Variability of warm/dry mixed conifer forests in southwestern Colorado, USA: Implications for ecological restoration

Abstract

There is a need to quantitatively describe forest types and their associated natural fire regimes in the western US to understand their variability and to develop strategies to restore fire dependent landscapes to reduce ecological problems that might ensue from forest structure and fire regime generalizations. We established three study sites in warm/dry mixed conifer across a west–east transect in southwestern Colorado to determine variability in the historical fire regime and fire–climate relationships and to quantify how diverse warm/dry mixed conifer forest composition and structure are along the same west–east transect. At each study site we collected cross-sections from dead fire-scarred trees to reconstruct fire history and established study plots to characterize forest structure. The three warm/dry mixed conifer sites exhibited different fire histories. One site was characterized by numerous smaller fires as well as larger fires that appeared to burn most of the study site with the other two study sites characterized by relatively infrequent, large fire events. There were more unique fire years at each site than synchronous fire years shared by any sites. Current forest stand structure, after extended fire exclusion and past logging across the three sites, also varied with a reverse-J distribution indicating strong dominance by small trees for two of the study sites and a truncated age distribution at the other site. Our research illustrates that historical fire regime variability exists within the same vegetation type in a relatively small, ∼50km, geographic locality emphasizing the role that other topographic variables play in determining fire regimes and forest structure. Our findings demonstrate the need to develop site-specific reference conditions and for managers to exercise caution when extrapolating fire regimes and forest structure from one geographic locality to another given a projected warmer climate making conditions more favorable to frequent, large wildfires.