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DFC Considerations
Southwest Mixed Conifer
This information was taken from various research and operational reports from across the
Southwest (see Bibliography at end of document). Some of the figures were rounded for
simplicity sake. Every site is different but these data give a good indication of condition,
influences, and effects within the ecosystem for developing an action (left side of NEPA triangle).
These are not objectives by themselves.
GENERAL
Depending on location, Douglas-fir, white fir, limber pine, and ponderosa pine form mixed stands
in this community. Quaking aspen, along with Gambel oak, is prominent in these forests
following disturbances. This community is typically called low elevation or “dry” mixed conifer
in contrast to higher elevation and moister, “wet/cool” mixed conifer that typically includes
Engelmann spruce and/or blue spruce. It is also differentiated from the true spruce-fir
communities of high elevations.
Understory conditions vary widely from dry, open-canopy forests with grassy undergrowth on
open slopes and ridges to moist, closed-canopied stands dominated by numerous herbaceous
plants in small openings, riparian areas, and ravines.
HISTORIC CONDITIONS
• Historic conditions varied widely and it is difficult to generalize about historic tree
density, fire regime, or forest structure and composition.
• Fire intervals were 5-25 years with a highly variable pattern of severity. Severe fires were
associated with drought conditions.
• Due to a mixed-severity fire regime, stands were patchy and irregular in size, age
structure, and composition. Most were multi-aged and multi-storied.
• Drier sites (lower elevations, south and west aspects) were more open than moist sites
due to the presence of more frequent fire.
• Most stand-replacing fires occurred at moist locations (higher elevations, north and east
aspects), where fire was less frequent but more intense under droughty conditions.
• Stand-replacing fires were localized to clumps, groups and stands, and tended not to
occur at landscape scales.
• Aspen was the key vegetative component following stand-replacing fire. Aspen
dominated fire-created openings for periods in excess of 100 years while playing a role as
a nurse stand for the regenerating conifer species.
• Almost all regeneration was created due to disturbance activities. Most important of these
disturbances was fire, but insects, disease, windthrow, drought and floods, either
separately or in combinations, all played a role.
• Understory herbaceous vegetation was limited, especially at the higher elevations where
grasses and forbs were restricted to small openings and riparian areas.
CHANGES
• Forest stand inventory data from Arizona and New Mexico show an 81% increase in the
area of mixed-conifer forests between 1962 and 1986.
• Shade-tolerant coniferous species have increased dramatically due to the absence of fire.
Many communities, now classified as mixed conifer, were ponderosa pine with scattered
fir stands in historical times.
Ecological Restoration Institute/Northern Arizona University
2007
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Mixed-conifer stands are more closed than in historic times. Historic basal areas were 3545% of today’s basal areas and trees per acre were 15-25% of today’s numbers.
Crown biomass has increased 120% at low elevations and as much as 270% at higher
elevations.
Herbaceous understories have been reduced because of denser canopies and needle litter;
nutrient cycles have been disrupted.
Heavy surface fuels and a vertically continuous ladder of dead branches have developed,
resulting in increased risks of crown fires.
PRESENT CONDITIONS AND TREATMENTS
• The quaking aspen component has been reduced from historic times due to the lack of
stand-replacing fire.
• In some locations more than 55% of the high-elevation meadows have disappeared due to
conifer encroachment.
• Higher-elevation, “cool/wet” mixed conifer has changed less radically from historic times
in the terms of composition, but its structure has become much less open and the all-aged
stands have been replaced by younger, even-age stands.
• Due to crown continuity, aspect, slope, and topographic locations (steep slopes, canyons,
sky islands), mixed-conifer forests are vulnerable to crown fire, and attacks by insects
and diseases.
• Due to the amount of dead fuel on the ground, mixed-conifer ecosystems are susceptible
to very high wildfire intensities and fire residency times. These two factors can easily
create hydrophobic soils, which repel moisture.
• Mixed-conifer stands are highly susceptible to windthrow when the stands are opened. In
thinning practice, it is not desirable to over-thin interdependent clumps or to split pairs.
• Mistletoe, especially in Douglas fir, can be a major problem. In thinning practices, it
might be desirable to manage for alternate species with fewer health problems.
• It is desirable to retain young healthy, vigorous trees in order to maintain vertical
diversity.
• It is generally desirable to favor fire-adapted species (ponderosa pine more so than
Douglas fir, Douglas fir more so than southwestern white pine, and southwestern white
pine more than white fir).
BIBLIOGRAPHY
Dahms, C.W. and B.W. Geils. 1997. An assessment of forest ecosystem health in the Southwest. USDA
Forest Service General Technical Report RM-GTR-295.
Dwyer, D.M. WUI within mixed conifer within the Sacramento Mountains of southern New Mexico and
the tools silviculture can provide. PowerPoint Presentation.
Fulé, P.Z., J.E. Crouse, A.E. Cocke, M.M. Moore, and W.W. Covington. 2004. Changes in canopy fuels
and potential fire behavior 1880-2040: Grand Canyon, Arizona. Ecological Modeling 175:231248.
Fulé, P.Z., J.E. Crouse, T.A. Heinlein, M.M. Moore, W.W. Covington, and G. Verkamp. 2003. Mixedseverity fire regime in a high-elevation forest of Grand Canyon, Arizona, USA. Landscape
Ecology 18:45-46.
Jones, J.R. 1974. Silviculture of southwestern mixed conifers and aspen: The status of our knowledge.
USDA Forest Service Research Paper RM-122.
Kaufmann, M.R., L.S. Huckaby, C.M. Regan, and J. Popp. 1998. Forest reference conditions for ecosystem
management in the Sacramento Mountains, New Mexico. USDA Forest Service General
Technical Report RMRS-GTR-19.
Land Use History of North America-Colorado Plateau (LUHNA-CP).
http://www.cpluhna.nau.edu/Biota/mixedconifer.htm. Accessed 12/15/2006.
Ecological Restoration Institute/Northern Arizona University
2007