Age Structure of Curl-leaf Mountain Mahogany Woodlands in Utah’s Bear River Mountains Seth Ex, R. Justin DeRose, and James N. Long
Department of Wildland Resources and Ecology Center
Acknowledgements
Keith Christensen, Richie Gardner, Antonin Kusbach, Dana Perkins,
Lindsay Winkler, the USU Undergraduate Research and Creative
Opportunity and Academic Opportunity Funds, the USU College of
Natural Resources, the Logan Ranger District, and the Ecology
Center
Abstract
Cercocarpus ledifolius (curl-leaf mountain mahogany) is a small,
slow-growing, evergreen tree common throughout the Great Basin in
the western United States. Young mountain mahogany trees are an
important winter food source for ungulates, though foliage in mature
stands is often out-of-reach of browsers, and regeneration is
generally sparse. Successional histories and stand age structures of
mountain mahogany woodlands are largely speculative.
This research uses basic forest mensuration and dendrochronology
to explore stand development and structure in this vegetation type.
We found tree ages ranging from 11 to over 400yrs. Age distributions
were not inconsistent with relatively discrete establishment periods
lasting from approximately 40 to over 100yrs, after which successful
recruitment of new individuals may not occur for decades. Individual
tree basal area and canopy projection area increased with age,
suggesting relative stand density may increase over time until sites
become fully occupied. Once a site becomes fully occupied, intra-
specific competition would make over-story disturbance necessary
for regeneration. While stand replacing disturbance could lead to
cover change or another even-aged mahogany stand, death or
breakage of individual trees may create old growth conditions by
liberating growing space and allowing an uneven-aged structure to
develop.
Measuring canopy projection
area using a plumb bob and
logger’s tape on the Temple
Fork site
Cutting cross-sections with a chainsaw on
the Sidehill Spring site; irregular stem
shapes, decay pockets and extremely hard
wood made sampling with an increment borer
impossible
Measuring and counting rings using a
boom arm stereozoom microscope,
stage, and digital measuring device
Methods
Sampling:
• Five research plots (sizes ranged from 225m2 to 375m2) were established in fall of 2008
• Basal diameter, canopy projection area, and height were recorded for all live trees
• Cross-sections were collected from all live trees
Sample preparation / data analysis:
• Cross sections were sanded to 400-grit with a belt sander, than polished to 1500-grit using a random orbital sander
• Growth rings were counted and measured to the nearest 0.001mm using a boom-arm stereozoom microscope, stage with
digital measuring device, and the measurement software Measure J2X
• Ages derived from ring counts were checked for accuracy using the software COFECHA
• Canopy projection areas were calculated by using the average of four radial measurements to calculate the area of a circle
Conclusions
Mountain mahogany woodlands in the study area appear to exist in
both even and uneven-aged conditions. Age structures of sample
stands suggest establishment periods are typically long, lasting from
approximately 40 to over 100yrs. Establishment periods may end
when relative stand density increases enough for intraspecific
competitive pressure to prevent establishment of new individuals in
the absence of canopy disturbance. Old growth (uneven-aged) stand
structures may develop in the absence of stand-replacing
disturbance; individual tree death and breakage over time likely
liberates growing space, which results in successful regeneration at a
small scale. High browse availability in mountain mahogany
woodlands may be synonymous with young stands of short stature.
Once stand height plateaus at 100+yrs, individual tree longevity, slow
growth, and episodic seed production appear to make regeneration
rare.
Introduction
Curl-leaf Mountain Mahogany: • Pure stands typically occur in the Bear River Mountains on rocky,
well-drained southwest to southeast aspects at elevations from
~1800m to ~2700m
• Stands often appear structurally simple, though individual stem
diameters vary
• Prior research suggests intraspecific relationships may strongly
influence recruitment within established stands
• Seed production is variable, with large seed crops every 2-10yrs
• Seeds germinate readily, but seedling survival is typically low
• Mountain mahogany does not readily sprout, and efforts to
regenerate trees vegetatively by burning, etc. have been largely
unsuccessful
Questions: • Is age structure similar between stands?
• Does regeneration occur without stand-replacing disturbance?
Results
• Live stem ages ranged from 11 to 447yrs
• Age range within a single plot ranged from 38 (Mahogany Range) to 394yrs (Sidehill Spring)
• Live stem diameters ranged from 0.4 to 55cm
• Individual tree canopy projection area ranged from 0.01 to over 40m2
• Tree heights ranged from 0.1 to 7.6m
Above, canopy projection area, basal diameter, and height for all
samples are plotted against age; canopy area also is plotted against
diameter. Chart symbols distinguish between trees from different plots.
Overall tree size tends to increase with age. Diameter is closely related
to both age (upper right) and canopy projection area (lower right).
Canopy projection area generally increases with age, though trees older
than 200yrs often have small canopies (upper left). This may reflect
damage (snow breakage, animal damage, etc.) incurred over time.
Height increases with age for 100+ years before reaching a plateau at
several meters (lower left).
Results, cont’d:
Five study sites were chosen to capture a spectrum of
mountain mahogany stand structures in the Bear River
Mountains. Sites are located in the Uinta-Wasatch-Cache
National Forest in northern Utah.
Utah
Age structures of the five sample sites show distinct differences. Mean ages range from less than 50 to over 200yrs (upper left,
error bars represent 95% confidence interval of the mean). Individual site age structures are presented for comparison with
samples grouped by establishment decade and converted to trees per hectare. Two sites (Cottonwood Spring and Mahogany
Range) appear to capture lengthy establishment periods with distinct beginnings and ends. The resulting stands, despite the
wide range of tree ages, can be considered even-aged in comparison to old growth stands. The youngest site (Lone Pine), also
even-aged, appears to be at or near the end of an establishment period. Temple fork is an older stand that may be in transition
from even to uneven-aged structure. Sidehill Spring appears to be an old growth stand with many relict trees.
n=13 n=13
n=31 n=14 n=12