Understory vegetation indicates historic fire regimes in ponderosa pine-dominated ecosystems in the Colorado Front Range

Question: Can current understory vegetation composition across an elevation gradient of Pinus ponderosa-dominated forests be used to identify areas that, prior to 20th century fire suppression, were characterized by different fire frequencies and severities (i.e., historic fire regimes)? Location: P. ponderosa-dominated forests in the montane zone of the northern Colorado Front Range, Boulder and Larimer Counties, Colorado, USA. Methods: Understory species composition and stand characteristics were sampled at 43 sites with previously determined fire histories. Indicator species analyses and indirect ordination were used to determine: (1) if stands within a particular historic fire regime had similar understory compositions, and (2) if understory vegetation was associated with the same environmental gradients that influence fire regime. Classification and regression tree analysis was used to ascertain which species could predict fire regimes. Results: Indicator species analysis identified 34 understory species as significant indicators of three distinct historic fire regimes along an elevation gradient from low- to high-elevation P. ponderosa forests. A predictive model derived from a classification tree identified five species as reliable predictors of fire regime. Conclusions: P. ponderosa-dominated forests shaped by three distinct historic fire regimes have significantly different floristic composition, and current understory compositions can be used as reliable indicators of historical differences in past fire frequency and severity. The feasibility demonstrated in the current study using current understory vegetation properties to detect different historic fire regimes, should be examined in other fire-prone forest ecosystems.