Evidence, exaggeration, and error in historical accounts of chaparral wildfires in California.

For more than half a century, ecologists and historians have been integrating the contemporary study of ecosystems with data gathered from historical sources to evaluate change over broad temporal and spatial scales. This approach is especially useful where ecosystems were altered before formal study as a result of natural resources management, land development, environmental pollution, and climate change. Yet, in many places, historical documents do not provide precise information, and pre-historical evidence is unavailable or has ambiguous interpretation. There are similar challenges in evaluating how the fire regime of chaparral in California has changed as a result of fire suppression management initiated at the beginning of the 20th century. Although the firestorm of October 2003 was the largest officially recorded in California (approximately 300,000 ha), historical accounts of pre-suppression wildfires have been cited as evidence that such a scale of burning was not unprecedented, suggesting the fire regime and patch mosaic in chaparral have not substantially changed. We find that the data do not support pre-suppression megafires, and that the impression of large historical wildfires is a result of imprecision and inaccuracy in the original reports, as well as a parlance that is beset with hyperbole. We underscore themes of importance for critically analyzing historical documents to evaluate ecological change. A putative 100 mile long by 10 mile wide (160 x 16 km) wildfire reported in 1889 was reconstructed to an area of chaparral approximately 40 times smaller by linking local accounts to property tax records, voter registration rolls, claimed insurance, and place names mapped with a geographical information system (GIS) which includes data from historical vegetation surveys. We also show that historical sources cited as evidence of other large chaparral wildfires are either demonstrably inaccurate or provide anecdotal information that is immaterial in the appraisal of pre-suppression fire size. Since historical evidence is inadequate for reconstructing a statistical distribution of pre-suppression fire sizes to compare with post-suppression data, other more propitious methods of evaluating change are discussed.