Improving the use of early timber inventories in reconstructing historical dry forests and fire in the western United States

Early timber inventories in dry forests of the western United States offer detailed data sets that might provide historical information to guide restoration and preparation for future forests, but inventories have errors, biases, and limitations. We reviewed early documentation of errors and estimated errors by comparing inventory estimates to nearby tree-ring and plot estimates. In a case study in the Greenhorn Mountains, southern Sierra Nevada, California, we studied how selection and use of evidence affects findings and compared timber-inventory, land-survey, and other early evidence about historical forests and fire. Early documents showed inventories were unreliable, often with large underestimation errors from poor visual estimates, requiring correction multipliers of 2.0-2.5. Comparing inventory estimates to treering estimates, we found commonly used two-chain-wide inventories required correction multipliers of about 1.4-3.2, consistent with, but wider than the 2.0-2.5 range. These needed corrections were not applied in any study. The case study showed (1) tree-density estimates from narrower one-chain-wide inventories could be more accurate, (2) data are often available, but unused, that provide quantitative evidence about historical high-severity fires consistent with nearby historical reports, and (3) differences in forest structure between inventories and land surveys may be explained by tree growth, stand development, and especially a significant fire. Our review also documented biased placement of inventories in merchantable timber, often excluding younger forests, chaparral, and other indicators of preceding mixed/high-severity fires. We found added significant bias introduced by omitting areas burned in mixed/high-severity fires, or by missing evidence of these fires on parts of forms or associated archival materials. Use of early timber inventories could be improved by (1) avoiding use of unreliable two-chain-wide inventories or applying correction multipliers to inventory estimates, (2) completing an accuracy test of one-chain-wide inventories, (3) locating and using notes, maps, and other data about small trees and high-severity fires often available in inventory archives, or omitting conclusions about these, (4) deriving an envelope model of inference space for inventories, and (5) specifying a large area, then including all available inventory data within it, or using unbiased selection criteria. These improvements could help bring timber-inventory data into congruence with other historical sources.