Representing anthropogenic gross land use change, wood harvest, and forest age dynamics in a global vegetation model ORCHIDEE-MICT v8.4.2

摘要

Land use change (LUC) is among the main anthropogenic disturbances in theglobal carbon cycle. Here we present the model developments in a globaldynamic vegetation model ORCHIDEE-MICT v8.4.2 for a more realisticrepresentation of LUC processes. First, we included gross land use change(primarily shifting cultivation) and forest wood harvest in addition to netland use change. Second, we included sub-grid evenly aged land cohorts torepresent secondary forests and to keep track of the transient stage ofagricultural lands since LUC. Combination of these two features allows thesimulation of shifting cultivation with a rotation length involving mainlysecondary forests instead of primary ones. Furthermore, a set of decisionrules regarding the land cohorts to be targeted in different LUC processeshave been implemented. Idealized site-scale simulation has been performed formiombo woodlands in southern Africa assuming an annual land turnover rate of5 % grid cell area between forest and cropland. The result shows that themodel can correctly represent forest recovery and cohort aging arising fromagricultural abandonment. Such a land turnover process, even though without anet change in land cover, yields carbon emissions largely due to theimbalance between the fast release from forest clearing and the slow uptakefrom agricultural abandonment. The simulation with sub-grid land cohortsgives lower emissions than without, mainly because the cleared secondaryforests have a lower biomass carbon stock than the mature forests that areotherwise cleared when sub-grid land cohorts are not considered. Over theregion of southern Africa, the model is able to account for changes indifferent forest cohort areas along with the historical changes in differentLUC activities, including regrowth of old forests when LUC area decreases.Our developments provide possibilities to account for continental or globalforest demographic change resulting from past anthropogenic and naturaldisturbances.