POLLEN- AND MODEL-BASED BIOME RECONSTRUCTIONS FOR COLOMBIA: AN APPRECIATION OF ECOLOGICAL COMPLEXITY

Pollen data from forty sites in Colombia are presented as biomes at 3000 year intervals from 18,000 to 6000 yr BP radiocarbon years before present (yr BP), and then 1000 year intervals through to the modern reconstruction. At 18,000 yr BP cool grassland / shrub reflects a generally cool and dry environment, extending to much lower altitudes than presently recorded. This response is reversed after 9000 yr BP when there is a shift to biomes reflective of warmer and wetter conditions. These conditions are strongly recorded at 6000 yr BP; most sites recording more xeric vegetation than the present day. A period of environmental change is recorded around 4000 yr BP when there is a relatively sharp transition to more mesic vegetation. Over the Late Holocene, the changes between sites are not synchronous; in part, reflecting methodological artifact as assignments 'flicker' between biomes. Additionally, the reconstruction is influenced by human-induced vegetation change. We determine the spatial pattern to this by focusing on ruderal components. These increase at 5000 yr BP at a coastal location followed by expansion within savanna areas and rapid expansion from 2000 yr BP to Andean locations.

Focusing on periods of change apparent from the pollen data, we compare results from pollen-based reconstruction against output from two global climate models, GENISIS-IBIS and LMD-5, scaled to Colombia. A second comparison runs a vegetation model (BIOME 4.2) under different environmental scenarios at regional and local scales. Model runs with temperature and precipitation reductions relative to the present of -10 °C and -700 mm yr-1 respectively. To investigate the role of CO₂ as a forcer of vegetation change; the model is run with concentrations of atmospheric CO₂ ([CO₂]_atm.) from 170 to 340 ppmV. The model results are compared with the data-based reconstructions to understand how vegetation response inter-linked [CO₂]_atm., temperature and precipitation. Although Colombia has the greatest concentration of palaeoecological records in the tropics, results show that efforts to compare data- and model-based environmental reconstructions remain hampered by low site density and problems of inter-dependencies between environmental and climate change.