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| | The inviting openness of the Sierra woods is one of their most distinguishing characteristics. The trees of all the species stand more or less apart in groves, or in small, irregular groups, enabling one to find a way nearly everywhere, along sunny colonnades and through openings that have a smooth, parklike surface . . . John Muir 1894
Tools and LinksManagement Forest Type Descriptions General Resources DiscussionThanks to Guild Member Jay Francis for his suggestions on this page Mixed conifer forest cover the largest area of the Sierra Nevada mountains (nearly 8 million acres) and provide habitat to more different invertebrate species than other forest types in the region (North and Chen 2005). Situated on the upper slopes of the mountain they are also called upper montane forests (Helms 1980). These forests protect the headwaters of drinking water supplies and provide recreational opportunities to millions of people. Decades of fire exclusion have altered stand structure and function so that one of the main concerns of ecological forestry in Sierran mixed conifer forests is how to restore more natural fire regimes (Minnich et al. 1995, Franklin et al. 2006, Noss et al. 2006). Fire suppression has also increased the impact of natural stresses on mixed conifer forests (Savage 1997).
The mixed conifer forest type is difficult to discuss in generalities because it varies in composition based on elevation and aspect. Ponderosa pine (Pinus ponoderosa) and incense-cedar (Calocedrus decurrens) dominate in warmer sites (lower elevation and south aspect) while white fir (Abies concolor) dominates on cooler and wetter sites (higher elevation and north aspect) (Helms 1980). Sugar pine (P. lambertiana) is indicative of mesic, high quality sites, Jeffrey pine (P. jeffreyi) of upper elevations and serpentine soils, and California red fir (A. magnifica) of the highest elevations within the forest type (Helms 1980). At the north end of the range of Sierran mixed conifer Douglas-fir (Psuedotsuga menzisii) becomes the most common species (Allen 2005). An important subtype of the Sierran mixed conifer, sequoia (Sequoiadendron giganteum) mixed conifer forests, is not directly discussed here. Most sequoia groves are now protected as parks such as Sequoia and Kings Canyon National Park. A related forest type, southwestern mixed conifer, is discussed in a separate article.
The mixed conifer forests of the Sierra Nevada experience a strong seasonal drought; 90% of annual precipitation comes as snow during the winter. By late summer forests can become extremely dry, especially during La Niña years. The natural fire regime of Sierran mixed conifer forests before decades of fire exclusion was 11 to 17 years (North and Chen 2005) with fires more frequent in La Niña years (North et al. 2005a). The federal description of the natural fire regime in Sierran mixed conifer forests is " primarily short-interval (e.g., 10-20 yr) surface fires with occasional mixed severity- and replacement fires (e.g., 30-100 yr intervals)" (Barrett et al. 2004). Fire frequency and severity changes with forest composition, aspect and elevation (Beaty and Taylor 2001), so averages obscure diversity in fire ecology through out the forest type. However, fire recurrence generally decreases with elevation and the pine-oak forest below mixed conifer forests tended to burn frequently (2 to 6 years) at low intensity (Beaty and Taylor 2001, Swetnam and Baisan 2002). There is also a complex interaction of fire recurrence and extent with climate over long periods of time. During colder, wetter times fires were less frequent and larger in extent, while warmer periods experience more frequent, extensive fires (Swetnam and Baisan 2002).
With fire exclusion, the physical structure, composition, and age structure of mixed conifer forests have changed (Franklin et al. 2006). Canopy cover has increased while gap size and abundance has decreased (Gray et al. 2005). For example, in the San Bernardino Mountains fire exclusion resulted in a ten-fold increase small diameter trees, a switch from ponderosa and Jeffrey pine to white fir and incense-cedar, and a dramatic increase in the youngest tree cohort (Minnich et al. 1995).
Fuels in Sierran mixed conifer forests have built up to 27,000 kg/ha (70 to 80 t/ac) in stands with a greater pine component and 16,000 kg/ha (40 to 50 t/ac) in stands with more fir (Allen 2005). Fire in mixed conifer forests that have experienced changes structure and composition due to fire exclusion tend to be stand replacing (Franklin et al. 2006). Regeneration
Mixed conifer stands generally initiate after a stand replacing fire when trees often compete with prostrate ceanothus or mahala mat (Ceanothus prostrates) and bearclover or mountain misery (Chamaebatia foliolosa) (Helms 1980). While prostrate ceanothus will allow or even facilitate conifer seedling survival, bearclover can out-compete other species through efficient water capture (McDonald et al. 2004). After stand initiation, shrub species can dominate mixed conifer sites until shade tolerant conifers, such as white fir, are able to grow above the shrub canopy. In some cases, shade intolerant species such as ponderosa pine, sugar pine, or Douglasfir, are able to seed into a site at stand initiation, capture growing space, and become a part of the new stand (Helms 1980).
Seedlings establish in both material soil and forest floor litter (Gray et al. 2005). White fir, sugar pine, and black oak seedlings have all shown a positive association with litter (Gray et al. 2005). While pines may preferentially establish on bare mineral soil, a post-logging study showed lower survival in bare mineral soil (Stark 1965). Most conifer seedlings are associated with intermediate moisture availability microsites, but incense-cedar is associated with high-moisture and Jeffrey pine with low-moisture (Gray et al. 2005). Direct sunlight on fir and cedar seedlings reduces their survival, probably because of the decreased water availability (Gray et al. 2005). Establishment is better in wet years (El Niño years) for both Jeffery pine and red fir and Jeffery pine establishment is also better post fire (North et al. 2005a). It is not possible to identify a single best approach to the silviculture of this type because of the great diversity of in stand structure, composition, and management objectives. Helms 1980 Silviculture
The scientific literature agrees on the uncharacteristic density of trees, particularly white fir and incense-cedar, where fire has been suppressed and on the need to restore fire to Sierran mixed conifer forests. However there is less agreement on appropriate silvicultural systems for sierran mixed conifer forest. Recent work suggests that at least in the southern portion of the Sierra Nevadas mixed conifer forests were not naturally gap regeneration systems. The mix of shade-tolerant and -intolerant species in forest patches, treeless gaps within the forest, different influences of climate on species establishment, and diversity of tree ages in forest patches are some of the pieces of evidence researchers use to reject the theory that mixed conifer forest naturally regenerate as even aged cohorts in gaps (North et al. 2005a, North et al. 2005b). However, farther north in the Bloddget Experimental Forest a full suite of mixed conifer species grew well when planted in gaps of 0.3 to 0.5 ha (.7 to 1.2 ac) (York et al. 2007).
The 6 to 14 ha (15 to 35 ac) clearcuts formerly used by the US Forest Service are poor surrogates for the natural processes of stand replacement and no longer socially acceptable (Helms 1980). On the other end of the spectrum, single tree selection is likely to perpetuate the current unnaturally dense conditions and favor shade tolerant white fir and incense-cedar. Therefore, if single or group selection methods are employed they should be combined with prescribed fire or thinning treatments and then prescribed fire to reduce the density of shade tolerant trees in the understory. Silvicultural treatments that reduced canopy cover, shrub cover, litter, and slash are most likely to increase herbaceous cover and species diversity (Collins et al. 2007). Fuel Reduction
Effective fuel reduction treatments focus on (1) reducing surface fuels, (2) increasing height to live crown base, (3) decreasing crown density, and (4) retaining the largest trees in the stand through thinning (Agee and Skinner 2005). In Sierran mixed conifer forests, treatments that employ fire (prescribed fire and mechanical thinning and fire), which implemented surface fuel, ladder fuel, and crown fuel reduction while retaining the largest trees in the stand were most effective at reducing modeled potential for crown fires (Miller and Urban, Agee and Skinner 2005). Prescribed fire helped return a sequoia-mixed conifer forest to more natural characteristics (Keifer et al. 2000). Without active fuels treatment (slash removal or burning), single tree selection, overstory removal, low thinning, and unmanaged forest in mixed conifer forests all created significant surface fuels (Stephens 1998, Stephens and Moghaddas 2005).
In many areas, particularly those with high fuel loads, early season (spring) burns are more appropriate than later season burns. "Burning areas with high fuel loads in early season when fuels are moister may lead to patterns of heterogeneity in fire effects that more closely approximate the expected patchiness of historical fires," (Knapp and Keeley 2006). Off-season prescribed fire had a greater positive effect on species diversity when additional fuel had been created by a preparatory mechanical thinning (Collins et al. 2007). References
Agee, J. K., and C. N. Skinner. 2005. Basic principles of forest fuel reduction treatments. Forest Ecology and Management 221(11):83-96. Allen, B. H. 2005. Sierran Mixed Conifer. California Wildlife Habitat Relationships System. California Department of Fish and Game. California Interagency Wildlife Task Group.
http://www.dfg.ca.gov/bdb/cwhr/pdfs/SMC.pdf
Barrett, S., N. Sugihara, R. Siemers, and H. Safford. 2004. Fire Regime Condition Class (FRCC) Interagency Handbook: California Mixed Conifer Reference Conditions.
http://www.frcc.gov/docs/PNVG/West/MCON_Description.pdf
Beaty, R. M., and A. H. Taylor. 2001. Spatial and temporal variation of fire regimes in a mixed conifer forest landscape, Southern Cascades, California, USA. Journal of Biogeography 28(8):955–966.
doi:10.1046/j.1365-2699.2001.00591.x
Collins, B. M., J. J. Moghaddas, and S. L. Stephens. 2007. Initial changes in forest structure and understory plant communities following fuel reduction activities in a Sierra Nevada mixed conifer forest. Forest Ecology and Management 239(1-3):102-111.
http://dx.doi.org/10.1016/j.foreco.2006.11.013
Franklin, J., L. A. Spears-Lebrun, D. H. Deutschman, and K. Marsden. 2006. Impact of a high-intensity fire on mixed evergreen and mixed conifer forests in the Peninsular Ranges of southern California, USA. Forest Ecology and Managment 235(1-3):18-29.
http://dx.doi.org/10.1016/j.foreco.2006.07.023
Gray, A. N., H. S. J. Zald, R. A. Kern, and M. North. 2005. Stand Conditions Associated with Tree Regeneration in Sierran Mixed-Conifer Forests. Forest Science 51(5):198 –210.
http://www.ingentaconnect.com/content/saf/fs/2005/00000051/00000003/art00001
Helms, J. 1980. The California Region. Pages 391-446 in J. Barrett, editor. Regional Silviculture of the United States. John Wiley and Sons, New York, NY. Keifer, M., N. L. Stephenson, and J. Manley. 2000. Prescribed fire as the minimum tool for wilderness forest and fire regime restoration: a case study from the Sierra Nevada, California. Pages 266-269 in D. N. Cole, S. F. McCool, W. T. Borrie, and J. O’Loughlin, editors. Wilderness science in a time of change conference-Volume 5: Wilderness ecosystems, threats, and management. P-RMRS-015, Missoula, MT.
Knapp, E. E., and J. E. Keeley. 2006. Heterogeneity in fire severity within early season and late season prescribed burns in a mixed-conifer forest. International Journal of Wildland Fire 15(1):37-45.
http://www.publish.csiro.au/?act=view_file&file_id=WF04068.pdf
McDonald, P. M., G. O. Fiddler, and D. A. Potter. 2004. Ecology and manipulation of bearclover (Chamaebatia foliolosa) in northern and central California: The status of our knowledge. Gen. Tech. Rep. PSW-GTR-190. Pacific Southwest Research Station, USDA Forest Service, Albany, CA.
http://www.fs.fed.us/psw/publications/documents/psw_gtr190/psw_gtr190.pdf Miller, C., and D. L. Urban. Modeling the Effects of Fire Management Alternatives on Sierra Nevada Mixed-Conifer Forests. Ecological Applications 10(1):85-94.
http://www.esajournals.org/esaonline/
Minnich, R. A., M. G. Barbour, J. H. Burk, and R. F. Fernau. 1995. Sixty Years of Change in Californian Conifer Forests of the San Bernardino Mountains. Conservation Biology & Philosophy 9(4):902-914.
http://links.jstor.org/
North, M., and J. Chen. 2005. Introduction to the Special Issue on Sierran Mixed-Conifer Research. Forest Science 51(5):185-186.
http://www.ingentaconnect.com/content/saf/fs/2005/00000051/00000003/art00001
North, M., M. Hurteau, R. Fiegener, and M. Barbour. 2005a. Influence of fire and El Niño on tree recruitment by Sierran mixed conifer. Forest Science 51(3):187-197.
http://www.fs.fed.us/psw/publications/north/captured/psw_2005_north001.pdf
North, M., B. Oakley, R. Fiegener, A. Gray, and M. Barbour. 2005b. Influence of light and soil moisture on Sierran mixed-conifer understory communities. Plant Ecology 177:13–24.
http://www.fs.fed.us/pnw/pubs/journals/pnw_2005_north001.pdf
Noss, R. F., J. F. Franklin, W. L. Baker, T. Schoennagel, and P. B. Moyle. 2006. Managing fire-prone forests in the western United States. Frontiers in Ecology 4(9):481–487.
http://www.frontiersinecology.org/
Savage, M. 1997. The Role of Anthropogenic Influences in a Mixed-Conifer Forest Mortality Episode. Journal of Vegetation Science 8(1):95-104.
http://links.jstor.org/
Stark, N. 1965. Natural regeneration of Sierra Nevada mixed conifers after logging. Journal of Forestry 63:456–461. Stephens, S. L. 1998. Evaluation of the effects of silvicultural and fuels treatments on potential fire behaviour in Sierra Nevada mixed-conifer forests. Forest Ecology and Management 105(21-35).
http://www.cnr.berkeley.edu/stephens-lab/Publications/
Stephens, S. L., and J. J. Moghaddas. 2005. Fire Hazard and Silvicultural Systems:
25 Years of Experience from the Sierra Nevada. Biological Conservation Biology 125:369-379. Swetnam, T. W., and C. H. Baisan. 2002. Fire Regimes in Sierran Mixed-Conifer Forests. Sierra Nature Notes 2.
http://www.yosemite.org/naturenotes/FireRegime.htm
York, R. A., J. J. Battles, and R. C. Heald. 2007. Gap-based silviculture in a sierran mixed-conifer forest: effects of gap size on early survival and 7-year seedling growth. Pages 181-191 in R. F. Powers, editor. Restoring fire-adapted ecosystems: proceedings of the 2005 national silviculture workshop. Albany, CA, Gen. Tech. Rep. PSW-GTR-203. Pacific Southwest Research Station, USDA Forest Service. |
