Long rotation swidden systems maintain higher carbon stocks than rubber plantations

Conversion of shifting cultivation to rubber (Hevea brasiliensis) plantations is one of the dominant land use changes in montane mainland areas of Southeast Asia, with the area of rubber expected to quadruple by 2050. However, the impacts of this transition on total ecosystem carbon stocks are poorly quantified. We undertook a chronosequence study to quantify changes in ecosystem carbon stocks following conversion from swidden agriculture to rubber plantations in Northern Laos. We measured above-ground biomass stocks and collected volume specific soil samples across rubber plantations established between 2 and 18 years prior to the study, and fallows used in a swidden system. The carbon stock in the upper 40 cm of the soil was almost 20% lower after 18 years of rubber than in the swidden system fallows, suggesting a SOC loss of 0.74 ± 0.2 Mg C ha−1 yr−1. Rates of biomass accumulation in fallows were 1.5 ± 0.12 Mg C ha−1 yr−1 and 1.9 ± 0.14 Mg C ha−1 yr−1 in rubber plantations. When comparing time-averaged carbon stocks of swidden systems to rubber plantations with 30 year rotation periods, the stocks of swidden systems with rotation times of 5 and 10 years were 19% and 13% lower respectively; the stock of swidden systems with a rotation time of ∼15 years was approximately equal to rubber; and the stock of swidden systems with a rotation time of 30 years was 11% higher than in rubber. Therefore, we conclude that the replacement of swidden agriculture with rubber leads to soil carbon losses, but the overall effects on ecosystem carbon stocks depend on the rotation intensity of the swidden system that is being replaced.