The University of Arizona

Research Outcomes | CLIMAS

Research Outcomes

This study was limited to a single watershed, which precluded interpreting regional fire and climate associations from the alluvial fan stratigraphy. However, the data was reviewed with respect to other regional fire history records. In addition, the tree-ring record assisted in interpreting the fine-grained sediment deposits, by defining the fire regime in the watershed over the past 300–400 years.

Tree-ring fire scar records from the watershed show evidence of frequent surface fires over most of the past 400 years, followed by virtual cessation of widespread fire since 1879. The fires appear to be more frequent and patchy during the late 1600s and 1700s, followed by a shift to more synchronous and widespread fires in the 1800s. This pattern of fire occurrence is common to southwestern fire history studies of this forest type and elevation range, including sites in southern Arizona and New Mexico.

It is difficult to link fires observed in the tree-ring record with individual alluvial deposits observed in the exposed channel. This is partly because the alluvial deposits have been dated with radiocarbon dating, which is difficult to calibrate with calendar years over the past 500 years B.P. However, general patterns of the two records show the following:

  • The dominance of fine-grained deposition in the channel represents generally stable conditions in the watershed over the past 2,000 years.
  • There is no evidence of erosion of channel material or deposition of coarser-grained deposits, which would indicate high-intensity fires.
  • The tree-ring record reflects this type of sediment deposition with the pattern of frequent surface fires and little evidence of high-intensity crown fire.

The combined data provide a context for comparing past fires with the recent Missionary Ridge Fire, which burned a large proportion of the watershed at high severity burn conditions. The geomorphic response of the Missionary Ridge Fire included several debris flow and flood events, which completely incised the older channel deposits. This geomorphic response sharply contrasts the period of fine-grained deposition, which characterizes the past 2,000 years.