The Role of Louisiana’s Forest Ecosystems in Carbon Sequestration

Linda Benedict  |  6/28/2006 7:09:03 PM

Carbon Dioxide and Global Climate Change
Since the late 19th century, the global average temperature of the Earth has increased by 0.7 to 1.4 degrees Fahrenheit. The significant global warming has been attributed to human activities such as fossil fuel combustion and land use change, which lead to the increase of the concentrations of greenhouse gases, especially carbon dioxide (CO2).

Total annual carbon emission from the Earth’s surface and from fossil fuel use is estimated to be about 7 billion metric tons, of which 2 billion metric tons are taken by the oceans and 1.8 billion metric tons are sequestered by vegetation growth, resulting in an annual increase of 3.2 billion metric tons in the atmosphere. Over the past 150 years, humans have put nearly 300 billion metric tons of carbon into the air. The atmospheric CO2 level is continuously rising and has recently surpassed 380 parts per million by volume (ppmv).

The Intergovernmental Panel on Climate Change (IPCC), established by the United Nations World Meteorological Organization (WMO) and Environment Programme (UNEP), has predicted that if anthropogenic emissions of carbon dioxide continue at present-day levels, the atmospheric concentration of carbon dioxide would increase to 460-560 ppmv by 2100. The global surface temperature would then be at least 0.7 degree Fahrenheit warmer than the levels in 1990.

Role of Forests in Global Carbon Budget
Trees are referred to as carbon sequesters because they take CO2 from the atmosphere and store it in biomass – live and dead tree trunks, branches, foliage and roots – and soils. About half of a tree is made up of carbon. The world’s forest cover is estimated to be about 9.4 billion acres. The amount of carbon stored in forest biomass alone is about 283 billion metric tons. Together with deadwood, litter and soil, the world’s forests store about 50 percent more carbon than the amount of carbon in the atmosphere, making a significant influence on the global carbon cycle.

Forest ecosystems in the United States contain about 53 billion metric tons of carbon, of which 41 percent is stored in biomass and 59 percent in forest soils. Spatial and temporal changes in this large carbon storage are complex and can be a result of: (a) local site conditions such as climate, geomorphology and soil, which determine long-term carbon sequestration potential; (b) disturbances such as changes in land use, previous and current forest management practices, and natural disasters, which alter carbon pools; (c) vegetation types as consequences of (a) and (b); and (d) forest growth phases, which determine temporal biomass accumulation rates.

In the context of global climate change, two substantial challenges exist:
  1. How to maintain and increase the forest carbon storage.
  2. How to predict its future change, given the large uncertainties of the inherent variability in forest ecosystems.

Forest Carbon Storage and Distribution in Louisiana  
To answer those questions, scientists at the LSU AgCenter have been assessing space-time variation of the forest biomass carbon in Louisiana and the predictability of potential future change of the carbon stock. As watersheds are increasingly becoming primary planning units for natural resource management, the research takes full advantage of advanced GIS technology and rich sources of spatially referenced databases to analyze forest ecosystem carbon stock and dynamics at the watershed scale.

The study shows that forest carbon density in Louisiana increases from the southeast Mississippi delta, to the lower coastal plain, and to the northwest upland region (Figure 1). The spatial distribution reflects the land use and management practices in the state. As of 2003, Louisiana’s forests store 268 million metric tons of carbon in all biomass components. Pine forests and oak-gum-cypress bottomland forests are two communities that present the largest stock of biomass carbon in Louisiana.

Louisiana Forest Carbon Change in the Past and Future  
During the period from 1991 to 2003, carbon stock of Louisiana’s forests underwent considerable change (Figure 2). Biomass carbon stocks in all forestry types, except for the longleaf-slash pines, have decreased by 2.7 percent to 18.4 percent. In total, the state lost 9.3 percent (or 27 million tons) of the carbon stored in the forests in 1991. The largest percentage loss was observed in the elm-ash-cottonwood (18.4 percent) and oak-hickory forest types (13.8 percent). The largest mass change in biomass carbon, however, occurred in the oak-gumcypress forests (9.4 million tons), whereby a 5 percent decrease of the land area by this forest type was also evidenced. This is a further alarming sign for the rapid loss of Louisiana’s bottomland hardwoods. Biomass carbon stock in the loblolly-shortleaf pine forests – the state’s major commercial timber source – also decreased by 9.6 percent from 1991 to 2003. However, the land area of this forest type increased by 17.5 percent. For the entire state, the forested land area increased by 1.7 percent from 1991 to 2003 (Figure 3).

According to the Louisiana Department of Agriculture & Forestry, the state’s landowners (industrial and non-industrial) reforested the land with more than 128 million seedlings in 2004. This includes about 55,000 acres of land (usually marginal farm land) converted to forestry. Another 10,345 acres were regenerated naturally. This increase of forested land areas with seedling trees indicates that the growth of the Louisiana’s forests will sequester a large amount of carbon in the coming 20-30 years, if managed properly.

LSU AgCenter researchers continue to develop and analyze a geographically referenced database of terrestrial ecosystem carbon to fully understand the spatiotemporal dynamics of the forest biomass carbon under a changing environment, especially land use change and climate change.

Yi Jun Xu, Associate Professor, and Fugui Wang, graduate student, School of Renewable Natural Resources, LSU AgCenter, Baton Rouge, La.

(This article was published in the spring 2006 issue of Louisiana Agriculture.)
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