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.
Shimamoto et al. selected 10 common species, 6 fast-growing and 4 slow-growing, and identified 6-19 individuals of each species. The sampled individuals were found in forests ranging from early restoration (7-11 years old) to late-stage secondary forests (20-60 years old). They measured each individual for height, diameter, wood density, and age (using small trunk cores), and used these measurements to calculate the amount of biomass and thus carbon present. They also measured canopy cover and vegetation density in circular plots around the trees, to determine if these had an effect on biomass. To compare the relationships between tree measurements, they used linear, polynomial, and logarithmic models, and determined which type fit best. They then used an analysis of covariance to compare the amount of carbon sequestered between fast-growing and slow-growing species. Patterns varied between species, but in general, older trees were taller and wider, with taller and denser trees accumulating the most biomass. This resulted in a variable but general pattern of biomass accumulation increasing with age. They also found that both fast and slow-growing species accumulated more biomass in areas with higher vegetation density, though only fast-growing species increased with higher canopy cover. Most importantly, they found that the type of growth pattern sequestering the most carbon changed over time. Initially, fast-growing species sequestered more carbon than slower-growing species, probably due to their rapid increases in height and diameter. This pattern persisted until approximately 38 years after replanting, when it switched, and the denser, slower-growing species began sequestering more carbon, a pattern that then persisted. The enhanced carbon capture capacity of slow-growth species caused older forests to accumulate significantly more carbon than younger forests, making them more valuable in efforts to alleviate climate change.
The selection of species for restoration efforts is difficult, and depends on the priorities of the specific site. Faster-growing species provide habitat for their species more quickly, increasing native biodiversity, arresting soil erosion, filtering water, and sequestering carbon with their rapid addition of biomass. However, slow-growth species are necessary to continue to accumulate benefits over the long term, and to provide a more complex assemblage that will further enhance species richness. Tree selection will differ for the needs of each specific project, but the models of Shimamoto et al. suggest that if the goal is effective long-term carbon accumulation, a mix of species will be most beneficial. Thus, restoration of native forests has the potential to provide numerous services and sequester significant amounts of carbon, and should be carefully considered as a strategy to mitigate global climate change.
Shimamoto, C.Y., Botosso, P.C., & Marques, M.C.M., 2014. How much carbon is sequestered during the restoration of tropical forests? Estimates from tree species in the Brazilian Atlantic forest. Forestry and Ecological Management 329, 1–9. doi:10.1016/j.foreco.2014.06.002