by Stephen Johnson
Forested landscapes in the tropics are often highly dynamic, with natural forest being replaced by a shifting mosaic of plantations, agriculture, pasture, and settlements, which are in turn occasionally replaced by ecological restoration. This produces landscapes that vary in their capacity to sequester and store carbon. In South East Asia, as in many parts of the tropics, natural forests are commonly converted to plantations of rubber, Eucalyptus, pine, and hardwood for timber. The carbon storage capacity of a forest depends on the species of trees, which vary in density and size. Consequently, the type of forest, artificial and natural, determines the carbon storage capacity of a landscape. On Hainan Island in Southern China, land cover has been continuously altered over the past century, with artificial plantation forests replacing natural rain forest. Ren et al. (2014) analyzed how carbon storage capacity differs between land use types and how the total quantity of stored carbon has changed through time. Using both remote sensing and forest inventory plots, they quantified the carbon stored in woody vegetation, understory, herbs, leaf litter, and soil in each type of land use. They found that carbon storage capacity is highest in natural forests, and while these forests have been reduced over the years, the proliferation of plantation forests has actually increased the total carbon storage of the island. Furthermore, they found that 75% of the forest’s carbon was stored in soil rather than woody biomass, emphasizing the importance of maintaining soil communities.
Ren et al. used historical forestry survey data obtained from the Chinese Government to estimate the extent and type of forests from the 1940’s onwards. To determine the capacity of these forests to store carbon, they established survey plots in forests of each type. They determined the dominant species, the average diameter and tree height, and collected litter and understory herbaceous plants. They also took soil cores to a depth of one meter. To determine the carbon storage capacity, the researchers calculated the forest stand volume in cubic meters, and then multiplied that by a species-specific conversion factor (similar to density) to determine the amount of biomass and hence carbon. Understory plants and leaf litter were dried and weighed, and soil samples were assayed for their organic matter content. These measures allowed Ren et al. to estimate the amount of carbon stored in an average hectare of forest of each type. Additionally, they estimated the number of trees present in urban areas and used this to estimate the carbon storage capacity of cities. They then used the calculated carbon storage capacity and spatial data detailing the extent of each forest type to determine the total amount of carbon stored on Hainan Island both historically and in the present day. The data revealed significant decreases in natural forest area throughout the 20th century, as well as the proliferation of plantation forests. Natural rainforests stored more carbon than any other forest type. However, while natural forests declined, total island-wide carbon sequestration increased from 1993 to 2008 as a result of the plantation forests. Understory plants and leaf litter were found to contain relatively little of the forests’ carbon. Aboveground woody biomass constituted less than 23% of forest carbon; soil contained almost 75% of the carbon. Total carbon storage was approximately 280 teragrams, with an additional possible storage capacity of 77 teragrams if natural forest ecosystems were restored.
Carbon storage is, unsurprisingly, highest in undisturbed ecosystems with high biomass. However, plantation forests strike a favorable balance between economic profitability and carbon storage, and due to the former, are much more likely to be viewed as an acceptable form of land use. Restoring natural ecosystems would enhance carbon storage capacity, but due to the lack of financial incentives, natural rainforests are largely confined to remote mountainous regions. Regardless, the high levels of carbon on Hainan emphasize the ability of anthropogenic ecosystems to serve as tools to mitigate climate change. This research also shines a light on the importance of soil carbon. Three quarters of forest carbon is stored in the soil, so land use policies that prevent soil erosion and degradation should be encouraged. Ecological restoration of natural forests is best for carbon storage, but plantation forestry offers an attractive way to maintain soil and woody biomass while generating economic profit for private landholders.
Ren, H., Li, L., Liu, Q., Wang, X., Li, Y., Hui, D., Jian, S., Wang, J., Yang, H., Lu, H., Zhou, G., Tang, X., Zhang, Q., Wang, D., Yuan, L., Chen, X., 2014. Spatial and Temporal Patterns of Carbon Storage in Forest Ecosystems on Hainan Island, Southern China. PLoS One 9, e108163. doi:10.1371/journal.pone.0108163
Hai Ren, Linjun Li, Shuguang Jian, Jun Wang, Hongfang Lu, Guoyi Zhou, Xuli Tang, Qianmei Zhang, Lianlian Yuan, Xubing Chen, Key Laboratory of Vegetation Restoration and Management of Degraded Ecosystems, South China Botanical Garden, Chinese Academy of Sciences, Qiang Liu, College of Life Science, Hainan Normal University, Xu Wang, College of Environment and Plant Protection, Hainan University, Yide Li, Huai Yang, Research Institute of Tropical Forestry, Chinese Academy of Forestry, Dafeng Hui, Department of Biological Science, Tennessee State University, Dong Wang, School of Life Sciences, Central China Normal University, Carbon Storage, Forest Carbon, Agricultural Carbon, Degraded Ecosystems, Tropical Forests, Sustainable Agriculture, Climate Change.