Carbon budget determination using field techniques and modeling of small holder tree farms in Leyte Island, Philippines

The role of terrestial ecosystem in mitigating the effects of climate change entails the assessment of carbon stocks in various pools. This study predicted the carbon storage and sequestration potential of common tree farm species in Leyte Island, Philippines using models that were parameterized to smallholder farm conditions. Data gathered from field measurements was used to fit the Chapman-Richards growth function to predict the volume and biomass increment of Gmelina arborea and Swietenia macrophylla tree farms until they reached their respective rotation ages. Predicted values, secondary sources and default values served as inputs to the CO2 Fix model to stimulate the carbon stocks and fluxes in the above-ground biomass, soil and the products for three rotation periods. Results showed that biomass and carbon density values varied with age, type of species, site conditions and silvicultural treatments applied in the stand. Although farm age had no relation with its soil carbon storage, this pool had greater storage capacity than the above-ground biomass and roots. By fitting the Chapman-Richards function, results showed that the average maximum growth was attained after 10 years for G. arborea and 13 years for S. macrophylla. Volume growth started to slow down when the tree species reached almost half its rotation age. The same trend was observed for the biomass and carbon density of each farm. The maximum mean annual increment of both farms was attained before the expected maximum growth year. Growth increment decreased as the species reached its rotation age. The total C storage capacity of a 15-year-old G. arborea tree farm was estimated at 64 Mg C/ha while a 25-year-old S. macrophylla was estimated at 159 Mg C/ha. The results from the CO2 Fix model run showed that soil and tree biomass contained high amounts of C than in the products. The G. arborea farm had difficulty in recovering from soil C loss for every rotation period as compared to S. macrophylla which had a constant soil C loss for three rotation periods. The study estimated that CO2 from the atmosphere can be absorbed by seven-year-old G. arborea farms, and nine-year-old S. macrophylla farms. The use of S. macrophylla as tree farm species was found to effectively store and sequester more carbon in the atmosphere, above-ground, soil and products as compared to farms planted with G. arborea.