Assessing fire regimes on Grand Canyon landscapes with fire-scar and fire-record data

Fire regimes were reconstructed from fire-scarred trees on five large forested study sites (135 - 810 ha) on the North and South Rims at Grand Canyon National Park. Adequacy of sampling was tested with cumulative sample curves, effectiveness of fire recording on individual trees, tree age data, and the occurrence of 20th Century fires which permitted comparison of fire-scar data with fire- record data, a form of modern calibration for the interpretation of fire- scar results. Fire scars identified all 13 recorded fires > 8 ha on the study sites since 1924, when record keeping started. Records of fire season and size corresponded well with fire- scar data. We concluded that the sampling and analysis methods were appropriate and accurate for this area, in contrast to the suggestion that these methods are highly uncertain in ponderosa pine forests. Prior to 1880, fires were most frequent on low-elevation 'islands' of ponderosa pine forest formed by plateaus or points ( Weibull Median Probability Intervals [WMPI] 3.0-3.9 years for all fires, 6.3-8.6 years for 'large' fires scarring 25% or more of the sampled trees). Fires were less frequent on a higher-elevation 'mainland' site located further to the interior of the North Rim ( WMPI 5.1 years all fires, 8.7 years large fires), but fires tended to occur in relatively drier years and individual fires were more likely to burn larger portions of the study site. In contrast to the North Rim pattern of declining fire frequency with elevation, a low-elevation 'mainland' site on the South Rim had the longest fire- free intervals prior to European settlement (WMPI 6.5 years all fires, 8.9 years large fires). As in much of western North America, surface fire regimes were interrupted around European settlement, 1879 on the North Rim and 1887 on the South Rim. However, either two or three large surface fires have burned across each of the geographically remote point and plateau study sites of the western North Rim since settlement. To some extent, these sites may be rare representatives of nearly-natural conditions due to the relatively undisrupted fire regimes in a never-harvested forest setting.