Historical Fire Regime Patterns in the Southwestern United States Since AD 1700

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Contribution to Book

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Fire Effects in Southwestern Fortest : Proceedings of the 2nd La Mesa Fire Symposium

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Fire-scar chronologies from a network of 63 sites in the southwestern United States are listed and described. These data characterize the natural range and variability of fire regimes from low elevation pine forests to higher elevation mixed-conifer forests since AD 1700. A general pattern of increasing length of intervals between low intensity surface fires was observed along gradients of low to high elevations, and from the relatively drier pine sites to the wetter mixed-conifer sites. However, large variability in the measures of central tendency and higher moments of the fire interval distributions suggest that elevation and forest type were often weak determinants of fire frequency. Some of the variations in fire interval distributions between similar elevation of forest types were probably due to unique site characteristics, such as landscape connectivity (i.e., ability of fires to spread into the sites), and land-use history. Differences in the sizes of sampled areas and fire-scar collections among the sites also limited our ability to compare and interpret fire interval summary statistics. Comparison of both the fire-scar network data (1700- to 1900) and documentary records of area burned on all southwestern region National Forests (1920 to 1978) with a Palmer Brought Severity Index time series clearly shows the association between severe droughts and large fire years, and wet periods and small fire years. Moreover, important lagging relations between climate and fire occurrence are also revealed. In particular, large fire years in ponderosa pine dominated forest were typically preceded by wet conditions in the prior one to three years. In contrast, large fire years in mixed-conifer forest were associated with extreme drought years, but no consistent lagging relations ere observed. We hypothesize that both fuel production (especially grasses and pine needles) and fuel moisture were important climate-linked factors in ponderosa pine fire regimes, While fuel moisture was the primary factor controlling mixed-conifer fire regimes. These results provide two important types of information for management: (1) Baselines of fire regime ranges and variations are documented across the most economically important and widespread forest types in the southwest. These data will be useful for guiding, developing, and justifying ecosystem management plans, particularly for the restoration of fire regimes and forest structures to improve forest health and sustainability. (2) The fire-climate relations suggest that a long-range fire hazard forecasting model could be developed that would be a valuable tool for planning and implementing both prescribed fire and fire suppression programs in the southwest.


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