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2011 Literature Review Archives - Atmospheric Composition

Donato, D.C., J.Boone-Kaufman, D. Murdyarso, S. Kurnianto, M., Stidham, and M. Kanninen. 2011. Mangroves among the most carbon-rich forests in the tropics. Nature Geoscience, Vol 4, pp 293-297, doi: 10.1038/NGE1123.

Estimates of whole-ecosystem carbon storage in mangrove forests indicate that they are amongst the most carbon-rich forests in the tropics storing 4-20 Pg C (petagrams of carbon) globally. Estimates based on current deforestation rates suggest that they release 0.02-0.12 Pg C per year (roughly 10% of total annual global deforestation emissions).

Mangrove forests are found along ocean coastlines in 118 tropical countries yet there is little information on whole-ecosystem carbon storage in these forests. Donato and colleagues attempt to address this knowledge gap by measuring above- and below-ground carbon pools at a range of mangrove stands (estuarine and oceanic) in the Indo-Pacific region. The authors found that mangroves are amongst the most carbon ( C) dense forests in the tropics; estuarine and oceanic sites contained on average 1,074 and 990 Mg (megagrams) C per hectare respectively. Most of the carbon storage was found below-ground (49-98% across all sites; soil depth ranged from 0.5 to >3 m). The high carbon storage and vast pan-tropical distribution of mangroves indicate that they are a globally important carbon reserve. The authors estimate that globally, mangroves store 4-20 Pg C. Mangroves are currently subject to widespread deforestation and the authors estimate that 112-392 Mg C are released per hectare cleared (varying with how deeply the land use change affects C storage). Given the published rates of mangrove deforestation this yields an estimate of global emissions of 0.02-0.12 Pg C yr-1 or around 10% of the total annual global emissions from deforestation (mangroves account for only ~0.7% of tropical forested area). Future coastal carbon stores may be further impacted by land use changes as well as changes in sea level associated with anthropogenic climate change.

Stinson, G., W.A. Kurz, C.E. Smyth, E.T. Neilson, C.C. Dymond, J.M. Metsaranta, C. Boisvenue, G.J. Rampley, Q. Li, T.M. White and D. Blain. 2011. An inventory-based analysis of Canada’s managed forest carbon dynamics, 1990 to 2008. Global Change Biology 17: 2227-2244.

An updated assessment of the carbon budget of Canada’s managed forests shows they were close to carbon neutral over the period 1990-2008 although year-to-year variability was high. On average, carbon gains from forest productivity were offset by carbon losses through disturbance (primarily fires) and transfers of carbon into harvested wood products. Since much of the carbon in harvested wood products is sequestered for many years, indicators of the exchange of CO2 alone between the forests and the atmosphere over the study period show Canada’s managed forests were a modest CO2 sink.

A key issue for understanding the fate of human-emitted CO2 is determining the capacity of land-based and oceanic sinks to take up that emitted carbon. Estimating the magnitude and the regional distribution of forest carbon sinks and the likelihood of these becoming carbon sources is an active research topic. This study, led by scientists with the Canadian Forest Service (CFS), provides an update of the carbon budget for Canada’s managed forests (a defined area encompassing 2.3 x 106 km2 of a total of 3.5 x 106 km2 of forest) from 1990-2008 using new forest inventory data and improved modeling (Carbon Budget Model of the CFS version CBM-CFS3). Values for several different indicators were estimated to report on trends in carbon stocks, fluxes to and from the atmosphere and transfers from the ecosystem into harvested wood products. Net ecosystem production (NEP; net primary productivity minus heterotrophic respiration - a measure of landscape averaged forest ecosystem productivity - was estimated at 31 gC/m2/yr, a relatively low value indicative of an older forest. About two thirds (~20 gC/m2/yr) of this carbon was transferred out of the ecosystem into harvested wood products (HWP) and about one third (~10 gC/m2/yr) was emitted directly to the atmosphere by fire, leaving Canada’s managed forests very nearly ‘carbon neutral’ over the study period (net biome productivity (NBP) = NEP – disturbance losses) = ~1 gC/m2/yr or 2 TgC/yr). In fact, there were two distinct periods: 1990-1999 during which the forests were carbon sinks, and 2000-2008 during which they were a source, with the latter period affected strongly by the mountain pine beetle outbreak in the west. Despite the very small gain in carbon over the 19 year study period as measured by NBP, the Canadian managed forest was, at the same time, a modest sink for CO2, with an uptake of 50 ± 18 TgC/yr (measured as net ecosystem exchange of CO2 (NEECO2 )) This discrepancy is largely due to different accounting of the carbon in HWP in the two indicators (NBP and NEECO2). Emissions from HWP were not accounted for when calculating NEECO2 because much of the carbon transferred to HWP is sequestered for many years.

Xiao, J., Q. Zhuang, B. E. Law.2011. Assessing net ecosystem carbon exchange of U.S. terrestrial ecosystems by integrating eddy covariance flux measurements and satellite observations. Agricultural and Forest Meteorology, Vol 151, p. 60-69, doi: 10.1016/j.agrformet.2010.09.002.

The terrestrial carbon sink over the coterminous U.S. is estimated to be 0.63 pg C per year on average over the period 2001-2006. This uptake is sufficient to offset 40% of U.S. annual fossil fuel carbon emissions.

An improved understanding of the terrestrial carbon cycle can improve projections of future atmospheric CO2 concentrations and associated climate change. Numerous studies have demonstrated that terrestrial ecosystems in the coterminous U.S. represent a net carbon sink but uncertainty remains about the overall size and distribution of this sink as well as the magnitude of year-to-year variations. Xiao and colleagues integrate flux tower measurements of ecosystem-level CO2 exchange from a range of U.S. biomes and climate types with satellite observations to provide an estimate of net ecosystem carbon exchange (NEE) in the coterminous U.S. for the period 2001-2006. The results indicate that, on average, the total annual carbon sink in the terrestrial ecosystems of the coterminous U.S. is 0.63 pg C with most of this sink in regions dominated by forests (evergreen and deciduous) and savannas. This result suggests that U.S. forests and other terrestrial vegetation (excluding agriculture) could sequester at maximum ~40% of current annual U.S. fossil fuel emissions. This is higher than a previous estimate that suggested the number was around 30%. The greatest interannual variations within the study period were related to major disturbances and climate extremes such as wildfires and drought both of which are projected to occur with greater frequency and/or severity in a future warmer world. Droughts in 2002 and 2006 were observed to reduce annual sequestration across the coterminous U.S. by ~20%.

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