Agroforestation of grasslands in Southeast Asia: WaNulCAS model scenarios for shade-based imperata control during tree establishment

In the stage of land use evolution where smallholder tree-based systems are desirable as replacement of Imperata cylindrica (and similar) grasslands, agroforestry can provide a gradual and rewarding approach to the transition. There tends to be, however, a gap between the last opportunity for food crop interplanting and canopy closure providing shade-based control of grass and weed growth. In such period, regrowth of Imperata enhances the risk of fire and failure of tree establishment. We analyzed the duration of this ‘Imperata regrowth window’, for a range of planting patterns and choice of tree species in Lampung (Indonesia) and northern Mindanao (the Philippines). Simulations of agroforestation scenarios with the WaNuLCAS model (‘water, nutrient and light capture in agroforestry systems’) focuss on the Imperata regrowth window as the period between 50 percent and 15 percent of ground-level light availability. The simulation results first of all confirm a well-known fact: young trees of most species are not able to compete with Imperata and partial weeding around the tree stem base is absolutely necessary to get most trees started, with the possible exception of Paraserianthes falcataria. Although Acacia mangium is a fast growing tree, a more intensive weeding regime will double tree growth. The improvement of initial tree growth speeds up tree canopy closure and reduces subsequent Imperata regrowth window by two to more than five years according to the model, with periods longer than five years associated with slow initial growth rates. There is, according to the model, only limited opportunity to reduce risk exposure by modifying tree spacing.

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