Large Trees Drive Carbon Sequestration in Degraded Tropical Forests

by Stephen Johnson

Deforestation is responsible for 15% of human-caused carbon release and hence is a key driver of global climate change. However, less known is the role that degradation plays. Forests become degraded by persistent human use or through selective logging, decreasing biodiversity and potentially hindering ecological dynamics. In the Amazon basin, degradation may account for up to 25% of carbon emissions by land use. Selective logging commonly targets the largest trees, which by definition contain the most biomass and carbon. By removing these, logging often substantially reduces forest carbon stocks. Relatively little is known, however, about how this disturbance affects biomass dynamics among size classes at a tree stand level. Sist et al. (2014) address this deficit by following biomass changes among trees of various size classes through 8 years after selective logging. They surveyed 18 experimental plots every two years, collecting data on biomass changes within individual trunk diameter categories and on mortality or morbidity in each category. They found that while small trees increased in biomass, large trees are the key drivers of ecosystem carbon storage. Large trees account for close to half of total carbon storage and experienced high post-logging mortality, which caused significant carbon losses. In order to compensate for this, the authors conclude that logging intensity may need to be reduced and a maximum diameter cutting limit should be adopted. 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