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The Preindustrial control should be the same as the one used for CMIP5 experiments. It is thus the same reference as the one for 20th century simulations and future climate projections (see taylor_cmip5_design_december_09.pdf).
Depending on the complexity of the model used the vegetation will be either:
Land use should be included in the PI vegetation map. In that case, care should be taken that the land use map used in the PI simulation is consistent with the one used for land use evolution either for the 20th century or the last Millennium. Please follow CMIP5 recommendations (see http://cmip-pcmdi.llnl.gov/cmip5/forcing.html?submenuheader=2#land-use_data/landuse data).