PAIRED CHARCOAL AND TREE-RING RECORDS OF HIGH-FREQUENCY FIRE FROM TWO NEW MEXICO BOG SITES

ALLEN, C.D., ANDERSON, R. S. JASS, R. B., TONEY, J. L., AND BAISAN, C. H.

We have developed long-term records of fire activity from paired and replicated charcoal and tree-ring proxies of fire at two bog sites in the Jemez Mountains, northern New Mexico.  Chihuahueños Bog occupies a 2.3 ha upland basin near the lip of a canyon amidst mixed conifer forest.  The bog surface is currently dominated by large sedges and some grasses.  Alamo Bog is a linear wetland extending for >1 km along the valley bottom axis of Alamo Canyon in Valles Caldera.  North-facing slopes are currently blanketed in dense mixed conifer forest, while the south-facing slopes exhibit more open stands of ponderosa pine and Gambel oak, with pockets of mixed conifer species.  The surface of Alamo Bog today is dominated by a large wetland bunchgrass, Deschampsia cespitosa. 

Both locations have high-resolution charcoal records, with replication of charcoal concentrations in the top meter of a second core.  In the ~ 14,000 cal year Chihuahueños Bog record, background and peak charcoal concentrations increase markedly in the early Holocene, ranging from 100's to >15,000 particles/cc until ca. AD 1890, when concentrations decline to zero in the top of both cores.  Cross-dated fire scars from 13 trees (3 live, 10 dead) in the forest adjoining the bog confirm frequent spreading fires at this site until AD 1902, with ten spreading fires (at least 2 trees scarred) recorded between AD 1617 - 1902, and fire dates on one old sample back to AD 1454.

The ~ 8,700 cal year Alamo Bog record also shows very high concentrations of charcoal throughout the Holocene, except from the topmost portions of the cores (i.e., since ca. AD 1900), where charcoal is essentially absent.  Cross-dated fire scars from 52 sampled trees (10 live, 42 dead) in the forests adjoining and upslope of Alamo Bog record fires back to AD 1422, and confirm the cessation of widespread fires since the late 1800s.  Pre-1900 fire frequencies in this watershed varied by forest type and landscape position, with higher-frequency spreading fires in ponderosa pine settings (4-27 year return intervals, mean ~11 years from 1696-1879), versus 9-45 year intervals in mixed conifer stands.

A number of challenges constrain joint interpretation of the long charcoal and shorter tree-ring records from these two bogs.  Diverse vegetation patterning in the Alamo Bog watershed and uncertain charcoal deposition processes in both bogs call for caution in directly linking the fire-scar and sedimentary charcoal patterns.  Extremely high charcoal concentrations, abundant fine fuels of the grassy wetland vegetation, and the fire-scar history of relatively frequent surface fires in adjoining forests suggests that pre-1900 surface fires likely spread through aerial herbaceous fuels across the bogs in many years.  Thus past surface fires might have produced abundant in situ charcoal, as well as charcoal influx from surrounding forested slopes.  Surface burning of the bog may have eliminated some of the sedimentary record during extreme droughts.  The high background concentrations of charcoal reflect the high frequency of fire activity affecting these bogs relative to the temporal resolution of 1 cc sampling intervals, further complicating the interpretation of observed charcoal peaks as discrete fire "events" (in contrast to relatively direct calculations of event frequency for long-interval stand-replacing fires from subalpine forest lake basins).

Still, the historic cessation of fire since ~AD 1900 seen in the paired and replicated charcoal and tree-ring records at these sites is consistent with many other tree-ring fire histories from the Jemez Mountains and the Southwest as a whole.  The near-absence of modern charcoal deposition replicated within and between these two bog sites increases the robustness of the interpretation that this post-1900 lull in fire activity is anomalous at millennial time scales for at least these two localities.  Determining the geographic extent of this pattern will require the development of regional networks of additional charcoal sediment records from historically high-frequency fire sites (generally drier, lower elevation, unglaciated landscapes), where unmanipulated, persistently wet basins that are necessary to foster long-term sediment records are relatively scarce in the western US.