Plantation Forests Increase Carbon Storage on Hainan Island, Southern China

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. Continue reading

Coffee Agroforests Can Store Significant Levels of Carbon

by Stephen Johnson

Over 40% of the world’s terrestrial surface is covered by agricultural activities, and approximately half of that area is agroforestry. Agroforests, agricultural areas that are at least 10% covered by tree shade, run the gamut from areas with a few exotic species to structurally complex, highly diverse ecosystems that mimic natural forests. Woody vegetation biomass is approximately 50% carbon, so incorporating trees into agricultural areas significantly improves the ability of these systems to sequester and store carbon. Given that agroforests cover almost half a billion hectares, they may represent a significant and underestimated carbon sink. The amount of carbon that can be stored is determined by a variety of factors, including the number of trees and the density of their wood. The type of and number of the trees present in turn depends on the individual management of the farm. In Ethiopia, Tadesse et al. (2014) investigated how different management regimes affected the species of trees present and the amount of carbon stored, compared to natural forests. They measured the density and species of trees in smallholder coffee farms, state-owned plantations, and forest fragments, and used these measurements to determine the carbon storage capacity of each forest. They also interviewed farmers to see how and why species are selected for inclusion in plantations. Tadesse et al. found that agroforests can store 50-62% of the carbon that natural forests can store. They also found that farmers tended to prefer and harvest denser-wooded species, though less dense species were used for some limited purposes. Continue reading

Carbon Storage in Restored Forests is Species and Age Dependent

by Stephen Johnson

Deforestation in tropical rainforests is a significant and growing conservation concern, and for good reason: as well as harboring high levels of biodiversity, tropical forests are estimated to store 59% of global terrestrial carbon. The capacity of woody plants to store carbon, which constitutes 50% of their biomass, makes them an indispensible consideration in the effort to mitigate global climate change. Of course, forests can’t store carbon if they don’t exist. In the past 14 years alone, more than 100 million hectares of tropical forest have been lost—an area greater than Texas and Arizona combined. This continued destruction has prompted interest in the ability of ecological restoration—replanting forests—to provide ecosystem services such as carbon sequestration and biodiversity habitat. In attempting to rapidly revitalize damaged ecosystems, fast-growing, pioneer species with low wood density are often chosen to replant, though slower-growing, denser species may be required for long-term carbon storage and ecosystem health. To help resolve this question, Shimamoto et al. (2014) examined the biomass accumulation of ten tree species with different ages and growth patterns. By comparing measurements of fast and slow-growing trees in forests of different ages, they were able to determine carbon sequestration through analysis of covariance tests as well as linear and non-linear models. They found that in the first 35-40 years, fast-growing species accumulate the most carbon, but after 40 years, slow-growing species accumulate more carbon, and older forests overall sequester more carbon than young forests. Continue reading