Modeling the environmental and economic impact of land-use change in tropical Imperata areas

Grasslands in tropical areas have expanded rapidly and encroached indiscriminately due to deforestation and consequent proliferation of shifting cultivation. Pure grasslands in the Philippines is estimated to occupy 1.8 million ha and another 10.8 million ha or 33 percent of the country's land area are under extensive cultivation mixed with grasslands and brushlands. Imperata cylindrica is the dominant species in these grasslands which generally represent degraded, acidic, low organic matter content and dry areas susceptible to soil erosion. Conversion of these grasslands into upland crop farms planted to rice, maize, and cassava is triggered by the interacting factors of rapidly increasing population, landholding policies and declining area of arable land per farmer in the lowlands. Likewise, attractive market-driven demand for fast growing timber species like Gmelina arborea is the main driving force for the development of smallholder timber plantations. The on-site and off-site biophysical and economic consequences of land-use change from Imperata grasslands to continuous maize cropping and Gmelina plantation systems in Northern Mindanao, Philippines were determined using Soil Changes Under Agroforestry (SCUAF) model linked to a cost-benefit spreadsheet. Simulated results have shown that least reduction in yield, total soil C, total soil organic N and total soil organic P as well as least cumulative soil loss through erosion occurred in the Gmelina plantation system. Consequently, carbon sequestration in Gmelina system was found to be higher than the other systems considered in the study. The maize system, on the other hand, showed the highest reduction in total soil C, total soil organic N and total soil organic P as well as the highest cumulative soil loss. Cost-benefit analysis have shown that long term financial profitability of the Gmelina system is superior compared with the other two systems. However, it takes a longer time for the investment in Gmelina to be translated into positive revenues due to the long-term nature of its output. The imputed value of the carbon sequestered in the soil and biomass is substantially higher in the Gmelina system