington.