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Fire and Forest Structure Across Vegetation Gradients in San Juan
National Forest, Colorado: A Multi-Scaled Historical Analysis
Peter M. Brown, Director, Rocky Mountain Tree-Ring Research
Rosalind Wu, Fire Ecologist, San Juan National Forest
Project Funded by:
Project Summary 
Managers and planners on the San Juan National Forest in southwestern Colorado require better information about how fire regimes and forest structure have varied across spatiotemporal scales and in response to regional climate variability and land use changes.  We used tree-ring data to document fire regimes, forest structure, and fire/climate relationships across gradients in forest types, elevation, and landscape physiography across three landscapes in the Piedra River watershed, located between Durango and Pagosa Springs.  This was an ideal area for this study in that it is largely unharvested and provided ideal reference conditions for assessing tree establishment and fire histories applicable to comparable forests in the region.  We developed forest histories across a range of forest types, including ponderosa pine, mixed-conifer, aspen, and subalpine forests.  We used both tree recruitment dates and fire scars to reconstruct and compare fire timing, frequency, spatial patterning, severity, and seasonality, and resulting forest age structure, density, and composition across two study landscapes next to the Piedra River and at Archuleta Mesa, just to the east of the Piedra near Chimney Rock.  Methods and results from the study have direct applicability to both on-going and proposed forest restoration and fuels treatment programs in the San Juan and adjacent National Forests, and provide baseline data on historical patterns and current conditions associated with fire suppression and land use over the past century.

We sampled cores or cross sections from ~3,700 trees in 122 plots.  Plots were sampled across gradients in elevation, aspect, and forest type in the three landscapes, Archuleta Mesa (23 plots in a dry ponderosa pine forest), Sheep Creek (56 plots spanning mixed-conifer to subalpine forests), and Bear Park (43 plots spanning wet mixed-conifer to subalpine forests).  A paper on fire and establishment patterns from Archuleta Mesa was published in 2005 (P.M. Brown and R. Wu, 2005, Climate and disturbance forcing of episodic tree recruitment in a southwestern ponderosa pine landscape, Ecology 86:3030-3038), and others will be submitted during 2009.  All fire history data from the Archuleta plots were submitted in November, 2005, to the International Multiproxy Paleofire Database (IMPD). A final report on the project was submitted to the Joint Fire Science Program on June 30, 2006 (PDF; warning, large file!).

Some photos of study areas and basic sampling techniques
October 2001: Archuleta Mesa, an unlogged old-growth ponderosa pine forest located between Durango and Pagosa Springs.  The stand is very open, with lots of structural diversity and many snags and logs.  We collected stand structure, age, and fire-scar data from over 700 trees in this dry pine site.
October 2001: The forest at Archuleta Mesa has a lot of grass/herbaceous and shrub diversity; quite a beautiful spot especially when the oak is turning. 
August 2002: This was one of the driest years on record for southwestern Colorado (and much of the Southwest and central Rockies), and these aerial views give some idea of the dry conditions on Archuleta.  This photo was taken not long after the nearby Missionary Ridge fire northeast of Durango was finally contained. 
August 2002: Note the steep cliffs on both sides of the mesa.  That plus no water on top means that impacts from historic livestock grazing are likely minimal, although we are seeing 20th century cessation of fire dates.  The last fire on the Mesa was in 1871.  Most of the older, larger ponderosa seen in the aerial views and in the first two photos established in the late 1500s and early 1600s, during and shortly after an extended "mega-drought" centered in the 1580s. 
August 2002: The Piedra River just above First Fork.  Note how much of the banks are exposed.  This was a very dry year in southwestern Colorado.
August 2002: Mixed-conifer forest in the Piedra drainage. This is a view of Bear Park Ridge where one of our transects extends from subalpine spruce/fir/aspen stands down through mixed-conifer forest.  
Ros Wu collecting an increment core with a power increment borer (an Atom drill attachment driven by a Stihle 044 chainsaw motor).  This is the only way to go when sampling thousands of trees for age analyses!  Note that we are coring close to base height on all trees to obtain near-germination dates.  Our sampling protocols also call for cores to be no more than a field-estimated 5 years from pith, which may require coring more than once on off-center trees (but not more than 3 times to keep potential damage to trees to a minimum).

We have found that a three-person crew is ideal when using the power borer.  The crew typically will have 2-3 increment borers in trees at any one time.  One person handles the motor and borer head, one is backing out the cores and checking for closeness to pith, and one is strawing the cores once a good one is taken.  We are using n-tree density sampling methods to sample 30 trees in each plot, and with a seasoned crew we can get all our plot data in around 2 hours.

Another tool needed for use with the power borer is a quad-B (Brown's Bent Boomerang Borer) handle.  Here's James Riser demonstrating.  Regular borer increment handles are too long to core right near the base of the tree or if you angle the borer it runs into the side of the tree if too big.  


A fire-scarred stump (this one had over 20 scars visible).  Because of widespread harvest in many ponderosa pine and mixed-conifer forests throughout the western US, the fire history of many, many forests is contained in the stumps.  Do not trust any fire history that does not also collect and analyze remnant (dead) trees, and to do that one must use dendrochronological crossdating to find fire-scar (and tree-recruitment) dates! 
Page last updated: December 2010
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