Tree-ring based fire histories in southwestern ponderosa pine forests have typically focused on fire frequency and timing while ignoring spatial components of fire severity and spatial patterning. This is at least partially the result of the types of paleo-fire evidence used for reconstruction. Past studies have focused on fire-scar records - which document non-lethal burning at the scale of single trees - without comparison to tree-origin data that may have resulted from past stand opening by lethal fires. We used both fire-scar and tree-origin dates collected from ~800 trees to reconstruct the fire regime of an old-growth (un-harvested) ponderosa pine forest growing on Archuleta Mesa in southwest Colorado. The study investigates the spatial components of burning by comparison of the fire-scar record with age structure of the current and historical stand. We also compare targeted versus systematically sampled fire-scarred trees to determine differences in fire frequency and timing that may be related to methodological approach.

Data reconstructed by this study strongly supports previous conclusions about fire regimes in ponderosa pine forests of the Southwest. Trees in the 300+ ha stand recorded abundant fire-scar evidence, with a surface fire frequency of c.13 yrs from 1700 to 1900 (last fire in 1871). Tree germination dates revealed distinct even-aged cohorts of trees, with pulses of ponderosa pine establishment during the late 1500s, early 1600s, early 1700s, and middle 1800s. However, spatial comparison of age data with fire-scar locations suggests that cohorts are not necessarily related to broad-scale lethal burning, while temporal comparison with drought records suggests that establishment is more likely related to optimal moisture conditions. Abundant regeneration that occurred during rare episodes of optimal climate may not have been limited by any existing overstory because open stands were maintained for long periods by recurrent surface fires and other disturbances. If this were the case, mortality and regeneration were largely uncoupled processes and even-aged structure may never be definitive evidence of stand-replacing fires in ponderosa pine forests. A comparison of targeted vs. systematically collected fire-scarred trees also did not reveal any significant differences in reconstructed frequency or spatial patterning based on methodological approach.