Three-hundred-year spatial reconstruction of fire patterns and fire effects in mixed-conifer/aspen forests of the San Juan Mountains, southwestern Colorado

Background/Question/Methods

Understanding historical fire regimes is fundamental to understanding the factors guiding heterogeneity in forest structure within stands and across landscapes. Although mixed-conifer forests are extensive in western North America and commonly noted for their fine-grained mosaics of varying stand structure and species composition, the role of fire in generating and maintaining those mosaics is poorly understood. To characterize the historical fire regime of a representative mixed-conifer system, we conducted a study in two, 1,500-ha study areas in the San Juan Mountains of southwestern Colorado. Both study areas span the continuum from relatively dry forests dominated by Douglas-fir and ponderosa pine to mesic white fir forests. Trembling aspen forms nearly pure stands in both areas and is present in most conifer-dominated stands. Aerial photos were used to subdivide each study area into stands (10s of ha) that are relatively homogeneous in canopy structure and overstory composition. Forest structure and tree ages were sampled in each stand. Altogether, 80 stands were sampled, > 4,000 tree cores were collected, and 88 wedges were collected from fire-scarred trees. These data were used to reconstruct the spatial extent and internal heterogeneity in fire severity of the major fire events over the last 300 years.

Results/Conclusions

Although fire scars are a primary source for evaluating historical fire regimes, scars were not evenly distributed across the study areas. Scars were best recorded and preserved on ponderosa pine trees in the driest stands, but few scars were found in mesic stands dominated by white fir or aspen. However, these stands typically have one or more abrupt pulses of tree establishment that correspond to fire-scar dates found in nearby stands. Therefore, fire scars and establishment pulses initiated synchronously across contiguous stands were used to trace the extent of each major fire event. Spatial variation in fire severity within these events was inferred by comparing the density of trees that predate the fire across stands. Fire severity varied widely over short distances, producing a fine-grained mosaic composed predominantly of multi-cohort stands with an abrupt pulse of regeneration by aspen and/or conifers within the first 2-4 decades following fire beneath older conifers at densities ranging from 30-120 trees/ha. Low-severity patches typically have less distinct pulses of post-fire regeneration, whereas high-severity patches formed even-aged aspen stands. At the stand level, fire severity varied over successive events, promoting heterogeneity in stand structures and a shifting landscape mosaic over time.