HOLOCENE VEGETATION & FOREST FIRE REGIMES IN SUBALPINE & MIXED CONIFER FORESTS, SOUTHERN COLORADO & NORTHERN NEW MEXICO

ANDERSON, R.S. (1), ALLEN, C.D. (2), TONEY, J.L. (3), JASS, R.B (4). AND BAIR, A.N. (4)

(1) Center for Environmental Sciences & Education, & Quaternary Sciences Program, Northern Arizona University, Flagstaff, AZ 86011; (2) USGS-BRD, Bandelier National Monument, Los Alamos, NM 87544; (3) Quaternary Sciences Program, Northern Arizona University, Flagstaff, AZ 86011; (4) 4014A Lewis Lane, Austin, TX, 78756

We have produced several Holocene-length and longer records of vegetation and fire for high elevation sites in the southern Rocky Mountains and Jemez Mountains of Colorado and New Mexico, USA.  Our elevational transect of sites includes locations at the alpine - treeline boundary, within the Picea engelmannii - Abies lasiocarpa forest, the mixed conifer forest, and the Quercus - Pinus woodland. 

Two records - one each from subalpine and mixed conifer forests - illustrate vegetation change over the last ca. 15,000 years.  At Hunter's Lake (subalpine site) an open Picea forest with Artemisia grew around the pond until ca. 12,000 cal BP.  Pinus pollen increased briefly during the period of the Younger Dryas.  Vegetation around the pond during the Holocene was a Picea engelmannii - Abies lasiocarpa forest, with little variation in composition.   At Chihuahueños Bog (mixed conifer site) an open Picea forest grew around a small pond until ca. 11,500 cal BP when Pinus ponderosa became established.  This shallow pond lasted until ca. 9,000 cal BP.  From ca. 8,600 to 6,200 cal BP the pond dessicated in what must have been the driest period of the Holocene there.  Wetter conditions returned after 6,200 years ago, with conversion of the site to a sedge bog.  Little change in the overstory species occurred throughout the Holocene, with mixed conifer forest dominating throughout. 

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 Picea - Abies forest, extreme "peakedness" is apparent in the charcoal records, each peak corresponding to stand-replacing fires within the watershed.  Preliminary fire event frequency calculations are on the order of 150 - 200 years.  Two upper treeline sites have the lowest deposition rates of charcoal, which may be due to their open nature and significant portions of the drainage basin above treeline that contributes less charcoal to the record when burned.  Lower elevation mixed conifer sites show significantly higher charcoal concentrations throughout the Holocene (one to two orders of magnitude) than at the Picea - Abies sites.  "Peakedness" is less prominent in the charcoal records from the mixed conifer sites, where surface fires may have been more common, although the stand-replacing regime becomes more pronounced in the late Holocene.  Periods of highest fire event frequency occur from ca. 1,000 to 2,000 and ca. 9,000 to 12,000 calendar years ago. 

The most significant changes in the fire record occur within sediments deposited in the late 19^th and early 20^th centuries, 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 a fire suppression period characteristic of most locations in the western United States.  Major shifts in the pollen record occur contemporaneously with these important environmental changes, including an increase in Quercus at many sites within the mixed conifer forest and Quercus - Pinus woodland.