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.
In order to determine the biomass changes taking place, Sist et al. established 18 one-hectare experimental plots in a forest designated for logging. They performed an initial survey of the plots, measuring tree density, basal area, and tree diameter. From the measurements of diameter and previously recorded density, they estimated the amount of biomass and hence the amount of carbon. The plots were then logged. Three months after logging, they were resurveyed to determine mortality and injury to all trees present. For the next 8 years, the plots were resurveyed approximately every two years, and the biomass calculations were performed again. Sist et al. found that prior to logging, large trees represented less than 10% of the tree density, but stored 49% of the carbon. Following logging, which targeted large trees, the plots experienced a significant loss of carbon. In the eight years after the logging event, biomass changes differed between size classes. Smaller trees tended to gain in biomass, and increased their rate of gain. However, medium and large trees tended to lose biomass as a result of residual mortality due to injury from the logging. This mortality in the larger size classes balanced biomass gains by small trees and led to a relatively constant overall biomass balance. The researchers also found that small trees sequester about 12 kilograms of carbon per year, while medium trees sequester 30 kilograms and large trees sequester 53 kilograms, more than 4 times as much as small trees. Sist et al. estimated that it would take more than 50 years for the forest to recover its biomass, but if logging intensity were reduced from 6 to 3 trees per hectare, recovery would only take 15 years. This recovery would be aided by preventing the cutting of the largest trees.
This research demonstrates the significant impact of large trees on carbon dynamics in all forests, and the downsides of removing them via selective logging. They are often selected for logging due to raw timber volume; however, many have structural defects that limit the amount of wood that can be recovered. Thus, leaving some of the largest trees may be less costly to loggers than timber volume would suggest. Reducing the logging intensity and leaving the largest trees remaining would significantly improve the ability of the stand to regenerate the lost biomass quickly, though this would present a cost to loggers. To help defray these costs, carbon credits could be issued that pay to keep the trees alive and the carbon sequestered. In order to make the prices competitive, the credits would have to be issued for at least $6.50 per ton of carbon. Damage to the stand overall could also be reduced by selecting only trees that show an increased risk of mortality, easily assessed via crown damage, liana infestation, etc. Better training and supervision of logging crews could in this way greatly reduce the damage done by selective logging to forest carbon storage.
Sist, P., Mazzei, L., Blanc, L., Rutishauser, E., 2014. Large trees as key elements of carbon storage and dynamics after selective logging in the Eastern Amazon. For. Ecol. Manage. 318, 103–109. doi:10.1016/j.foreco.2014.01.005
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