Mixed-severity fire regimes may be the most extensive yet poorly understood fire regimes of western North America. Understanding their long-term spatiotemporal dynamics is central to debates regarding altered fire regimes and the need for restoration in the context of changing climate and nearly a century of active fire suppression. However, the complexity of fire patterns and forest stand and landscape structures characteristic of mixed-severity regimes poses a substantial challenge to understanding their long-term dynamics. In this study, we develop analysis methods aimed at understanding the fire-driven forest dynamics of mixed-severity systems and apply them in the San Juan Mountains of southwestern Colorado. We sampled fire scars, stand structure, and >4,300 tree ages across two 1,340-ha landscapes (Williams Creek and Squaretop Mountain) that span the environmental gradient of montane mixed-conifer and aspen forests. New approaches were applied to identify pulses of tree recruitment, evaluate climate and fire as potential drivers of synchronous recruitment pulses, and combine fire scar and recruitment data to reconstruct fires. The reconstructions provided detailed fire history for each stand, which in turn was used to develop a fire-severity metric, compare fire frequency and severity by forest type, and develop a simulation procedure to evaluate the degree to which tree regeneration depended on fire by species within each forest type. Twenty fires were reconstructed since 1685 at Williams Creek and 13 fires since 1748 at Squaretop Mountain. Patterns of fire severity varied within each fire and over successive events, including high-severity patches of 100s of ha in both study areas. Dry mixed-conifer forests experienced relatively short fire intervals (mean 21 years) and low fire severity, and regeneration of the main conifer species was largely dependent on open conditions sustained over successive fires. Moist mixed-conifer forests experienced longer fire intervals (mean 32 years) and a broader range of severities, and fire-caused canopy openings were important for initiating pulses of tree recruitment. Most (83%) aspen stands included two or more post-fire cohorts. The methods presented here can be adapted to other mixed-severity systems to better understand their long-term spatial and temporal dynamics and develop restoration priorities.

Tepley&Veblen2015_transect_dataForest stand- and age-structure-data were collected across two ca. 1,340-ha study areas (Squaretop Mountain and Williams Creek) located within the San Juan National Forest of southwestern Colorado as part of a study to evaluate historical fire regimes and fire-driven forest dynamics across the moisture gradient of mixed-conifer and aspen forest. Within the data file, a separate sheet is provided for each of the following datasets: (1) transect locations, (2) overstory (> 15 cm dbh) species composition and size structure, (3) ages of overstory trees, (4) sapling (1.5–15.0 cm dbh) density, (5) the forest type for each transect, and (6) codes for species used in the other datasheets.