DOI: 10.25921/9ve9-3325
title: NOAA/WDS Paleoclimatology - Tepley fire data from Squaretop Mountain, southwestern Colorado, IMPD USSQM001

description: 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 3 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. 
description: The fire-scar dataset was collected as part of a study using fire scars and forest stand- and age-structure data to reconstruct the fire history and fire-driven forest dynamics across the gradient from dry to moist mixed-conifer and aspen forests in the San Juan Mountains of southwestern Colorado. The Squaretop Mountain study area is 1,340 ha located on the north side of Rio Blanco, within the Pagosa Ranger District of the San Juan National Forest. Sampling of forest stand- and age-structure data was conducted in 47 contiguous stands across the study area. Fire-scar samples were collected and crosssdated from 48 trees. Ponderosa pine accounts for 87% the samples, limber pine accounts for 9%, and white fir and Gambel oak comprise the remaining 4% samples. Fire scars were collected by searching while hiking to and from each transect, searching the area around each transect, and by making additional trips at the end of the field season to help fill spatial gaps in the dataset. Scars were sampled preferentially from dead trees, but live trees were also sampled to better represent recent fires and to provide a fire record in stands where ponderosa pine was absent and scars were poorly preserved on dead stems of the more rapidly decaying species. The fire-scar data were analyzed along with the age-structure data to reconstruct the extent and severity of each major fire event since the mid-18th century.  Lowest elevation: 2421 meters, Highest elevation: 3263 meters. Individual sample information is located at: http://www.ncei.noaa.gov/pub/data/paleo/firehistory/firescar/northamerica/supplemental/ussqm001-individual-sample-information.csv . Alan J. Tepley (Alan.Tepley@Colorado.edu or tepleya@gmail.com).