Vertical migrations of upper forest lines in the northern Andes identified by soil organic matter fingerprints (Ecuador)

To reconstruct past upper forest line (UFL)positions in the northern Andes (Ecuador)the programme comprises an interdisciplinary effort to obtain a very significantly improved signal of the UFL in palaeo-records from the Andes of Ecuador.High-resolution data on soil organic matter (SOM),fossil pollen content of soils and sediments,recent pollen rain and vegetation composition will be obtained along altitudinal transects from (3000)3200 to 4000 (4300)m containing UFL ecotones with highly organic volcanic ash soils. This project encompasses a molecular and isotopic approach to identify the past vegetation changes along ecotones from forested sites to páramo grasslands between which the UFL has shifted in the past.Information on the former composition of the vegetation at the various sites sampled along these ecotones will be obtained by establishing the contribution of UFL characteristic species from the soil lipid composition,using these lipids as plant-specific fingerprints.Similar,but independent information will be obtained by assessing the contribution of C3 and C4 plants to the SOM,based on the different isotopic 13 C/12 C ratios of C3 and C4 species,usually expressed as d 13 C relative to V-PDB (approx.-27 ‰for C3 and approx.- 12 ‰for C4).Since the vertical distribution of SOM is disturbed by bioturbation and root derived SOM,d 13 C values of bulk SOM may provide less precise estimates of the ratio between C3 and C4 plants. Therefore, isotopic 13 C/12 C ratios of specific compounds,i.e.n -alkanes,will be estimated. SOM in volcanic ash soils is stabilized through binding with the mineral soil particles, in particular allophanes and amorphous aluminium hydroxides,giving rise to characteristic Andosols.This stabilization may affect the SOM composition and particularly that of the different lipid compounds.From the SOM fractions isolated by selective extractions that are typical of the various types of SOM-mineral bonds in Andosols the chemical composition will be determined to elucidate the mechanisms of SOM stabilization and that of the lipids and their related vegetative fingerprints.