USING VEGETATIVE ANALYSIS TO DETERMINE
Michael J. Jenkins, Ph.D.
Elizabeth G. Hebertson
Department of Forest Resources
The success of potential rescue operations requires the development and implementation of plans based on knowledge of the frequency and maximum runout of large avalanche events. Dendrochronology and vegetative analysis may prove useful in augmenting historical records and existing maps to determine past avalanche activity.
Avalanches can cause wounds when rock and debris carried from upslope strike standing trees resulting in scars on the uphill side of stems and branches. Trees form callous tissue around the margin of scars, and gradually add new wood and bark. A wedge cut from either side of the scar allows rings of new wood formed after scarring to be counted, and the date of injury determined.
New vertical stems may develop from release of lateral branches or dormant buds on the upper surface in response to tilting or stem breakage. Approximate avalanche dates can be determined from ages obtained from these new stems by counting the annual rings.
The uprooting of trees by avalanche impact creates gaps in the forest canopy. With a reduction of shoot and root competition, residual trees may experience enhanced growth indicated by wider ring widths. Patterns of release may be particularly evident in undamaged trees, or trees bordering the new trimline.
Each method described above has limitations in its ability to provide accurate dates. Soil creep, landslides, and other geomorphic processes can tilt trees resulting in reaction-wood formation and growth suppression. Other agents, including lighting and wind, can break trees. Insects and diseases can cause reduced growth, and site improvement can enhance growth. Finally, reaction wood formation, release, and sprouting can all be delayed leading to misinterpretations. Where avalanche activity cannot be determined from historical records however, vegetative analysis, particularly dendrochronology provides reliable dates for the occurrence of avalanche events (Bryant et al. 1989).
The objective of the study was to determine the extent and frequency of maximum run-out in Little Pine East avalanche path using techniques of dendrochronology and vegetative analysis.
The total sample contained 50 conifers including Engelmann spruce (Picea engelmannii), white fir (Abies concolor), alpine fir (Abies lasiocarpa), and 28 aspen (Populus tremuloides). Up to four increment cores were taken from the uphill and downhill sides of each sample tree. The majority of the aspen sampled had heart rot caused by a fungus that decays the wood tissue in the center portion of the stem making infected trees useless for study. Other aspen cores were twisted making it difficult to see the annual rings. Consequently, only conifer cores were used for tree-ring analysis. Trees observed to have bole damage were sampled by cutting a wedge from the scar, or by taking a core sample using the technique described by Arno and Barrett (1988).
From modified skeleton plots, it was possible to establish approximate dates of avalanche events. To obtain more precise dates, increment cores were cross-dated. Using a dissecting scope, and a Model 4 Digital Display Unit (Fred C. Henson Co. Mission Viejo, CA) with a Tree-Ring Increment Measuring System (Madera
Software, Tucson, AZ) the ring widths of each sample core were measured. Skeleton plots were constructed using computer generated graphs of ring widths for each core. Because sample cores had not been collected from trees in off-path locations, a chronology was developed from a sample of cores with-no evidence of morphological response to avalanche. Growth patterns observed in skeleton plots of core samples were compared with those derived from the master chronology in addition to those of other cores. From this information, dates were assigned to the rings and years of reaction wood formation.
Aspen discs were sanded to reveal the annual growth rings. Beginning with the outermost ring, rings were counted inward to the pith to determine the age of the sample tree. The same procedure was used to determine the age of disc samples cut from the new alpine fir stems. One increment core taken from a new stem had reaction wood on what would have been the underside of the branch surrounded by wood formed following years of release.
RESULTS AND DISCUSSION
Estimating dates of earlier events was more difficult. Only a small number of older trees were still living and available for sampling. With time there is also an increased probability of encountering false and missing rings, and morphological changes in response to damage caused by other agents. From modified skeleton plots, however, dates for earlier events could be approximated.
Skeleton plots derived from graphs of measured ring widths proved useful in substantiating these dates. Although the original purpose of these plots was to cross-date samples taken from different trees, the variability in ring widths made cross-dating difficult. Skeleton plots did, however, reveal patterns of suppressed growth associated with years of reaction wood formation and other morphological responses when compared to the modified skeleton plots (Fig. 1). Abrupt decreases in growth increment in avalanche areas suggesting avalanche activity has been reported in other studies (Carrara 1979). Where comparisons were in close agreement, dates could be adjusted producing a better estimate of the actual date of avalanche occurrence.
A total of 13 avalanches reached maximum run-out in the Little Pine East avalanche path between 189S and 1991. The dates determined for these events along with the maximum extent of runout are shown in Figure 2. Four large Engelmann spruce growing at the bottom margin on the east flank provided the longest record of avalanche activity. Reaction wood was formed in these trees as a result of 1898, 1901, and 1906 avalanche events. Narrow ring patterns produced during the same years that younger trees initiated reaction wood formation (1911-1916, the early 1930's,1935, early to mid 1940's, early 1950's, and 1965) indicated that patterns of reduced growth were probably a consequence of damage caused by avalanche impact. Perhaps the largest avalanche event detected occurred in the early 1920's (1920 or 1921). Trees sampled from the far east flank of the avalanche path, to west of the White Pine trailhead had evidence of reaction wood or reduced growth. Other avalanches more typically ran to the extent of the dashed lines shown in Figure 2. The most frequently recorded event was the 1965 avalanche, where sixteen trees had reaction-wood or reduced growth.
Other vegetative analysis and historical information provided verification for some dates. The age of aspen sampled from different age-classes along the west flank corresponded with dates from the 1920-1921 and 1965 avalanches. The minimum year of establishment for the aspen along the far west flank of the avalanche path was 1924. The aspen sampled from the younger age class was approximately 28 years old suggesting that these trees were established following the 1965 avalanche. The ages determined for aspen growing in the center island and toe of the avalanche path verified the 1951 date. Finally, the Engelmann spruce scar was dated to 1945 indicating some avalanche activity during the mid 1940's.
Historical records of avalanche activity in Little Cottonwood Canyon were used for further verification. The 1983, and the 1965 events were confirmed by historical records (UDOT 1987). Kalatowski (1988) cited accounts of avalanches occurring in Alta in 1898, 1906, and 1911. The agreement between this documentation and tree-ring analysis supports other findings suggesting that morphological responses are useful for dating past avalanche events with reasonable accuracy (Bryant et al. 1989, Shroder 1976, Burrows and Burrows 1976, Hansen-Bristow and Birkland 1989).
Avalanche frequency was calculated by dividing the number of years examined by the total number of avalanches. Calculations determined that large avalanche events occurred in the Little Pine East path once every eight years prior to 1991. The frequency of avalanche events prior to 1960 was once every six years, and decreased to once every 15 years between 1960 and 1991. The increase in avalanche return interval is perhaps a consequence of avalanche control which reduces the likelihood of avalanches running to their maximum extent.
Arno, S.F., and S.W. Barrett. 1988. Increment borer methods for determining fire history in coniferous forests. Intermountain Research Station General Technical Report INT-244, U.S. Forest Service, 16 pp.