Climate Change and...

Annotated Bibliography

Carbon Dynamics

Tropical Forests

Z. Ben-Zhi, F. Mao-Yi, X. Jin-Zhong, Y. Xiao-Sheng, L. Zheng-Cai (2005). Ecological functions of bamboo forest: research and application. Journal of Forestry Research 16 (2): 143-147

ABSTRACT: Bamboo forest is an important forest type in subropical and tropical areas. Due to its biological characteristic and growth habits, bamboo is not only an ideal economic investment that can be utilized in many different manners but also has enormous potential for alleviating many environmental problems facing the world today. This review describes ecological functions of the bamboo forest on soil erosion control, water conservation, land rehabilitation, and carbon sequestration.

Brown, S., Hall, C.A.S., Knabe, W., Raich, J., Trexler, M.C., Woomer, P. (1993). Tropical forests: their past, present, and potential future role in the terrestrial carbon budget. Water, Air, and Soil Pollution 70 (1-4): 71-94

ABSTRACT: In this paper we review results of research to summarize the state-of-knowledge of the past, present, and potential future roles of tropical forests in the global C cycle. In the pre-industrial period (ca. 1850), the flux from changes in tropical land use amounted to a small C source of about 0.06 Pg yr–1 . By 1990, the C source had increased to 1.7 ± 0.5 Pg yr–1 . The C pools in forest vegetation and soils in 1990 was estimated to be 159 Pg and 216 Pg, respectively. No concrete evidence is available for predicting how tropical forest ecosystems are likely to respond to CO2 enrichment and/or climate change. However, C sources from continuing deforestation are likely to overwhelm any change in C fluxes unless land management efforts become more aggressive. Future changes in land use under a business as usual scenario could release 41–77 Pg C over the next 60 yr. Carbon fluxes from losses in tropical forests may be lessened by aggressively pursued agricultural and forestry measures. These measures could reduce the magnitude of the tropical C source by 50 Pg by the year 2050. Policies to mitigate C losses must be multiple and concurrent, including reform of forestry, land tenure, and agricultural policies, forest protection, promotion of on-farm forestry, and establishment of plantations on non-forested lands. Policies should support improved agricultural productivity, especially replacing non-traditional slash-and-burn agriculture with more sustainable and appropriate approaches.

Brown, S., Hall, M., Andrasko, K., Ruiz, F., Marzoli, W., Guerrero, G., Masera, O., Dushku, A., DeJong, B., Cornell, J. (2007). Baselines for land-use change in the tropics: application to avoided deforestation projects. Mitigation and Adaptation Strategies for Global Change 12 (6): 1001-1026

ABSTRACT: Although forest conservation activities, particularly in the tropics, offer significant potential for mitigating carbon (C) emissions, these types of activities have faced obstacles in the policy arena caused by the difficulty in determining key elements of the project cycle, particularly the baseline. A baseline for forest conservation has two main components: the projected land-use change and the corresponding carbon stocks in applicable pools in vegetation and soil, with land-use change being the most difficult to address analytically. In this paper we focus on developing and comparing three models, ranging from relatively simple extrapolations of past trends in land use based on simple drivers such as population growth to more complex extrapolations of past trends using spatially explicit models of land-use change driven by biophysical and socioeconomic factors. The three models used for making baseline projections of tropical deforestation at the regional scale are: the Forest Area Change (FAC) model, the Land Use and Carbon Sequestration (LUCS) model, and the Geographical Modeling (GEOMOD) model. The models were used to project deforestation in six tropical regions that featured different ecological and socioeconomic conditions, population dynamics, and uses of the land: (1) northern Belize; (2) Santa Cruz State, Bolivia; (3) Paraná State, Brazil; (4) Campeche, Mexico; (5) Chiapas, Mexico; and (6) Michoacán, Mexico.

A comparison of all model outputs across all six regions shows that each model produced quite different deforestation baselines. In general, the simplest FAC model, applied at the national administrative-unit scale, projected the highest amount of forest loss (four out of six regions) and the LUCS model the least amount of loss (four out of five regions). Based on simulations of GEOMOD, we found that readily observable physical and biological factors as well as distance to areas of past disturbance were each about twice as important as either sociological/demographic or economic/infrastructure factors (less observable) in explaining empirical land-use patterns.

We propose from the lessons learned, a methodology comprised of three main steps and six tasks can be used to begin developing credible baselines. We also propose that the baselines be projected over a 10-year period because, although projections beyond 10 years are feasible, they are likely to be unrealistic for policy purposes. In the first step, an historic land-use change and deforestation estimate is made by determining the analytic domain (size of the region relative to the size of proposed project), obtaining historic data, analyzing candidate baseline drivers, and identifying three to four major drivers. In the second step, a baseline of where deforestation is likely to occur–a potential land-use change (PLUC) map—is produced using a spatial model such as GEOMOD that uses the key drivers from step one. Then rates of deforestation are projected over a 10-year baseline period based on one of the three models. Using the PLUC maps, projected rates of deforestation, and carbon stock estimates, baseline projections are developed that can be used for project GHG accounting and crediting purposes: The final step proposes that, at agreed interval (e.g., about 10 years), the baseline assumptions about baseline drivers be re-assessed. This step reviews the viability of the 10-year baseline in light of changes in one or more key baseline drivers (e.g., new roads, new communities, new protected area, etc.). The potential land-use change map and estimates of rates of deforestation could be re-done at the agreed interval, allowing the deforestation rates and changes in spatial drivers to be incorporated into a defense of the existing baseline, or the derivation of a new baseline projection.

Cleveland, C. C., Townsend, A. R. (2006). Nutrient additions to a tropical rain forest drive substantial soil carbon dioxide losses to the atmosphere. Proceedings of the National Academy of Sciences 103 (27): 10316-10321

ABSTRACT: Terrestrial biosphere–atmosphere carbon dioxide (CO2 ) exchange is dominated by tropical forests, where photosynthetic carbon (C) uptake is thought to be phosphorus (P)-limited. In P-poor tropical forests, P may also limit organic matter decomposition and soil C losses. We conducted a field-fertilization experiment to show that P fertilization stimulates soil respiration in a lowland tropical rain forest in Costa Rica. In the early wet season, when soluble organic matter inputs to soil are high, P fertilization drove large increases in soil respiration. Although the P-stimulated increase in soil respiration was largely confined to the dry-to-wet season transition, the seasonal increase was sufficient to drive an 18% annual increase in CO2 efflux from the P-fertilized plots. Nitrogen (N) fertilization caused similar responses, and the net increases in soil respiration in response to the additions of N and P approached annual soil C fluxes in mid-latitude forests. Human activities are altering natural patterns of tropical soil N and P availability by land conversion and enhanced atmospheric deposition. Although our data suggest that the mechanisms driving the observed respiratory responses to increased N and P may be different, the large CO2 losses stimulated by N and P fertilization suggest that knowledge of such patterns and their effects on soil CO2 efflux is critical for understanding the role of tropical forests in a rapidly changing global C cycle.

Fearnside, P. M., Imbrozio Barbosa, R. (1998). Soil carbon changes from conversion of forest to pasture in Brazilian Amazonia. Forest Ecology and Management 108 (1-2): 147-166

ABSTRACT: Soils in Brazilian Amazonia may contain up to 136 Gt of carbon to a depth of 8 m, of which 47 Gt are in the top meter. The current rapid conversion of Amazonian forest to cattle pasture makes disturbance of this carbon stock potentially important to the global carbon balance and net greenhouse gas emissions. Information on the response of soil carbon pools to conversion to cattle pasture is conflicting. Some of the varied results that have been reported can be explained by effects of soil compaction, clay content and seasonal changes. Most studies have compared roughly simultaneous samples taken at nearby sites with different use histories (i.e., `chronosequences'); a clear need exists for longitudinal studies in which soil carbon stocks and related parameters are monitored over time at fixed locations. Whether pasture soils are a net sink or a net source of carbon depends on their management, but an approximation of the fraction of pastures under 'typical' and 'ideal' management practices indicates that pasture soils in Brazilian Amazonia are a net carbon source, with the upper 8 m releasing an average of 12.0 t C/ha in land maintained as pasture in the equilibrium landscape that is established in the decades following deforestation. Considering the equilibrium landscape as a whole, which is dominated by pasture and secondary forest derived from pasture, the average net release of soil carbon is 8.5 t C/ha, or 11.7x106 t C for the 1.38x106 ha cleared in 1990. Only 3% of the calculated emission comes from below 1 m depth, but the ultimate contribution from deep layers may be substantially greater. The land area affected by soil C losses under pasture is not restricted to the portion of the region maintained under pasture in the equilibrium landscape, but also the portion under secondary forests derived from pasture. Pasture effects from deforestation in 1990 represent a net committed emission from soils of 9.2x106 t C, or 79% of the total release from soils from deforestation in that year. Soil emissions from Amazonian deforestation represent a quantity of carbon approximately 20% as large as Brazil's annual emission from fossil fuels.

Fisher, R. A., Williams, M., Da Costa, A. L., Malhi, Y., Da Costa, R. F., Almeida, S., Meir, P. (2007). The response of an Eastern Amazonian rain forest to drought stress: results and modelling analyses from a throughfall exclusion experiment. Global Change Biology 13 (11): 2361-2378

ABSTRACT: Warmer and drier climates over Eastern Amazonia have been predicted as a component of climate change during the next 50-100 years. It remains unclear what effect such changes will have on forest-atmosphere exchange of carbon dioxide (CO2 ) and water, but the cumulative effect is anticipated to produce climatic feedback at both regional and global scales. To allow more detailed study of forest responses to soil drying, a simulated soil drought or 'throughfall exclusion' (TFE) experiment was established at a rain forest site in Eastern Amazonia, Brazil, for which time-series sap flow and soil moisture data were obtained. The experiment excluded 50% of the throughfall from the soil. Sap flow data from the forest plot experiencing normal rainfall showed no limitation of transpiration throughout the two monitored dry seasons. Conversely, data from the TFE showed large dry season declines in transpiration, with tree water use restricted to 20% of that in the control plot at the peak of both dry seasons. The results were examined to evaluate the paradigm that the restriction on transpiration in the dry season was caused by limitation of soil-to-root water transport, driven by low soil water potential and high soil-to-root hydraulic resistance. This paradigm, embedded in the soil-plant-atmosphere (SPA) model and driven using on-site measurements, provided a good explanation (R2 > 0.69) of the magnitude and timing of changes in sap flow and soil moisture. This model-data correspondence represents a substantial improvement compared with other ecosystem models of drought stress tested in Amazonia. Inclusion of deeper rooting should lead to lower sensitivity to drought than the majority of existing models. Modelled annual GPP declined by 13-14% in response to the treatment, compared with estimated declines in transpiration of 30-40%.

Foley, J. A., Asner, G. P., Costa, M. H., Coe, M. T., Defries, R., Gibbs, H. K., Howard, E. A., Olson, S., Patz, J., Ramankutty, N., Snyder, P. (2007). Amazonia revealed: forest degradation and loss of ecosystem goods and services in the Amazon basin. Frontiers in Ecology and the Environment 5 (1): 25-32

ABSTRACT; The Amazon Basin is one of the world's most important bioregions, harboring a rich array of plant and animal species and offering a wealth of goods and services to society. For years, ecological science has shown how large-scale forest clearings cause declines in biodiversity and the availability of forest products. Yet some important changes in the rainforests, and in the ecosystem services they provide, have been underappreciated until recently. Emerging research indicates that land use in the Amazon goes far beyond clearing large areas of forest; selective logging and other canopy damage is much more pervasive than once believed. Deforestation causes collateral damage to the surrounding forests – through enhanced drying of the forest floor, increased frequency of fires, and lowered productivity. The loss of healthy forests can degrade key ecosystem services, such as carbon storage in biomass and soils, the regulation of water balance and river flow, the modulation of regional climate patterns, and the amelioration of infectious diseases. We review these newly revealed changes in the Amazon rainforests and the ecosystem services that they provide.

Glenday, J. (2006). Carbon storage and emissions offset potential in an East African tropical rainforest. Forest Ecology and Management 235 (1-3): 72-83

ABSTRACT: Forestry based carbon emissions offset projects have potential to both mitigate climate change and foster sustainable forest management. Degraded African tropical forests could sequester large amounts of additional carbon, but the lack of empirical data limits the feasibility of initiating carbon offset projects in many threatened forests. This study examines the potential to increase carbon stocks in the Kakamega National Forest of western Kenya, a threatened biodiversity hotspot and Kenya's only remaining rainforest. Carbon density values for indigenous forest and plantations were estimated based on forest inventory data from 95 randomized plots distributed throughout the forest. Total ecosystem carbon was estimated using allometric equations for tree biomass, destructive techniques for litter and herbaceous vegetation biomass, and Dumas combustion and spectroscopy for soils. Land cover maps for 1975, 1986, and 2000 were used to estimate both current carbon stocks and the influence of past land use changes. Mean carbon density in indigenous forest was 330 ± 65 Mg C/ha, greater than that of the forest's hardwood plantations (280 ± 77 Mg C/ha) and significantly greater that that of softwood plantations (250 ± 77 Mg C/ha). The distribution of carbon densities within the indigenous forest and the variation between plantation types suggest management practices could feasibly increase Kakamega's carbon stock. Deforestation between 1975 and 1986 and limited reforestation from 1986 to 2000 have resulted in a net loss of 0.4–0.6 Tg C. If this loss were reversed, the value of possible associated carbon credits dwarfs the current operational budget for managing and protecting the forest, even at low carbon prices. Additional income could help address resource needs of impoverished communities surrounding the forest and promote sustainable protection of Kakamega's high biodiversity.

Houghton, R. A., Hackler, J. L. (2006). Emissions of carbon from land use change in sub-Saharan Africa. Journal of Geophysical Research - Biogeosciences 111 (G02003): doi:10.1029/2005JG000076

ABSTRACCT: Previous estimates of the flux of carbon from land use change in sub-Saharan Africa have been based on highly aggregated data and have ignored important categories of land use. To improve these estimates, we divided the region into four subregions (east, west, central, and southern Africa), each with six types of natural vegetation and five types of land use (permanent crops, pastures, shifting cultivation, industrial wood harvest, and tree plantations). We reconstructed rates of land use change and rates of wood harvest from country-level statistics reported by the Food and Agriculture Organization (FAO) (1961–2000) and extrapolated the rates from 1961 to 1850 on the basis of qualitative histories of demography, economy, and land use. We used a bookkeeping model to calculate the annual flux of carbon associated with these changes in land use. Country-level estimates of average forest biomass from the FAO, together with changes in biomass calculated from the reconstructed rates of land use change, constrained the average biomass of forests in 1850. Comparison of potential (predisturbance) forest areas with the areas present in 1850 and 2000 suggests that 60% of Africa's forests were lost before 1850 and an additional 10% lost in the last 150 years. The annual net flux of carbon from changes in land use was probably small and variable before the early 1900s but increased to a source of 0.3 ± 0.2 PgC/yr by the end of the century. In the 1990s the source was equivalent to about 15% of the global net flux of carbon from land use change.

Kirby, K. R., Potvin, C. (2007). Variation in carbon storage among tree species: Implications for the management of a small-scale carbon sink project. Forest Ecology and Management 246 (2-3): 208-221

ABSTRACT: Despite growing evidence for an effect of species composition on carbon (C) storage and sequestration, few projects have examined the implications of such a relationship for forestry and agriculture-based climate change mitigation activities. We worked with a community in Eastern Panama to determine the average above- and below-ground C stocks of three land-use types in their territory: managed forest, agroforests and pasture. We examined evidence for a functional relationship between tree-species diversity and C storage in each land-use type, and also explored how the use of particular tree species by community members could affect C storage. We found that managed forests in this landscape stored an average of 335 Mg C ha−1 , traditional agroforests an average of 145 Mg C ha−1 , and pastures an average of 46 Mg C ha−1 including all vegetation-based C stocks and soil C to 40 cm depth. We did not detect a relationship between diversity and C storage; however, the relative contributions of species to C storage per hectare in forests and agroforests were highly skewed and often were not proportional to species’ relative abundances. We conclude that protecting forests from conversion to pasture would have the greatest positive impact on C stocks, even though the forests are managed by community members for timber and non-timber forest products. However, because several of the tree species that contribute the most to C storage in forests were identified by community members as preferred timber species, we suggest that species-level management will be important to avoiding C-impoverishment through selective logging in these forests. Our data also indicate that expanding agroforests into areas currently under pasture could sequester significant amounts of carbon while providing biodiversity and livelihood benefits that the most common reforestation systems in the region – monoculture teak plantations – do not provide.

R. D. Lasco, M. M. Cardinoza (2007). Baseline carbon stocks assessment and projection of future carbon benefits of a carbon sequestration project in East Timor. Mitigation and Adaptation Strategies for Global Change 12 (2): 243-257

ABSTRACT: Climate change is one of the most pressing environmental problems humanity is facing today. Forest ecosystems serve as a source or sink of greenhouse gases, primarily CO2 . With support from the Canadian Climate Change Fund, the Community-based Natural Resource Management for Carbon Sequestration project in East Timor (CBNRM-ET) was implemented to “maintain carbon (C) stocks and increase C sequestration through the development of community-based resource management systems that will simultaneously improve livelihood security”. Project sites were in the Laclubar and Remexio Sub-districts of the Laclo watershed. The objective of this study was to quantify baseline C stocks and sequestration benefits of project components (reforestation with fast-growing species, primarilyCasuarina equisetifolia , and agroforestry involving integration ofParaserianthes falcataria ). Field measurements show that mature stands (=30 years) ofP. falcataria andC. equisetifolia contain up to 200 Mg C ha-1 in above ground biomass, indicating the vast potential of project sites to sequester carbon. Baseline C stocks in above ground biomass were very low in both Laclubar (6.2 Mg C ha-1 for reforestation sites and 5.2 Mg C ha-1 for agroforestry sites and Remexio (3.0 Mg C ha-1 for reforestation and 2.5 Mg C ha-1 for agroforestry). Baseline soil organic C levels were much higher reaching up to 160 Mg C ha-1 in Laclubar and 70 Mg C ha-1 in Remexio. For the next 25 years, it is projected that 137 671 Mg C and 84 621 Mg C will be sequestered under high- and low C stock scenarios, respectively.

Laurance, W. F. (2000). Mega-development trends in the Amazon: Implications for global change. Environmental Monitoring and Assessment 61 (1): 113-122.

ABSTRACT: This paper describes four global-change phenomena that are having major impacts on Amazonian forests. The first is accelerating deforestation and logging. Despite recent government initiatives to slow forest loss, deforestation rates in Brazilian Amazonia have increased from 1.1 million ha yr–1 in the early 1990s, to nearly 1.5 million ha yr–1 from 1992–1994, and to more than 1.9 million ha yr–1 from 1995–1998. Deforestation is also occurring rapidly in some other parts of the Amazon Basin, such as in Bolivia and Ecuador, while industrialized logging is increasing dramatically in the Guianas and central Amazonia.

The second phenomenon is that patterns of forest loss and fragmentation are rapidly changing. In recent decades, large-scale deforestation has mainly occurred in the southern and eastern portions of the Amazon — in the Brazilian states of Pará, Maranho, Rondônia, Acre, and Mato Grosso, and in northern Bolivia. While rates of forest loss remain very high in these areas, the development of major new highways is providing direct conduits into the heart of the Amazon. If future trends follow past patterns, land-hungry settlers and loggers may largely bisect the forests of the Amazon Basin.

The third phenomenon is that climatic variability is interacting with human land uses, creating additional impacts on forest ecosystems. The 1997/98 El Niño drought, for example, led to a major increase in forest burning, with wildfires raging out of control in the northern Amazonian state of Roraima and other locations. Logging operations, which create labyrinths of roads and tracks in forsts, are increasing fuel loads, desiccation and ignition sources in forest interiors. Forest fragmentation also increases fire susceptibility by creating dry, fire-prone forest edges.
Finally, recent evidence suggests that intact Amazonian forests are a globally significant carbon sink, quite possibly caused by higher forest growth rates in response to increasing atmospheric CO2 fertilization. Evidence for a carbon sink comes from long-term forest mensuration plots, from whole-forest studies of carbon flux and from investigations of atmospheric CO2 and oxygen isotopes. Unfortunately, intact Amazonian forests are rapidly diminishing. Hence, not only is the destruction of these forests a major source of greenhouse gases, but it is reducing their intrinsic capacity to help buffer the rapid anthropogenic rise in CO2 .

Macedo, M.O., Resende, A.S., Garcia, P.C., Boddey, R.M., Jantalia, C.P., Urquiaga, S., Campello, E.F.C., Franco, A.A. (2008). Changes in soil C and N stocks and nutrient dynamics 13 years after recovery of degraded land using leguminous nitrogen-fixing trees. Forest Ecology and Management 255 (5-6): 1516-1524

ABSTRACT: In tropical forest areas with highly weathered soils, organic matter plays an important role in soil functioning and forest sustainability. When forests are clear-cut, the soil begins almost immediately to lose organic matter, triggering a series of soil degradation processes, the extent and intensity of which depends on soil management. Depending on the level of soil degradation, the rate at which the system can re-establish itself can be slow and may require the use of degraded land restoration techniques. This study aimed at evaluating the potential of pioneer leguminous nitrogen-fixing trees to recuperate degraded land. The area studied – located in the coastal town of Angra dos Reis in the State of Rio de Janeiro, Brazil – was planted with seven species of fast-growing leguminous nitrogen-fixing trees in 1991. The nutrient concentrations (Ca, Mg, P and K) and N and C stocks in the soil and litter were determined, in addition to the free- and occluded-light fractions of soil organic matter. Soil samples were also collected from two reference areas: (1) an area of undisturbed native forest; and (2) a deforested area spontaneously colonised by Guinea grass (Panicum maximum ). The nutrient stocks in the litter of the restored area were similar to those found in native forest. The recuperation technique used was able to re-establish the soil C and N stocks after 13 years. C and N increased by 1.73 and 0.13 Mg ha−1 year−1 , respectively. The free-light fraction was highest in the recuperated area and lowest in the deforested area. The occluded-light fraction of the recuperated area was higher than that of the native forest only in the 0–5 cm layer. Both the free-light and occluded fractions were higher in the native forest and recuperated areas than in the deforested area. Since the free-light and the occluded-light fractions are the result of litterfall and decomposition, these results – combined with the data of litter stocks and soil C and N stocks – indicate that the use of legume trees was efficient in re-establishing the nutrient cycling processes of the systems. These results also show that recovering degraded land with this technique is effective in sequestering carbon dioxide from the atmosphere at high rates.

Raich, J.W., Russell, A.E., Kitayama, K., Parton, W.J., Vitousek, P.M. (2006). Temperature influences carbon accumulation in moist tropical forests. Ecology 87 (1): 76-87

ABSTRACT: Evergreen broad-leaved tropical forests can have high rates of productivity and large accumulations of carbon in plant biomass and soils. They can therefore play an important role in the global carbon cycle, influencing atmospheric CO2 concentrations if climate warms. We applied meta-analyses to published data to evaluate the apparent effects of temperature on carbon fluxes and storages in mature, moist tropical evergreen forest ecosystems. Among forests, litter production, tree growth, and belowground carbon allocation all increased significantly with site mean annual temperature (MAT); total net primary productivity (NPP) increased by an estimated 0.2–0.7 Mg C·ha−1 ·yr−1 ·°C−1 . Temperature had no discernible effect on the turnover rate of aboveground forest biomass, which averaged 0.014 yr−1 among sites. Consistent with these findings, forest biomass increased with site MAT at a rate of 5–13 Mg C·ha−1 ·°C−1 . Despite greater productivity in warmer forests, soil organic matter accumulations decreased with site MAT, with a slope of −8 Mg C·ha−1 ·°C−1 , indicating that decomposition rates of soil organic matter increased with MAT faster than did rates of NPP. Turnover rates of surface litter also increased with temperature among forests. We found no detectable effect of temperature on total carbon storage among moist tropical evergreen forests, but rather a shift in ecosystem structure, from low-biomass forests with relatively large accumulations of detritus in cooler sites, to large-biomass forests with relatively smaller detrital stocks in warmer locations. These results imply that, in a warmer climate, conservation of forest biomass will be critical to the maintenance of carbon stocks in moist tropical forests.

Schedlbauer, J. L., Kavanagh, K. L. (2008). Soil carbon dynamics in a chronosequence of secondary forests in northeastern Costa Rica. Forest Ecology and Management 255 (3-4): 1326-1335

ABSTRACT: Secondary forests are becoming an increasingly important tropical landscape component with the potential to provide environmental services such as soil carbon storage. Substantial losses of soil carbon can occur with tropical forest conversion to pasture, but stocks can sometimes be restored with the development of secondary forest. Few studies have taken advantage of shifts in vegetation from C4 to C3 communities to determine soil carbon turnover following secondary forest development on pasture. Because trees quickly colonize abandoned pastures in northeastern Costa Rica, we expected to find evidence of increased soil carbon storage and gradual soil carbon turnover following pasture abandonment. Three early successional and nine late successional secondary sites ranging in age from 2.6 to 33 years, as well as four pastures were used in this study. At each site, mineral soil samples up to 30 cm depth were collected from three plots to determine bulk density, percent soil carbon, and stable carbon isotope values (d13 C). Thed13 C of soil respired CO2 was also determined at each site. Contrary to expectations, soil carbon storage did not increase with secondary forest age and was unrelated to increases in aboveground carbon storage. However, pastures stored 19% more carbon than early and late successional sites in the top 10 cm of mineral soil, and successional sites stored 14-18% more carbon than pastures between 10 and 30 cm.d13 C data indicated that most pasture-derived soil carbon in the top 30 cm of soil turned over within 10 years of pasture abandonment and subsequent colonization by trees. Overall, these data indicate that total soil carbon storage remains relatively unchanged following land use transitions from pasture to secondary forest. This is likely due to the presence of large passive pools of mineral-stabilized soil carbon in this region of Costa Rica. The contribution of these forests to increased carbon storage on the landscape is primarily confined to aboveground carbon stocks, though other environmental services may be derived from these forests. In the context of global carbon accounting, it appears that future carbon credits may be best applied to aboveground carbon storage in secondary forests regrowing on soils with large mineral-stabilized soil carbon pools.

Sierra, C. A., del Valle, J. I., Orrego, S. A., Moreno, F. H., Harmon, M. E., Zapata, M., Colorado, G. J., Herrera, M. A., Lara, W., Restrepo, D. E., Berrouet, L. M., Loaiza, L. M., Benjumea, J. F. (2007). Total carbon stocks in a tropical forest landscape of the Porce region, Colombia. Forest Ecology and Management 243 (2-3): 299-309

ABSTRACT: Detailed ground-based quantifications of total carbon stocks in tropical forests are few despite their importance in science and ecosystem management. Carbon stocks in live aboveground and belowground biomass, necromass, and soils were measured in a heterogeneous landscape composed of secondary and primary forest. A total of 110 permanent plots were used to estimate the size of these carbon pools. Local biomass equations were developed and used to estimate aboveground biomass and coarse root biomass for each plot. Herbaceous vegetation, fine roots, coarse and fine litter, and soil carbon to 4 m depth were measured in subplots. In primary forests, mean total carbon stocks (TCS) were estimated as 383.7 ± 55.5 Mg C ha−1 (±S.E.). Of this amount, soil organic carbon to 4 m depth represented 59%, total aboveground biomass 29%, total belowground biomass 10%, and necromass 2%. In secondary forests, TCS was 228.2 ± 13.1 Mg C ha−1 , and soil organic carbon to 4 m depth accounted for 84% of this amount. Total aboveground biomass represented only 9%, total belowground biomass 5%, and total necromass 1% of TCS in secondary forests. Monte Carlo methods were used to assess the uncertainty of the biomass measurements and spatial variation. Of the total uncertainty of the estimates of TCS, the variation associated with the spatial variation of C pools between plots was higher than measurement errors within plots. From this study it is concluded that estimates of aboveground biomass largely underestimate total carbon stocks in forest ecosystems. Additionally, it is suggested that heterogeneous landscapes impose additional challenges for their study such as sampling intensity.

Tian, H., Melillo, J. M., Kicklighter, D.W., McGuire, A. D., Helfrich, J. V. K., Moore, B., III, Vorosmarty, C. J. (1998). Effect of interannual climate variability on carbon storage in Amazonian ecosystems. Nature 396 (6712): 664-667

ABSTRACT: The Amazon Basin contains almost one-half of the world's undisturbed tropical evergreen forest as well as large areas of tropical savanna1,2 . The forests account for about 10 per cent of the world's terrestrial primary productivity and for a similar fraction of the carbon stored in land ecosystems2,3 , and short-term field measurements4 suggest that these ecosystems are globally important carbon sinks. But tropical land ecosystems have experienced substantial interannual climate variability owing to frequent El Niño episodes in recent decades5 . Of particular importance to climate change policy is how such climate variations, coupled with increases in atmospheric CO2 concentration, affect terrestrial carbon storage6, 7, 8 . Previous model analyses have demonstrated the importance of temperature in controlling carbon storage9,10 . Here we use a transient process-based biogeochemical model of terrestrial ecosystems3,11 to investigate interannual variations of carbon storage in undisturbed Amazonian ecosystems in response to climate variability and increasing atmospheric CO2 concentration during the period 1980 to 1994. In El Niño years, which bring hot, dry weather to much of the Amazon region, the ecosystems act as a source of carbon to the atmosphere (up to 0.2 petagrams of carbon in 1987 and 1992). In other years, these ecosystems act as a carbon sink (up to 0.7 Pg C in 1981 and 1993). These fluxes are large; they compare to a 0.3 Pg C per year source to the atmosphere associated with deforestation inthe Amazon Basin in the early 1990s12 . Soil moisture, which is affected by both precipitation and temperature, and which affects both plant and soil processes, appears to be an important control on carbon storage.

Van Noordwijk, M., Cerri, C. C., Woomer, P. L., Nugroho, K., Bernoux, M. (1997). Soil carbon dynamics in the humid tropical forest zone. Geoderma 79 (1-4): 187-225.

ABSTRACT: Conversion of natural forests to agriculture in the humid tropics leads to a reduction in ecosystem carbon storage due to the immediate removal of aboveground biomass and a gradual subsequent reduction in soil organic carbon. A considerable part of soil carbon is protected from microbial attack by a range of physical and chemical mechanisms and is not sensitive to landuse change. We analyzed the soils data base for Sumatra (Indonesia) developed by the Center for Soil and Agroclimate Research (CSAR) to estimate effects of landuse on soil C content. Sumatra has a considerable diversity of soils ranging from those of recent origin in the highlands, to older sedimentary and heavily leached soils in the pedimont peneplain and large areas of wetland soils along the coast. Peat soils (Histosols) and other wetland soils (Aquic and Fluvic suborders) contain the greatest soil C reserves, followed by young volcanic soils (Andisols). Agricultural use of these soils can have a disproportionately large effect on C release to the atmosphere. On the major part of the upland soils the difference in (top) soil C content between natural forest and agricultural land is in the range 0.5–1.0% C, equivalent to a change in total C stock of 10–20 Mg ha−1 . These results agree with data collected in S. Sumatra in the 1930s. Corg of forest soils is related to soil pH, and is lowest in the pH range 5.0–6.0. Wetland conditions, lower pH, higher altititude (lower temperature) and higher clay and silt content all contributed to higher soil C contents in a multiple regression analysis of the whole data set. Existing models and data sets are insufficient to predict changes in soil C contents under various landuse practices. Carbon isotope studies, and especially thed13 C method may be used to study the effects of landuse change, especially when the vegetation was changed from one dominated by C3 plants (most forest species) to one dominated by grasses and crops with a C4 photosynthetic pathway. Results from Brazil documented a gradual decline of organic matter originating from the forest system and its partial replacement by organic matter derived from inputs of sugarcane during the first fifty years of cultivation. Forest conversion to well managed grasslands may lead to an increased soil C storage, after an initial decline. The consequences of erosion on losses of soil C depend on the scale at which these losses are considered, because of sedimentation processes. When net erosion losses are not expressed per unit area, but per length scale to the power l.6, erosion losses appear to be largely independent of scale. The'fractal dimension of erosion’ (on average around 1.6) probably is a landscape characteristic and estimates of its value are needed for extrapolation. Better understanding of soil C deposition sites is needed to evaluate overall erosion effects and test whether or not erosion can contribute to net C sequestration.

J. M. Melillo, R. A. Houghton, D.W. Kicklighter, A. D. McGuire (1996). Tropical deforestation and the global carbon budget. Annual Review of Energy and the Environment 21: 293-310

ABSTRACT: The CO2 concentration of the atmosphere has increased by almost 30% since 1800. This increase is due largely to two factors: the combustion of fossil fuel and deforestation to create croplands and pastures. Deforestation results in a net flux of carbon to the atmosphere because forests contain 20–50 times more carbon per unit area than agricultural lands. In recent decades, the tropics have been the primary region of deforestation. The annual rate of CO2 released due to tropical deforestation during the early 1990s has been estimated at between 1.2 and 2.3 gigatons C. The range represents uncertainties about both the rates of deforestation and the amounts of carbon stored in different types of tropical forests at the time of cutting. An evaluation of the role of tropical regions in the global carbon budget must include both the carbon flux to the atmosphere due to deforestation and carbon accumulation, if any, in intact forests. In the early 1990s, the release of CO2 from tropical deforestation appears to have been mostly offset by CO2 uptake occurring elsewhere in the tropics, according to an analysis of recent trends in the atmospheric concentrations of O2 and N2 . Interannual variations in climate and/or CO2 fertilization may have been responsible for the CO2 uptake in intact forests. These mechanisms are consistent with site-specific measurements of net carbon fluxes between tropical forests and the atmosphere, and with regional and global simulations using process-based biogeochemistry models.

Jean Pierre H. B. Ometto, A. D. Nobre, H. R. Rocha, P. Artaxo, L. A. Martinelli (2005). Amazonia and the modern carbon cycle: lessons learned. Oecologia 143 (4): 483-500

ABSTRACT: In this paper, we review some critical issues regarding carbon cycling in Amazonia, as revealed by several studies conducted in the Large Scale Biosphere Atmosphere Experiment in Amazonia (LBA). We evaluate both the contribution of this magnificent biome for the global net primary productivity/net ecosystem exchange (NPP/NEE) and the feedbacks of climate change on the dynamics of Amazonia. In order to place Amazonia in a global perspective and make the carbon flux obtained through the LBA project comparable with global carbon budgets, we extrapolated NPP/NEE values found by LBA studies to the entire area of the Brazilian Amazon covered by rainforest. The carbon emissions due to land use changes for the tropical regions of the world produced values from 0.96 to 2.4 Pg C year−1 , while atmospheric CO2 inversion models have recently indicated that tropical lands in the Americas could be exchanging a net 0.62±1.15 Pg C year−1 with the atmosphere. The difference calculated from these two methods would imply a local sink of approximately 1.6–1.7 Pg C year−1 , or a source of 0.85 ton C ha−1 year−1 . Using our crude extrapolation of LBA values for the Amazon forests (5 million km2 ) we estimate a range for the C flux in the region of −3.0 to 0.75 Pg C year−1 . The exercise here does not account for environmental variability across the region, but it is an important driver for present and future studies linking local process (i.e. nutrient availability, photosynthetic capacity, and so forth) to global and regional dynamic approaches.

Phillips, O. L., L. E. O. C. Aragão, S. L. Lewis, J. B. Fisher, J. Lloyd, 61 co-authors, (2009). Drought sensitivity of the Amazon rainforest. Science 323 (5919): 1344-1347

ABSTRACT: Amazon forests are a key but poorly understood component of the global carbon cycle. If, as anticipated, they dry this century, they might accelerate climate change through carbon losses and changed surface energy balances. We used records from multiple long-term monitoring plots across Amazonia to assess forest responses to the intense 2005 drought, a possible analog of future events. Affected forest lost biomass, reversing a large long-term carbon sink, with the greatest impacts observed where the dry season was unusually intense. Relative to pre-2005 conditions, forest subjected to a 100-millimeter increase in water deficit lost 5.3 megagrams of aboveground biomass of carbon per hectare. The drought had a total biomass carbon impact of 1.2 to 1.6 petagrams (1.2 x 1015 to 1.6 x 1015 grams). Amazon forests therefore appear vulnerable to increasing moisture stress, with the potential for large carbon losses to exert feedback on climate change.

bottom right