Estimating Changes of Forest Carbon Storage in China for 70 years (1949-2018)

Abstract Stand level biomass carbon model is an important tool for forest resource inventory and monitoring. Developing biomass carbon models for major forest types in China not only provide a scientific basis for estimating forest carbon storage changes at national level, but also provide an important reference for estimating forest carbon storage and carbon sequestration potential at regional and global levels. In this paper, based on the measured data of 52,700 permanent plots from the 9th national forest inventory (NFI) of China, the biomass and carbon storage per hectare were firstly calculated according to the tree biomass models and carbon factors of 35 tree species groups. Then, stand level volume-derived biomass and carbon storage models were developed for 20 forest types by using the approach of simultaneous equations with error-in-variables. Secondly, based on the developed biomass and carbon models of 20 forest types, using the data of 1st to 9th NFIs of China and the area and volume data of different forest types in 1949 and 1950-1962, the forest carbon storages in different periods in the past 70 years and the carbon storages of planted forest in the past 40 years were estimated. Results showed that the determination coefficients (R2) of the developed biomass and carbon models of 20 forest types were more than 0.87, the total relative errors (TREs) were close to 0, and the mean prediction errors (MPEs) were less than 3% (less than 1% for 8 forest types). From the results of independent cross-validation, the TREs were within ±3% except one model, and the average systematic errors (ASEs) were all within ±5%. China's forest carbon storage was 5.89Pg in 1949, and decreased to the lowest level of 4.90Pg in the late 1970s. With the implementation of large-scale afforestation and key forestry programs such as natural forest protection, China's forest carbon storage gradually increased to 8.69Pg in the 9th NFI. Over the past 40 years, China's forest carbon storage has increased by 3.79Pg, of which 2.25Pg was from natural forest and 1.54Pg from planted forest. The applicability of three sets of published biomass models were tested with the data from this study, and it was found that there were large biases for all models, but the estimated results would be improved with the increase of modeling sample size. The biomass and carbon storage models of 20 forest types in China developed in this study would provide a quantitative basis for accurately estimating status and changes of forest carbon storage at national and regional levels. In the past 70 years, China's forest carbon storage has shown a U-shaped development trend. Both afforestation and natural forest protection have made great contributions to the increase of forest carbon storage in China.