Clusters Related to Controversy

Carbon Sequestration in Tropical Agroforestry Systems (2003) is an article identified by CiteSpace© as being a pivotal article related to the controversy. The articles below are research clusters connected to this pivotal article by A. Albrecht and S. Kandji. Each review describes what the research encompasses, what techniques are used in analysis, and the goal of the researcher(s). 

Literature Review

Note: Articles are in order by date of publication.

1. Paul, K.I., Polglase, P.J., Nyakuengama, J.G., Khanna, P.K., 2002. Change in soil carbon following Afforestation. Forest Ecology and Management. 168, 241-257.

About 75% of terrestrial carbon is stored in soils, and forest soils hold about 40% of all belowground carbon. This research analyzed 43 studies (data collected from 204 sites globally) where changes in soil carbon were monitored following afforestation (defined in text as ex-agricultural lands). “If afforestation only slightly affects soil carbon stocks at a local level, it could have a significant effect on the global carbon budget if enough agricultural land is converted”.

Studies varied in amount of time since forest establishment and initial carbon content in the soil, and therefore a weighed-average (sum of carbon change divided by sum of years since establishment) was used to account for these variations. Soils were grouped in three sampling depth categories (<10cm, >10cm and <30cm) to allow comparisons between results from different studies. Rates of change in soil carbon were overall fairly modest, varying from increasing 17% per year to decreasing by 12% per year, compared to expected carbon accumulation in tree biomass. Studies showed an initial carbon decrease in surface soil during the first 5 years, but more-or-less recovered after about 30 years, with a gradual increase in carbon following afforestation. Although soil carbon changes due to afforestation are low, the numbers still need to be taken into consideration when determining total carbon sequestration amounts. 

It was also determined that there are factors affecting the extent of changes in soil carbon. Factors include: site preparation, previous land use, climate, soil texture and clay content, site management (forest type, stocking, weed control, thinning, fertilizer application, nitrogen fixation, and fire management), and harvesting/rotation length.

2. Montagnini, F., Nair, P.K.R., 2004. Carbon Sequestration: An Underexploited Environmental benefit of agroforestry Systems. Agroforestry Systems. 61, 281-295.

Agroforestry has substantial potential to sequester carbon in multiple plant species and soils related to agricultural lands and reforestation. Proper design and management of agroforestry can make them effective carbon sinks. The extent of carbon sequestered will depend on the amounts of carbon in standing biomass, carbon in the soil, and sequestered in wood products. This article also suggests further research to be conducted in various regions to exploit agroforestry in both subsistence and commercial enterprises.

This paper classifies forest management in respect to reduction of atmospheric carbon into three categories: Carbon Sequestration (afforestation, reforestation, restoration of degraded lands, agroforestry), Carbon conservation (preserving carbon in biomass and soil in existing forests, improved logging techniques, fire protection), and Carbon substitution (increased use of bio-fuels, introduction of bio-energy plantations). Of the three strategies, carbon conservation has been regarded as the most effective method of rapidly mitigating climate change, whereas carbon sequestration takes places over a long period of time. Of carbon sequestration methods, agroforestry is of special importance because of its applicability in agricultural lands as well as in reforestation programs.

In tree plantations, there is variation of carbon sequestration among different plantation species, regions and management. “The main fast-growing, short rotation species are genera Eucalyptus and Acacia”. Coniferous species of the temperate and boreal regions, such as Pines, are of medium-rotation, but in North Carolina, an experiment with loblolly pine showed that after an initial growth spurt, trees grew slowly and did not sequester much carbon.  Average storage is 9, 21, 50 and 63 mg ha-1 in semiarid, sub-humid, humid, and temperate regions.

Agroforestry can increase the amount of carbon atmosphere, by including trees in agricultural systems. Agroforestry systems with perennial crops may be better sinks than annuals.”According to the estimates of the Intergovernmental Panel on Climate Change (IPCC), tropical forests are by far the largest carbon stock on vegetation, while boreal forests represent the largest carbon stocks in soils.” Being the largest pool of terrestrial carbon, 1.5 to 3 times more carbon appears in soils than in vegetation. Carbon in the soil and in the trees of agroforest systems depends on soil management, such as rotation cycles, and conservation practices, which could enhance storage in trees as well as in the soil.

3. Peichl, M., Arain. M.A., 2006. Above and belowground ecosystem biomass and carbon pools in an age-sequence of temperate pine plantation forests. Agric.and Forest Meterorology. 140, 51-63.

Although a trees biomass (stem, branches, foliage) contains the largest amount of carbon, other components may have contribution as well. When estimating forest carbon storage, many researchers do not include aboveground vegetation, belowground tree roots, forest floor, or soils. The goal of this paper was to include these carbon pools in an assessment of four white pine plantations in temperate forests located Ontario, Canada. Researchers have found that temperate forests have a high carbon stocks in their tree’s biomass, with pines, being among the highest. White Pines, in particular, are fast-growing and highly adaptable, and therefore considered a suitable species for plantations.

An age-sequence was applied to study successional development, at 2, 15, 30, and 65 years of age. Biomass from standing and dead trees, understory, forest ground vegetation, forest floor, and woody debris were determined by sampling. Root biomass and soil carbon were estimated from soil cores. Above-ground tree biomass dominated with increasing age. Understory, forest ground vegetation and woody debris biomass contributed very little to the carbon pool. Total tree root biomass increased with age and was significant overall, but small roots peaked at 15 year old stands. Forest floor carbon increased only in the first 15-30 years. Mineral soils did not seem to be age-dependent, but other undetermined factors, such as previous land management, soil properties, vegetation and climate, need to be further analyzed. When comparing over all ecosystem carbon in above and below ground biomass, aboveground increased more in its younger years, whereas belowground increased as tree matured, due to the total tree root biomass significantly rising with age.

4. Takimoto, A., Nair, P.K.R., Nair, V.D., 2007. Carbon Stock and sequestration potential of traditional and improved agroforestry systems in the West African Sahel. Agriculture Ecosystems & Environment. 125, 159-166.

Carbon sequestration through agroforestry has become widely known since the Kyoto Protocol renowned it as a greenhouse gas mitigation strategy. This study, conducted in Mali (buffer zone of the Sahara known as the Sahel region), examined five land areas, two traditional parkland systems, two improved agroforestry systems, and one abandoned land area. All areas were measured two ways: carbon in biomass only, and carbon in biomass and soil together. “Carbon stock in the biomass was estimated by algometric equations and soil carbon stock was determined at three depths (0-10cm, 10-40cm, and 40-100cm)”. In biomass alone, carbon storage ranged from 0.7 to 54.0 Mg ha-1, and total from biomass and soil ranged from 28.7 to 87.3 Mg ha-1. Results indicate that soil stores large amounts of carbon. Type of land also showed different rates of sequestration, with improved agroforestry systems sequestering more carbon, parkland containing more in its biomass (seemingly because the parkland contained larger trees), and with abandoned land containing the largest carbon stock. The authors believe this to be due to the clay and silt within the soil of the abandoned land of that area being able to trap the carbon better than other soil profiles.  

Why are these articles part of the Main Cluster?

There are two reasons why these articles are connected. The first reason is because they are related through direct and indirect citations. Here are a list of the authors from the pivotal article and connected articles documented by CiteSpace©:

Albrecht, A.K - Cited in Pivotal article (twice), Article 4
Arain. M.A. - Cited in Article 3 (twice)
Kandji, S.T. - Cited in Pivotal article, Article 3, Article 4
Khanna, P.K. - Cited in Article 1 (twice)
Montagnini, F. - Cited in Article 2 (6 times), Article 4
Nair, P.K.R - Cited in Pivotal article twice, Article 2 (5 times), Article 4 (3 times)
Nair, V.D. - Cited in Article 2, Article 4
Nyakuengama, J.G. - Cited in Article 1, Article 3
Paul, K.I. - Citied in Article 1, Article 3
Peichl, M. - Cited in Article 3 (3 times)
Polglase, P.J. - Cited in Article 1, Article 3
Takimoto, A. - Not cited, but this was the most current article

In my exploration of these journal articles, I found direct citations of many of these authors, with Nair, P.K.R., Kandji, S.T., and Montagnini, F. cited most frequently. Citation of eachother's papers shows there is a network of scientists interested in a common study, in this case, in agroforesty as a carbon offset method.

The second connection is within the research itself.  All of these articles are based on forest sequestration and carbon storage. They are related to the pivotal article because some an aspect of their research deals with agroforestry as a potential atmospheric carbon sink.

In addition, each article above is part of a sub-cluster. The first article was based on change in soil carbon following afforestation, with its main focus being on soil carbon being included in total carbon stocks. The second article focused on tree type and climate effecting carbon storage and sequestration. The third cluster deals with above and below ground carbon, including soils, as well as age of forests being a factor. The final article compared traditional agroforestry, improved foresty, and abandoned areas, concluding that soil profile and land type are important factors to consider.

There was a general theme related to soil carbon pools, and many used the same methods to analyze soil core samples at various depths in their research to determine how much carbon is stored. In addition, there seems to be a general consensus that soil carbon and below-ground biomass need to be included in total carbon stocks for the purpose of determining whether agroforestry is a good strategy for mitigating climate change. Finally, many  of these researchers took external factors, such as climate, tree species, and age of the forest, into account for their calculations.