What Are Some Of The Effects Of Land-Cover And Land-Use Changes?


Demonstration Of The Potential For Carbon Sequestration At Global Scales Using Forest Regeneration Or By Slowing Deforestation

Forest systems cover more than 4.1 billion hectares of the Earth's land area. Globally, forest vegetation and soils contain about 1150 billion tones (GtC) of carbon, with approximately 37% of this carbon in low-latitude forests, 14% in mid-latitudes, and 49% at high latitudes. Over two-thirds of the carbon in forest ecosystems is contained in soils and associated peat deposits. In 1990, deforestation in the low latitudes emitted 1.6 GtC per year, whereas forest area expansion and growth in mid- and high-latitude forests sequestered 0.7 GtC of carbon per year, for a net flux to the atmosphere of 0.9 GtC of carbon per year. Slowing deforestation, combined with an increase in forestation and other management measures to improve forest ecosystem productivity, could conserve or sequester significant quantities of carbon. Future forest carbon cycling trends attributable to losses and regrowth associated with global climate and land-use change are uncertain. Model projections and some results suggest that forests could be carbon sinks or sources in the future.
Reference: Carbon Pools and Flux of Global Forest Ecosystems, Dixon, R. K., S. Brown, R. A. Houghton, A.M. Solomon, M. C. Trexler, and J. Wisniewski, Science, Vol. 263, p. 185-190, 1994.

New Studies Suggest That Precision Farming Can Contribute To Climate Protection

The soil carbon in agricultural lands of the world is an important consideration to researchers trying to understand relationships between land use and climate change. It is generally thought that the conversion of natural ecosystems to agriculture results in the release of approximately 0.6 - 3.6 GtC/yr to the atmosphere as carbon dioxide (CO2). Generally, the tilling of soil associated with agriculture leads to increased oxidation of soil organic matter. However, field experiments at specific locations have demonstrated that there is a potential for reducing soil carbon losses from cultivated soils through changes in agricultural practices. For example, reduced or no-till methods, use of cover crops, and manure amendments can sometimes increase soil carbon levels and enhance soil fertility while maintaining crop yield.

A newly developed, process-based model of carbon and nitrogen in agricultural soils now allows both site-specific and large-scale evaluations of the quantitative impacts of changes in agricultural practices on soil carbon dynamics and CO2 emissions from agricultural soils. The results, for a variety of climate and soil conditions, showed that the best protection against soil carbon loss is manure amendments; however, the results were very sensitive to soil texture. Increased nitrogen fertilization and reduced tillage generally enhanced the retention of soil carbon. The ultimate soil organic content is sensitive to climate and increases with decreasing soil temperature, increasing clay content, nitrogen fertilization, manure amendments and crops with higher residue yields. Efforts to enhance carbon retention and buildup in agricultural soils require comprehensive understanding of soil biogeochemical dynamics.

Reference: Modeling Carbon Biogeochemistry in Agricultural Soils, Li, C., S. Frolking, and R. Harriss, Global Biogeochemical Cycles, Vol. 8, pp. 237-254, 1994.

Human Engineering Of Fresh Water Systems May Be Contributing To Sea Level Rise

Global compilations of tide records indicate that sea level has been rising throughout the twentieth century, with potentially serious consequences for low coastal areas if the rate of rise continues. Thermal expansion of ocean water, as well as the melting of alpine glaciers, are responsible for some of this change, but human activities such as ground water withdrawal, surface water diversion, and land-use changes may also have influenced sea level directly. Sea level is suggested to have risen 10 to 25 centimeters (cm) this century, of which about 1 cm (range -5 to +7 cm) may have been due to the direct effects of humans in changing land use. This rise in sea level would have been about 50% larger if large quantities of water had not been stored in reservoirs and channeled into aquifers by irrigation projects. Human activity also affects the water storage capacity of soils, forests, and wetlands. These and other human activities appear to have caused the net increase in sea level over this past century. The combination of ground water withdrawal, surface water diversion, and land-use changes has caused at least a third of the observed rise in sea level. This work suggests that we need to continually monitor and manage the movement of fresh water.
Reference: Direct Anthropogenic Contributions to Sea Level Rise in the Twentieth Century, Sahagian, D. L., F. W. Schwartz, and D. K. Jacobs, Nature, Vol. 367, p. 54-57, 1994.

Fire Distribution And Associated Emissions

Fire is an important ecosystem process and one which is poorly documented in the tropical world. Distributions of fire as a disturbance regime are needed as input for ecosystem process models. The extent of burning and change in the frequency of fire will have implications for the inventory of emissions and global biogeochemical cycles. Daily coarse resolution satellite data are being used to monitor the occurrence of fire and its timing in the southern African savannas. High resolution satellite data are being used to validate the fire maps and calibrate the data to derive improved estimates of total area burned over a yearly cycle. When linked to models of fuel load and ground measurements of emission factors these data can be used to generate improved estimates of trace gas and particulate emissions. The improved availability of global multiyear satellite data sets means that such information can now be provided on an annual basis.
Reference: Emissions of Trace Gases and Aerosol Particles Due to Vegetation Burning in Southern Hemisphere Africa, Scholes, R. J., D. Ward, and C.O. Justice, Journal of Geophysical Research, in press, 1995.

Soil Organic Carbon In The U.S. Found To Have Increased Since 1950

A study of soil carbon in agricultural ecosystems of the United States (40% of the land area and 60-70% of the agricultural cropland of the U.S.), has been conducted using the CENTURY model for a 124-year simulation (1907-2030). The results suggest that there was a continuous decrease in soil organic carbon following land conversion to agriculture in 1907, that this ended 1950, and was followed by a slight increase in soil organic carbon through 1970. Significant soil organic carbon increases are expected through 2030. It was also found that conservation tillage practices can significantly increase soil carbon, but the impacts are highly variable from region to region. Cover crops were found to lead to significant increases in soil carbon in crop soil and climate regimes where they are feasible and appropriate, especially in the southern and eastern regions of the U.S.
Reference: Alternative Management Practices Affecting Soil Carbon in Agroecosystems of the Central U.S., Donigian, A. S., T. O. Barnwell, Jr., R. B. Jackson IV, A. S. Patwardhan, K. B. Weinrich, A. L. Rowell, R. V. Chinnaswamy, and C.V. Cole, EPA Report 600/R-94/067, 1994.

Surveys Of Tropical Forests Find That They Are Changing Much More Rapidly Now Than They Have In The Past

The measured turnover rate of tropical forest trees since 1934 suggests that there has been a significant increase in forest turnover since about 1960. The increase appears to have been accelerated during the 1980s. The likely cause is environmental changes from increased extreme weather events, adjacent deforestation, and elevated productivity that has resulted from increased atmospheric CO2. This increased turnover rate has implications for tropical biodiversity and possibly for the tropical carbon cycle. Faster forest turnover could lead to a dominance of climbing plants and gap-dependent tree species, which are most likely to benefit from increased CO2. Many of these species have less dense wood than shade-tolerant species, suggesting that tropical forests could eventually become less of a sink for carbon than they are currently.
Reference: Increasing Turnover Through Time in Tropical Forests, Phillips, O. L., and A. H. Gentry, Science, Vol. 263, pp. 954-958, 1994.

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Last updated 04/10/96