Human activities are influencing the chemistry of the Earth=s atmosphere in ways that are not fully understood but which could ultimately affect forest ecosystems in significant ways. The buildup of greenhouse gases is accelerated by fossil fuel burning, deforestation, livestock production, agricultural activities, and the widespread use and release of chemical compounds such as CFC=s.
Forest productivity may be altered by the resulting changes in temperature, precipitation, and nutrient availability, as well as the direct effects of pollution. Changes in species composition and cover type may also occur. Scientists are predicting that insect and disease outbreaks could increase and water yield and streamflow timing may be altered. Soil, stream, and lake acidification has already occurred. Pollution, climate stress, and land use changes all interact to affect the responses of forests and rangelands to naturally occurring stresses. Increasing pressure to use land by the growing numbers of humans may exacerbate these natural and human-caused stresses.
Measurement of forest biomass focuses on the role of temperate and boreal forests in the Earth=s carbon cycle, and the response of forests to changes in the atmosphere, especially in levels of carbon. Ultimately, the information available through measurement of these indicators could lead to more effective mitigation measures and development of strategies to minimize atmospheric changes, thus helping to insure the sustainability of forests not only in the United States, but around the globe.
INDICATOR 26: Total forest ecosystem biomass and carbon pool, and if appropriate, by forest type, age class, and successional stage.
The accumulation of biomass as living vegetation, debris, peat, and soil carbon (carbon pool) is an important forest function in regulating atmospheric carbon. The production rate of biomass is also a measure of forest health and vitality.
Biomass estimates are developed from data compiled from inventories conducted by the Forest Service, Forest Inventory and Analysis (FIA) projects, using conversion factors derived from biomass studies. There are some areas of the nation that have not been inventoried, notably parts of Alaska and some reserved forests, and there is little information on lands of low productivity or >reserved= lands. A greater limitation is that there are no inventories statistically designed over large regions for actual biomass measurements, because it requires destructive sampling. Biomass may be estimated for 1977, 1987, and 1992. Inventories were not necessarily taken in the featured year. Inventorying just one State may take 2-3 years, so available data are updated to reflect what is believed to be the conditions in the featured years.
Estimates of biomass may be developed for groups of States and individual States using FIA's Eastwide and Westwide databases and appropriate conversion factors. The inventories are statistically designed to provide State-specific estimates. Tree carbon estimates are developed from data compiled from inventories conducted by the FIA projects using conversion factors derived from biomass studies, and wood-to-carbon estimates from forest products literature.
Estimates of carbon storage in the soil, on the forest floor, and in the understory were developed using models based on data from forest ecosystem studies. No inventories specifically designed to quantify carbon in different ecosystem components over large regions have been made, although the USDA Natural Resource Conservation Service (NRCS) is testing protocols for total carbon in a forest ecosystem to include measurements of the forest floor. Some FIA projects have recently begun measuring coarse woody debris.
Soil carbon may be estimated on a national level using NATSGO database from the Natural Resource Conservation Service, or at the State level using STATSGO. A county-level database is being constructed. These geographic databases are based on a combination of soil surveys and expert knowledge. Estimates of tree carbon may be developed for individual states and groups of states using FIA's databases and conversion factors.
Another source of biomass estimates is Cost, et al. (1990), but estimates are presented only by State and softwood/hardwood type, and do not include trees smaller than 12.7 cm dbh.
We believe this information is only sufficient to characterize the indicator for "commercial forest land" or productive forest lands. As discussed in the methodology section, many areas of U.S. forest land have not been adequately sampled by FIA. Some data for western national forests are unavailable. Estimates for non-tree carbon are based on models rather than original data.
Forest Inventory and Assessment and the Natural Resources Conservation Service inventories should be conducted on all forest lands rather than current focus on timberland. Data on non-tree components (forest floor, coarse woody debris, soil carbon, and understory vegetation) should be collected along with the statistical samples for timber. Meanwhile, we are developing better models and locating more comprehensive data sources for these ecosystem components.
INDICATOR 27: Contribution of forest ecosystems to the total global budget, including absorption and release of carbon (standing biomass, coarse woody debris, peat and soil carbon).
Generally, flux estimates are made indirectly, by taking the difference between inventories (as compiled for Indicator 26). Direct, instantaneous measurements of carbon fluxes are only applicable to very specific areas and are highly variable throughout the day.
Estimated carbon inventories have been made for 1977, 1987, and 1992. In flux estimates, volume on reserved lands was assumed to grow at the same rate as volume on similar unreserved lands, and for national forest lands harvest and growth rate projections were derived from national forest plans.
At present we cannot determine the flux for a specific year, but must infer the yearly flux from successive estimates of long-term trends over several decades. There are statistical methods available to apply to this estimation problem, but this has not yet been done rigorously. Direct flux estimates could be made for a network of sites, but the cost would be prohibitive. However, a network of sites is being implemented for the US and analysis of this data could prove fruitful.
INDICATOR 28: Contribution of forest products to the global carbon budget.
The ecological and sustainable management of production forests and the long lasting use of forest products can be a factor in controlling the amount of carbon entering the world=s atmosphere.
Estimates of carbon in forest products is based on removal estimates from FIA project inventories, from timber product surveys compiled at the national level, and from various other sources, such as special studies of organic material in landfills. These estimates, which are usually presented in volume units, have been converted to mass (carbon) using analogous procedures for Indicators 26 and 27. Estimates of harvested carbon are available at the regional and national levels. Annual products statistics have been fairly reliable since about 1950.
Researchers need more information about carbon decay rates in products, which may be measured in length of product life. There is considerable uncertainty about carbon released in burning fuelwood, as well as the amount emitted from logging residue. Estimated carbon inventories are approximated at 10- to 15-year intervals beginning with the year 1900. Reliable data are first available for around 1950.
Removals estimates from FIA data are subject to the same limitations as described for Indicator 26. Special studies of carbon decay rates in products are needed.
Some areas of the nation have not been inventoried for forest ecosystem biomass, notably parts of Alaska, and there is less information on lands of low productivity and reserved lands.
No inventories have been specifically designed to measure carbon in different ecosystem components over large regions.
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