biological resources such as wild fish, timber and other plants in unmanaged forests, and other uncultivated biological resources. The construction of accounts for agricultural, horticultural, and animal husbandry assets poses no major data issues, and the U.S. Department of Agriculture, together with the U.S. Bureau of the Census, has a comparatively full set of information on these issues. Similarly, data sources, though of varying accuracy, are available from which to estimate the market value of developed land.

Accounting for renewable resources such as forests encounters some of the same information issues and data gaps as does accounting for exhaustible resources. Managed forests other than plantations contain trees of heterogeneous ages, species, and other characteristics. Harvested trees will generally differ in unit value from the unharvested stock and from additions to that stock. Data on the heterogeneity of timber stocks are particularly important because harvesting is likely to be limited to the more valuable stocks, and therefore stumpage price estimates derived from such commercial operations cannot be reliably extrapolated to other unexploited stocks.

Though the national forests contribute a small share of total harvested timber, there are particular problems in accounting for wood extracted from these forests. Though standing timber is typically sold through auction bids, sales prices will not represent the market stumpage value of the timber for those sales that have only a single bidder. In such sales, the winning bid usually corresponds to the Forest Services's administratively determined minimum acceptable bid. Bids are also influenced by cost considerations. Logging contractors are required to operate under conditions imposed to protect other multiple-use environmental values, such as water quality, habitat protection, and recreational and aesthetic values. These conditions may increase logging costs and therefore reduce the amounts potential contractors are willing to bid for logging rights. Offsetting these upward pressures on costs in the national forests, the government bears some logging costs, notably those of road construction, which are financed out of road credits. Research will be necessary to determine whether transaction data based on bids for logging rights in national forests are an accurate source of information on stumpage values, or whether they would require some adjustment to be useful in the environmental accounts.

With respect to timber harvested on private lands, difficulties arise in allocating joint production costs in industrial forestry operations carried out by integrated pulp and paper or forest product companies. A substantial fraction of total timber harvested originates on lands owned and operated by such companies. In addition to problems of joint cost allocation, there are also problems of establishing or inferring prices for logs



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