The Santa Fe municipal watershed provides up to 40% of the city’s water and is at high risk of stand-replacing fire that could threaten the water resource and potentially cause severe ecological damage. The high fire risk is due to 1) increased forest density in lower elevation ponderosa pine and xeric mixed-conifer forests caused by over 160 years of fire exclusion and 2) naturally dense high elevation spruce-fir forests that historically burned in stand-replacing fires and will again in the future. The vegetation gradient, from ponderosa pine, through mixed-conifer/aspen, to spruce-dominated forests, provides a unique opportunity to test for the connectivity of fire regimes between forest types (from low severity, to mixed severity, to high severity). The objective of our study was to use dendroecological techniques to reconstruct fire history and fire-climate relationships along an elevation, forest type, and fire regime gradient in the upper Santa Fe River watershed and provide historical ecological data to guide management. We combined systematic (gridded) sampling of forest age structure with targeted sampling of fire scars, tree-ring growth changes/injuries, and death dates to reconstruct historic fire occurrence and severity in the 7016 ha study area (elevation 2330m – 3650m).
Fire scars from 147 trees (at 41 plots) and age structure of 438 trees (at 26 transects) were used to reconstruct 125 unique fire years (1296 - 2008). Widespread fire was more frequent in ponderosa pine (mean fire interval, 20% scarred = 18.19 years) compared to the mixed-conifer forest (27.43 years). Only 19% of fires in the ponderosa pine were recorded in the mixed-conifer forest, but these accounted for a large percent of all mixed-conifer fires (60%). Fire occurrence was associated with anomalously wet (and El Niño) years preceding anomalously dry (and La Niña) years in ponderosa pine, and unexpectedly, also in the mixed-conifer forest, which is interpreted as evidence of fire spread from ponderosa pine to the mixed-conifer forest. The last fire in the spruce forest (1685) was largely stand-replacing (1200 ha, 93% of sampled spruce area), recorded by fire scars at 68% of plots throughout the mixed-conifer and ponderosa pine forests, and burned during a severe, regional drought. The connectivity of forest ecosystems through the continuous, landscape-scale process of fire underscores the potential for fire suppression to affect ecosystems outside of the well-documented changes in ponderosa pine, including forests where stand-replacing fire is a natural component.