Seth Ex, R. Justin DeRose, and James N. Long Department of ... · Lindsay Winkler, the USU Undergraduate Research and Creative Opportunity and Academic Opportunity Funds, the USU

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