Soil carbon stocks decrease following forest-to-rubber plantation conversion
In sharp contrast to the IPCC tier 1 method, which assumes that soil carbon changes from forest-to-rubber plantation conversions are zero, a study by de Ble´court et al published in PLoS One, states that forest-to-rubber plantation conversion results in soil carbon losses of 19% in a period of 46 years. They add that if montane Southeast Asian rubber plantations, which now cover 1.5 million ha expand fourfold by 2050, as they are predicted to, there could be huge errors in overall ecosystem carbon fluxes—should scientists continue to assume that forest-to-rubber conversions cause a zero change soil carbon.
Rubber plantations in montane mainland southeast Asia cover more than 1.5 million ha and are rapidly expanding, causing large decreases in forest cover. It is expected that by 2050 rubber plantations will replace secondary forests, and swidden-related bushes and shrublands will occupy four times the area they do now.
An estimated 12–15% of global anthropogenic CO2 emissions result from deforestation and forest degradation in the tropics. Here, forest-to-rubber (Hevea brasiliensis) plantation conversion is a significant land-use change, but its impacts on soil carbon stocks have hardly been studied. Scientists from the University of Gottingen and the World Agroforestry Centre in Xishuangbanna, Yunnan Province, China, set out to quantify the changes in soil carbon stocks following conversion from secondary forests to rubber plantations; and to determine the biophysical factors that control soil carbon concentrations and soil carbon stock changes. The role of forests, trees and agroforestry in climate change mitigation is a key focus of the CGIAR’s Collaborative Research Project 6 on Trees, Forests and Agroforestry—of which the Centre is a key partner.
The study area of 4500 ha was located in Menglong township, Jinghong county of Xishuangbanna prefecture in Yunnan province, China. Study plots were located between 700 and 830 m above sea level. Eleven rubber plantations ranging in age from 5 to 46 years and seven secondary forest plots were sampled using a space-for-time substitution approach. The study found decreased soil carbon stocks under rubber plantations, not only in the top 0.6 m depth but also when considering the whole 1.2 m depth.
These soil carbon losses are related to:
- years since land-use change: there is an exponential decrease in soil carbon stocks in the top 0.15 m depth with years since land-use change
- vegetation cover: denser cover increases soil carbon input and soil stability
- management practices such as terrace maintenance, rubber tapping, fertilization and the removal of vegetation
- clay and silt content: the higher the clay and silt content of the soil, the higher the soil carbon concentrations
- total basal area: increases of above-ground biomass could increase soil carbon
- plantation age: soil carbon stocks decline with increasing plantation age
- altitude: soil carbon stocks increase with altitude
A soil carbon steady state was reached approximately 20 years after conversion, but soil carbon losses induced by land-use change in this region are expected to continue for a longer period of time. Forest conversion to rubber plantations had a much larger effect on soil carbon stocks than on above-ground carbon stocks.
Although the Intergovernmental Panel on Climate Change (IPCC) provides guidelines for the estimation of ecosystem carbon fluxes arising from land-use changes, the IPCC tier 1 method assumes soil carbon changes to be zero for the conversion from forests to rubber plantations. In contrast, this study found that forest-to-rubber plantation conversion results in soil carbon losses of 19% in a period of 46 years. This means soil carbon changes need to be included to avoid possibly large errors in the estimates of overall carbon fluxes. The authors also call for improved estimates of the effects that forest-to-rubber conversions have on soil carbon stocks, in greenhouse gas inventories from the Conference of Parties of the United Nations Framework of Climate Change.
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