HOLOCENE FOREST FIRE REGIMES IN THE SOUTHERN ROCKY MOUNTAINS AND JEMEZ MOUNTAINS OF COLORADO & NEW MEXICO

Higher elevations of the southern Rocky Mountains and Jemez Mountains of New Mexico and Colorado support a variety of forest types, each possessing a characteristic forest fire regime. Ongoing research there has produced several Holocene-length and longer records of vegetation and fire for the region. Our transect of sites spans a wide elevational gradient, with sites at the alpine - treeline boundary, within the engelmann spruce (Picea engelmannii) - subalpine fir (Abies lasiocarpa) forest, the mixed conifer forest, and the oak (Quercus) - pine (Pinus) woodland.

Our high-resolution fire history records are determined from fine interval sediment sampling and charcoal particle analysis. For most sites, especially those in and near the spruce - fir forest, extreme "peakedness" is apparent in the charcoal record, each peak corresponding to stand-replacing fires within the watershed. Preliminary fire event frequency calculations are on the order of 200 - 400 years. Two sites at upper treeline have the smallest deposition rate of charcoal of any of the sites. This may be due to the more open nature of the site, with significant portions of the drainage basin above treeline, which, when burned, contributes less charcoal to the record.

Lower elevation mixed conifer sites show significantly higher charcoal concentrations throughout the Holocene (one to two orders of magnitude) than at the spruce - fir sites. The pattern of charcoal record "peakedness" is less prominent at the mixed conifer sites, where surface fires may have been more common, although the stand-replacing regime becomes more pronounced in the late Holocene.

The most significant changes in the fire record occur within sediments deposited in the late 19th and early 20th century, when charcoal is essentially absent. There is no analog to this phenomenon in the earlier record. This period corresponds to initiation of widespread cattle and sheep grazing in the region, and the initiation of the fire suppression period. Major shifts in the pollen record occur contemporaneously with these important environmental changes, including an increase in oak at many sites within the mixed conifer forest.