Credits for sequestered carbon augment forests’ already considerable value as natural habitat and as producers of timber and biomass, making their accurate inventory more critical than ever before. Thisarticle examines discrepancies in inventories of forest attributes and their sources in four variables: area, timber volume per area, biomass per timber volume, and carbon concentration. Documented discrepancies range up to a multibillion-ton difference in the global stock of carbon in trees. Because the variables are multiplied together to estimate an attribute like carbon stock, more precise measurement of the most certain variable improves accuracy little, and a 10 percent error in biomass per timber levers a discrepancy as much as a mistake in millions of hectares. More precise measurements of, say, accessible stands cannot remedy inaccuracies from biased sampling of regional forests. The discrepancies and uncertainties documented here underscore the obligation to improve monitoring of global forests.
A new RFF Discussion Paper highlights multiple incongruities in national forest measures from across the globe that could present a stumbling block for the development of international forest policy. “Without accuracy, appraisals of timber will be discredited, assays of biomass will be deceptive, and claims of sequestered carbon may be fraudulent,” writes Paul Waggoner in “Forest Inventories: Discrepancies and Uncertainties”
To take one example, according to a Food and Agriculture Organization assessment, El Salvador’s forested land shrank 14 percent from 1990 to 2000, while a separate study reported that its dense forested land area expanded 25 percent in the same timeframe. Furthermore, measurement uncertainties are inherent in even the best practices approved by the Intergovernmental Panel on Climate Change, currently the most authoritative source on climate science, says Waggoner.
Many of the discrepancies and uncertainties in total forest assessments can be expressed in four variables that make up the Forest Identity: area, stock density (or volume), biomass, and carbon. Because of the interrelationships of these variables, uncertainties in their measures compound and potentially result in a final estimate that has significantly more uncertainty than any single variable.
The purpose of pointing out these discrepancies, according to Waggoner, is not to develop a perfectly accurate measurement for forests, but rather to develop measurements that are good enough and consistent enough for parties interested in forests. Says Waggoner, “…the discrepancies and uncertainties in forest surveys must next be evaluated against standards of good enough for, say, scientific debates, timber sales, or carbon credits. Then economical methods for meeting those standards must be established.”