Soil carbon is lost when forests become rubber plantations. Does it matter?
Soil carbon is lost when forests are cleared and rubber planted. But the Intergovernmental Panel on Climate Change doesn’t include this in its accounting guidelines. This needs to change if estimates of greenhouse gas emissions are to be accurate, say Marleen de Blécourt, Rainer Brumme, Jianchu Xu, Marife D. Corre and Edzo Veldkamp
We conducted a study that found that converting secondary forests to rubber plantations resulted in losses of carbon by an average of 37.4 Mg of carbon per hectare through the entire 1.2 m depth of soil over 46 years. This was equal to nearly 20% of the initial soil carbon in the secondary forests studied.
This decline in soil carbon was much larger than differences between the carbon in trees in rubber plantations and secondary forests. These ranged from a loss of 18 Mg of carbon per hectare to an increase of 8 Mg. In the topsoil, carbon stocks declined exponentially with the number of years since deforestation and reached a steady state at around 20 years.
These findings have implications for the Intergovernmental Panel on Climate Change’s tier 1 method for assessing carbon stock, which assumes that there is no change to soil carbon when forests are converted to plantations. We have shown that soil carbon needs to be included to avoid errors in carbon accounting.
We came to our conclusions through a study we carried out as part of the Making the Mekong Connected project, funded by Deutsche Gesellschaft für Internationale Zusammenarbeit. We examined the conversion of secondary forests to rubber plantations in Xishuangbanna, the southernmost prefecture of Yunnan province, China. Xishuangbanna was selected because of its long history of conversion to rubber. The first plantations were established there by the Chinese government in the late 1950s. The subsequent expansion has resulted in strong economic development. The area under rubber now totals around 424 000 ha. By 2050, this is predicted to increase fourfold, mainly by replacing secondary forests, swidden-related bushes and shrub lands.
We sampled 11 rubber plantations ranging in age from 5 to 46 years and seven secondary forest plots. Our objectives were 1) to quantify changes in soil carbon following conversion of secondary forests to rubber plantations; and 2) to determine the biophysical factors that controlled concentrations of, and changes to, soil carbon. We hypothesized that conversion would result in a decrease in carbon in the soil.
According to local plantation owners, the dominant land-use changes were 1) primary forest converted to swidden agriculture that reverted to secondary forest, which was converted to rubber plantations; and 2) primary forest converted to swidden agriculture and then directly to rubber plantation. The widespread practice of swidden cultivation in the past resulted in loss and degradation of primary forests. Nowadays, almost all swidden fields have been replaced by monocultural rubber plantations. Since primary forests and swidden agriculture were not present any more, we focused on the conversion of secondary forests to rubber plantations. Based on information from local plantation owners, we selected rubber plantations that had all been through this change. The forests had been cleared by hand; no heavy machinery was used; and afterwards the sites were usually burnt. During the first four years after planting, rubber trees were most often intercropped with maize, upland rice, peanuts and beans. Selected rubber plantations were both state-owned rubber plantations and plantations belonging to smallholder farmers.
Four forest remnants were extant: three ‘collective’ forests and one ‘watershed protection’ forest. We used these as reference points. These broadleaf forests were highly degraded owing to the continuing collection of firewood and extraction of timber in the past. The age of each forest remnant was estimated to be between 40 and 60 years. The size of the forest patches ranged 20–60 hectares.
Soil carbon was measured in clusters consisting of one reference secondary-forest plot and one-to-three plots in rubber plantations. Clusters were established around randomly selected secondary-forest plots. The forest plots were selected at least 20 m from the forest edge. Within each cluster, the rubber plantations were chosen based on biophysical conditions, land-use history and distance to the reference plot. We only selected rubber plantations that were established immediately after forest clearing. To keep biophysical conditions within a cluster as similar as possible, we selected rubber plantations with similar altitude, slope, aspect, soil colour and soil texture as the reference plot. Since we did not detect significant differences in soil texture between the secondary forests and rubber plantations within a cluster, we assumed that the soils were originally similar and that changes to the soil carbon could be attributed to changes in land use.
Cutting natural forests and replacing them with rubber plantations is a common practice throughout the tropics. This ‘conversion’ contributes to the estimated 12–15% of the world’s greenhouse gas emissions. Most of these emissions occur when the carbon stored in the trees is lost through burning or reprocessing. However, a significant proportion of emissions comes from the organic matter stored in the soil, which decomposes more quickly and is released as gas when the forest is cleared. But just how significant a contribution this soil organic carbon makes has never been properly understood. Until now.
Edited by Robert Finlayson
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This work is linked to the CGIAR Research Program on Forests, Trees and Agroforestry component on Landscape Management for Environmental Services, Biodiversity, Conservation and Livelihoods