Fire Regimes and Forest
Structure in the Mountains of
Northwestern Mexico and
Southern California
Scott Stephens
Department of Environmental Science, Policy,
and Management
University of California, Berkeley
Background and Objectives
• Coniferous forests in Northwestern Mexico
and Southern California
– Similar flora, soils, environment, bark beetles
– Very different management histories
• Northwest Mexico, limited fire suppression begins
1970, no harvesting, grazing
• Southern California, fire suppression 1910, multiple
harvests, grazing, air pollution
• Drought last 5 years in both areas
• Forest restoration
– Can forests in northwest Mexico help in USA?
Lake Arrowhead Sept, 2003
Mixed conifer forests
CDF
Forests in Northwestern Mexico
Sierra San Pedro Martir
Within the California floristic province
– unique to Northern Baja California
Forested area approximately 40,000 ha
Elevation upper plateau 2600 m
– 3 large plateaus
Jeffrey pine-mixed conifer forests
– Similar to areas of southern California
Image 1: Aerial
Fire History
What is past fire frequency, seasonality,
and extent in the SSPM?
Is it similar to California?
Study used standard dendrochronology
techniques including analysis of fire
scarred tree samples
Stephens et al. 2003
SSPM Fire History
FRI 10%
Forest Structure
Live trees (above 2.5 cm DBH)
– Average 149 trees/ha (60 trees/acre)
– Range 30 - 317 trees/ha
Average basal area 20 m2/ha (86 ft2/acre)
– range 5.8-51 m2/ha
Structural classification of forest (hierarchical cluster
analysis )
– 33% old forest, single stratum
– 24% young forest, multi-strata
– 43% old forest, spatially distinct multi-strata
Large amount of variability common
Stephens and Gill 2005 (in press)
Tree Mortality and Snags
Snags (2001)
– average 4.2 snags/ha (0-22.5 snags/ha)
Snags (2003)
– average 6.2 snags/ha (0-22.5 snags/ha)
Annual rate of tree mortality 0.162% /yr
native bark beetles: majority of mortality
Relatively large increase in snags but very small
compared to forests of Southern California
Stephens 2004; Stephens and Gill 2005 (in press)
How Common are Average Snag
Densities?
0 snags on 30% of plots
Less than average density on 65% of plots
Greater than 10 snags/ha in 14% of plots
Conclusion
– average densities are rare
– occur in approximately 12% of plots
Stephens 2004
Wildfire July 4, 2003
• Started in chaparral below forest
– In SSPM lightning ignited fires
suppressed by 8 person hand crew
• Very low hazards, suppression efficient
• Fire burned approximately 4000 ha
• Largest fire in 20 years
– Occurred during drought period
Wildfire Impacts
• Approximately 5% of dominant and codominant trees killed (study in progress)
• Fire was very patchy
– directly linked to heterogeneity of forest
structure and fuels
• produces fire with diverse effects
• continues high spatial heterogeneity in forest
• bark beetles killing a few trees
• Southern California forests will burn at
much higher intensity, much higher bark
beetle activity
Conclusions
SSPM can serve as a reference location for
some forests in the western USA
Great deal of variability common
Both spatial and temporal
Relatively intact disturbance regime important
USA federal standards and guides: average
structural characteristics desired everywhere
Critical to develop ability to assess, quantify,
and communicate spatial variation
Acknowledgements
Dr. Ernesto Franco
Centro de Investigacion Cientifica y de
Educacion Superior de Ensenada, B.C.
(CICESE)
Mr. Carl Skinner
USFS PSW Research Station
Dr. Samantha Gill, Cal Poly State University
California Department of Forestry and Fire
Protection
Email: stephens@nature.berkeley.edu