The Relative Importance of Snow Avalanche Disturbance and ...

The Relative Importance of Snow Avalanche Disturbance and Thinning on Canopy Plant Populations Author(s): E. A. Johnson Source: Ecology, Vol. 68, No. 1 (Feb., 1987), pp. 43-53 Published by: Ecological Society of America Stable URL: http://www.jstor.org/stable/1938803 Accessed: 28/09/2008 22:42 Your use of the JSTOR archive indicates your acceptance of JSTOR's Terms and Conditions of Use, available at http://www.jstor.org/page/info/about/policies/terms.jsp. JSTOR's Terms and Conditions of Use provides, in part, that unless you have obtained prior permission, you may not download an entire issue of a journal or multiple copies of articles, and you may use content in the JSTOR archive only for your personal, non-commercial use. Please contact the publisher regarding any further use of this work. Publisher contact information may be obtained at http://www.jstor.org/action/showPublisher?publisherCode=esa. Each copy of any part of a JSTOR transmission must contain the same copyright notice that appears on the screen or printed page of such transmission. JSTOR is a not-for-profit organization founded in 1995 to build trusted digital archives for scholarship. We work with the scholarly community to preserve their work and the materials they rely upon, and to build a common research platform that promotes the discovery and use of these resources. For more information about JSTOR, please contact [email protected].

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68(1),1987,pp.43-53 Ecology, ? 1987bytheEcological Societyof America

THE

RELATIVE DISTURBANCE

IMPORTANCE AND PLANT

OF

SNOW

THINNING

ON

AVALANCHE CANOPY

POPULATIONS1

E. A. JOHNSON Department of Biology and Kananaskis Centre for Environmental Research, University of Calgary, Calgary, Alberta T2N 1N4, Canada Abstract. Snow avalanche return intervals on two avalanche paths in the southern Canadian Rockies were estimated from scarred trees and shrubs. The interval between avalanches increased exponentially down each path. The tree and shrub diameters at which avalanches could cause breakage were predicted using both the mechanics of large deflections in tapered beams and the resulting stem bending stress, and these predictions were confirmed by comparison to actual diameters at breakage on avalanche paths. Lodgepole pine (Pinus contorta) and Engelmann spruce (Picea engelmannii) bend when small, but were broken by avalanches when larger than ~6 cm in diameter at the base. Glandular birch (Betula glandulosa) and willow (Salix glauca) never grew large enough to break. Breakage was influenced by size rather than wood elasticity or strength. Information on thinning mortality was reconstructed from live and dead stems in two Engelmann spruce populations and one lodgepole pine population. Dead trees were cross-dated, using ring widths, to determine time of death. Avalanche mortality of trees was more important than thinning mortality when the average interval between avalanches was < 150 yr. The shift from shrub- to tree-dominated growth habit down the avalanche path occurred when the average interval between avalanches was less than 15 to 20 yr. Key words: avalanche frequency; bending stress; Betula glandulosa; disturbance; mechanical strength; mortality; Picea engelmannii; Pinus contorta; plant populations; Rocky Mountains; Salix glauca; snow avalanches; thinning. Introduction Natural disturbances are postulated to be one of the important forces driving population and community dynamics (Wiens 1977, Paine 1979, White 1979, Oliver 1981). Evaluation of this assertion requires knowledge of the frequency and magnitude of disturbance and how the force of disturbance affects the extent of damage or mortality of organisms (Levin and Paine 1974, Runkle 1982). The objective of this paper is to address the questions: what is the chance that trees and shrubs will be broken when they are growing at a certain place on a snow avalanche path, and how does the chance of mortality from avalanche compare to mor? tality from thinning? The study was conducted in the Kananaskis Valley, a major north-south valley in the southern Canadian Rockies of Alberta (Johnson et al. 1985). Snow avalanches are driven by gravity acting on a snow mass and are resisted by snow and air friction, ploughing, and drag (Voellmy 1955, Mellor 1968, Leaf and Martinelli 1977, Perla 1980). The number of times a section of slope is overrun by avalanches is best determined by the age of the scars left on the trees and shrubs growing in the avalanche's path (Burrows and Burrows 1976). Snow avalanches are believed to create 1 Manuscript received 4 June 1985; revised 22 January 1986; accepted 5 February 1986.

a continuum of average avalanche return times (return time = 1/avalanche frequency), with the bottoms of avalanche-prone slopes being overrun by avalanches only at long intervals (Perla 1980). In Alberta, the vege? tation in these extreme runout positions consists predominantly of forests of lodgepole pine (Pinus contorta London var. latifolia Engelm.) and Engelmann spruce (Picea engelmannii Parry). The vegetation at the tops of avalanche-prone slopes, where avalanches occur very frequently, usually has a 1.5-m canopy of glandular birch (Betula glandulosa Michx.) and willow (Salix glauca L.). Saplings and broken small trees of Engel? mann spruce and lodgepole pine occur in this shrub canopy, but never reach a height much greater than the shrub canopy before being broken, permanently bent, or uprooted. The shrubs are rarely if ever broken Subalpine fir (Abies lasiocarpa by the avalanches. [Hook.] Nutt.) also occurs in the paths at higher ele? vations. In this study, avalanche frequency for different slope trees positions was estimated from avalanche-scarred and shrubs. Next, a model for the mechanical basis of avalanche-induced stem bending and breakage predicted the diameter at which breakage would start. This prediction was validated using the observed breakage pattern. Finally, thinning mortality was estimated in two spruce populations and one pine population, and comparisons to avalanche mortality were made.

44

Ecology, Vol. 68, No. 1

E. A. JOHNSON

Fig. 1. Avalanche paths in Kananaskis Valley.

Previous studies of avalanche frequencies (Potter 1969, Butler 1979, Carrara 1979, Johnson et al. 1985) have not determined the decrease in frequency at dif? ferent slope positions, but have instead determined the frequency only on the extreme runouts. Understanding the decrease in avalanche frequency down a path requires that the slope position be related to avalanche dynamics so that comparisons between avalanche paths can be made. My study compared two paths that were reasonably similar in path form but differed in steepness. In developing a mechanical model of tree breakage by snow avalanches, I incorporated the assumptions that tree boles are tapered and that both trees and shrubs are significantly bent before breakage. This approach differs from that of Mears (1975), who used large broken trees in extreme avalanche runouts to estimate the velocity and impact pressure of the ava? lanche. Study

Area

The upper Kananaskis River Watershed contains >250 recognizable snow avalanche paths (Gardner et al. 1983, Johnson et al. 1985). The two paths used in this study are located on the east slope of Mt. Lawson adjacent to the Kananaskis River (115?10' N, 50?47' W). Both paths are vegetated except for the steep scree back walls of the catchments (Fig. 1). The paths have decreasing slopes with single catch? (1) monotonically ments, where snow collects and from which it breaks away when an avalanche begins; (2) unconfined tracks where the speed of the avalanche may increase, de? crease, or stay constant but the snow mass remains relatively constant; and (3) well-defined runout zones at the bottom of the path where the snow mass decelerates and stops (cf. Perla and Martinelli 1976). The average slope angles were 24? and 26? (cf. definition of Johnson et al. 1985), and the path lengths from the middle of the catchments to the furthest identified run? out were 1140 and 2040 m, respectively. Avalanches start to occur on these paths in December and are possible through early June. Dry-snow av-

alanches predominate from approximately December to the end of February and early March, due to the cold continental polar air masses, which result in lowdensity snow. Mixed and wet-snow avalanches are ex? pected after early March, when Maritime Pacific air masses bring periods of large wet snowfalls (Janz 1976). Chinooks (Foehns) are frequent during the winter (Longley 1967) and melt the surface snow which, after re-freezing, creates planes in the snowpack along which shearing may occur. Early and midwinter snow release is often along zones of granular snow or hoarfrost crystals near the snow pack base. Late winter and spring snow release usually occurs when the whole snowpack is near 0?C. Avalanches are mostly surface slides, eroding the ground only rarely. Stone lines (Rapp 1960) set out on the two paths in 1979 showed no disturbance after being overrun by at least three avalanches. These paths are well vegetated throughout their length. High-elevation paths above tree line and on scree-covered slopes do show signs of erosion (Luckman 1978, Gardner 1983). Methods Dating avalanche

events

The recognition of an avalanche event depends on not confusing it with other causes of the same kind of damage (Burrows and Burrows 1976, Shoder 1977). Systematic observations were made of avalanche dam? age on paths during summers following winters when avalanches occurred, and comparisons were made with damage occurring from other causes, e.g., bears, porcupines, falling trees, wind, ice storms, soil creep, fire, and frost cracks. Not all avalanche injuries to trees and shrubs were equally useful in identifying the annual ring corresponding to the year in which the event occurred. Imtrunk and branchpact scars on the avalanche-exposed es were the most reliable evidence of avalanches. They were reasonably easy to differentiate from fire scars and animal damage although they could be confused with damage caused by falling trees. Avalanche impact scars were found everywhere within the flow height of the avalanches. Very old scars were often completely overgrown, leaving only a thin-line scar (which could be differentiated from a frost crack by its short length). Impact scars, except in very badly damaged specimens, also gave the best identification of events occurring in successive years. Other evidence frequently gave unreliable dates of avalanche events. The ring responses caused by growth release of trees and shrubs from overstory shading and by plagiotropic shoots or dormant buds released by broken terminal stems both often lag by a year or more behind the actual date of the avalanche event. Reaction wood was the most difficult to use in dating avalanches because tilted trees tend to return to vertical by redirecting the growth of the terminal leader; the formation

February 1987

AVALANCHES AND PLANT POPULATIONS

of reaction wood at the base then migrates around the stem as the tree straightens. This new reaction wood could be mistaken for evidence of another avalanche. In identifying avalanches, impact scars were generally so abundant that other less reliable indicators were not needed. If reaction wood, growth release, or broken terminals suggested an event, usually a careful search revealed scars. No matter how accurately the evidence for avalanch? es is interpreted, the date of the event may still be in error if only ring counting is used, primarily because some growth rings may be missing on part of the stem. Partially missing rings were particularly common in injured and bent stems. To resolve this problem, stan? dard dendrochronological techniques (Stokes and Smiley 1968, Fritts 1976) were used to establish the year of a particular annual ring as follows. Cross sections of the whole tree or shrub were used when at all possible. Cores and wedges are very difncult to use for dating because they do not allow the whole ring circuit to be examined and the exact start of scars, patterns of reaction wood, or growth release to be ascertained. The surface of each sample was first sanded with a mechanical sander and then finished with very fine sandpaper. To date annual rings, we first visually noted if any rings could be seen to converge. Rings were rarely miss? ing all the way around a disk. Next, we marked on the disk two radii, and on each radius marked decades with pin pricks. We then traced along each of the marked annual rings from one radius to the other to confirm that the decade marks corresponded. Finally, the ring widths were measured (taking into account missing rings) on both radii to the nearest 0.1 mm. These ringwidth series were then compared to master ring-width indices, a process called cross-dating. Master ring-width indices for a species consisted of ring-width series of 10 to 20 trees that had been standardized by having their growth- and site-specific trends statistically removed (cf. Fritts 1976). Cross-dating then allowed rings to be accurately dated. It was then relatively easy to date the annual ring in which the event of interest occurred. Cross-dating was not used in all cases, but we found it useful for difncult specimens, old events, and events based on few confirming records. Each avalanche slope was divided into segments of relatively homogeneous slope angle. Slope angle in each segment was measured as: 6 = arctan (VD/HD), where VD is the vertical drop in elevation and HD is the horizontal distance. In order to produce a complete record of avalanches for a slope segment, an Event Record Plot was constructed (modified from Shoder 1977). The record of avalanche events provided by each sampled tree or shrub was plotted: the horizontal axis showed the individual trees on which scars were found and the ver? tical axis the calendar year in which each scar was formed. When all the trees or shrubs with records of scars had been plotted for a path segment, a visual

45

comparison of event timing was performed. Trees or shrubs whose event records were not duplicated by other plants were re-examined for errors in dating or After these dating probquestionable identification. lems were resolved, a master record of avalanches for a segment was compiled from the Event Record Plot. The value of the event plot is not that it confirms the accuracy of the dates. Cross-dating presumably does that (Madany et al. 1982). The event plot is a means of evaluating the quality and quantity of evidence for an avalanche event. Completeness and accuracy of the record for the two paths was further checked using historical records of avalanches for the last 15 yr and maps of the extent of avalanches for the last 3 yr. In both of these paths, records of known avalanches were easily found in tree and shrub impact scars. A valanche frequency Frequency estimates for each path segment were calculated from the number of avalanche events divided by the number of years over which avalanches could be recognized, and the frequencies were compared to the extreme-value distribution, which has been found to give the best fit to a 70-yr record of avalanches in Roger's Pass, Alberta (Fitzharris 1981). The extremevalue distribution was also appealing because many other geophysical phenomena (e.g., flood levels, wind gusts) fit this distribution. The data were plotted on extreme-value probability paper, which gives a straight line when the fit to the distribution is good. Goodness of fit was tested visually using the median regression technique (Ferrill 1958). Visual testing is the only effective method for these data, as for hydrological data (Kite 1976). Chi-square tests of goodness of fit are and Kolmogorov-Smirnov not sensitive with so few sample points. Avalanche-induced

bending and breakage

A woody plant responds to the impact of an ava? lanche by bending. If it is flexible enough, it is deflected out of the way and will be largely undamaged by the avalanche's passage. On the other hand, if the plant is stiff, the stresses that build in the wood as it resists bending by the avalanche may exceed the breaking strength and then the bole will break. Flexural stiffness is defined by Young's modulus of elasticity of the material (wood) and the moment of inertia. Bending stress is defined as force per unit area of the stem as it is bent; breaking strength is the stress at which failure occurs. If the soil tensile stress on the roots is less than the breaking strength of the bole, the plant may be uprooted instead of broken. A distinction is necessary between material and structural properties. Young's modulus and the mod? ulus of rupture are material properties, in this case of wood. Flexural stiffness, bending stress, and breaking stress are structural properties, which depend on a combination of material properties and the assembly


E. A. JOHNSON

46

avalanche deflection

moment arm

P(a) = Fig. 2. Diagram showing a tapered tree bole (shaded area) being deflected by an avalanche. Variables are defined in Methods: Avalanche-induced Bending and Breakage. and mass distribution of the material. The relevant structure in this case is the stem of the tree or shrub. Breakage by bending was modeled for stems of smaller diameter. Shear is not considered to be signif? icant at these stem diameters but may be important in stems of larger diameter. The avalanche was assumed not to be armored with wood or other solid objects with impact characteristics significantly different from those of clean snow, and impact was assumed to act as a concentrated load at the center of gravity of the plant. The center of gravity is the point at which a load will have the most effect on the vertical stability of the stem. Clearly, an avalanche does not concentrate its load at a single point but has a load distributed over the bole and canopy of a smaller plant. However, this simplifying assumption was used because the actual profile of avalanche loads on trees and shrubs is not understood at this time. Perla (1980) showed that even avalanche impact pressure profiles are controversial, with impact pressure peaks having been reported by different investigators to occur at the top, middle, and bottom of the flowing avalanche. As a final assumption, impact pressures in the range 10 to 300 kPa, the range of empirical measurements (Mellor 1968, Schaerer to always give loads were considered Perla 1973, 1980), large enough to break the stems considered here. Bending. ?The bending of a tree or shrub as it is hit by an avalanche was treated as the large deflection of a tapered cantilever beam with load concentrated at the center of gravity (Fig. 2). The large deflection of a beam of uniform width can be written as: dfl _ P(a - x) ' ds EIQ

{ }

which states that the rate of change in bending angle (6) with respect to position (s) on the beam is propor? tional to the bending moment P(a - x) and inversely proportional to the flexural stiffness (EI0). The symbol

a refers to the moment arm and x is height above x) takes into account the shortground. The term (a ening of the moment arm at large deflection. The deflection is the result of the avalanche load (P) pushing with a leverage of (a - x) on the beam and the beam pushing back. The beam's ability to push back is given by Young's modulus of elasticity (E) for the wood = 7rr4/4, where multiplied by its moment of inertia I0 r is the radius of a uniform circular beam. Trees and shrubs are usually tapered towards the top rather than being beams of uniform diameter as assumed in this formula for 70. This means that the top of the beam will bend more than the base. That is, the beam does not bend with uniform curvature. To ac? count for this behavior, the formula for I0 was modified for a tapering diameter (cf. Kemper 1968) to calculate I, the tapered moment of inertia: l]4,

I=I0[ks+

(2)

rt)/rhL,

k=-(rb-

where I0 is now the moment of inertia at the base, rh is the radius at the base, rt is the radius at the center of gravity, and L is the height of the beam at the center of gravity. Following Kemper (1968), I was substituted for I0 in Eq. 1. Then, differentiating with respect to 5 and letting dx/ds = cos 6 gives d26_P_ ~ds2~~W0

cos 6_4k (ks + l)4

Through the substitution dimensional:

d6_ (ks+\)'ds'

? = s/L, Eq. 3 becomes

d26

PL2

cos e

d?

EI0

[{kL)$ + l]4

_

4(kL) ' d6_' (kL)? + 1 d$

non-

(4)

February 1987


47 c

CD & 0.041

? 0.04

20

40

60

3 cm dbh. The empirical mortality curves (-) pothesized seedling mortality (-).

February 1987


51

positions. The alternative view, often implicitly held, is that avalanches are accidents, i.e., completely unexpected. This view gives no consideration to the fact that the vegetation has experienced avalanches several times before and that its composition is shaped by, and therefore can reveal, the length of time between recurrences. Threshold of breakage The diameter at which breakage can occur marks an important boundary condition for plant populations on avalanche paths. Below this diameter an individual is not at risk of death from an avalanche. Above this diameter (Fig. 5) the path position (7) determines how long a plant can be expected to live before avalanche breakage. The threshold for each species will also have an effect on the growth habit of the canopy species. The thresholds of the paths studied here seem to be determined not by major differences in species wood strength or elasticity (modulus of rupture or Young's modulus) but by plant size. Pine and spruce are capable of growing to larger heights and diameters than the birch and wil? low, which gives them a competitive advantage over the shrubs at non-avalanche locations. However, shrubs have a competitive advantage when avalanches recur more often than every 15 to 20 yr. From Eq. 6, the probability of at least one avalanche in a year (qA) can be plotted against the path position (T) so that the curves describe the distribution of probabilities at which the trees can overtop the shrubs. Fig. 7 gives the distribution of qAfor 15 to 20 yr, when pine and spruce would be first overtopping the shrubs. As can be seen in this graph, trees near the top of the path, where the average time interval between avalanches is short, will always have very high probabilities of av? alanche and little chance to gain dominance over the shrubs before breakage. Trees lower on the path, where there are longer intervals between avalanches, will have lower probabilities of avalanche; this means that the trees have a better chance of overtopping and shading out the shrubs. The breakage threshold directly influences the probability of overtopping, since breakage threshold is mainly dependent on plant size and not on wood properties. Comparison

of avalanche

and thinning mortality

Whereas avalanches are catastrophic, affecting all individuals above the height threshold, thinning selects individuals by position in the canopy (Harper 1977). Furthermore, while avalanche probability increases with time since the last avalanche (Fig. 5), thinning is associated with a certain period in stand development (Fig. 6). The most significant difference between ava? lanche and thinning effects, however, is in the mortality probabilities. Thinning probabilities rarely get above 0.10 (Fig. 6); avalanche probabilities are >0.10 for

0

10

20

Average interval between

30 avalanches

40 (yr)

Fig. 7. The distribution of probabilities of at least one avalanche (qA)at the age when trees could be overtopping the shrubs; x and b as in Fig. 5.

trees 5 yr past the breakage threshold, at all slope po? sitions with average return time 0.10 for trees 10 yr past the breakage threshold, at positions with average return time < 150 yr (Eq. 7). Thinning is approximately as important as the probability of av? alanche mortality when the average avalanche return interval is 150 yr (compare Fig. 6 to Fig. 5). As shown by Johnson et al. (1985) and confirmed in this study, average avalanche return intervals > 130 yr were never found. Therefore, path positions with a hypothetical 150-yr average avalanche return interval can be as? sumed never to experience avalanches. We are now in a position to consider the question "What is the relative role of avalanche disturbance and in determining the canopy on thinning competition avalanche-prone slopes?" This is a problem of the scale of the two processes. By scale, I mean the specific eco? logical consequences of changing the time and spatial dimensions of the disturbance and competition process. The breakage threshold and the thinning thresh? old establish the domain in time and space in which avalanche disturbance can affect the canopy for ava? lanche-prone slopes. The breakage threshold is defined by plant size and the mechanics of breakage in tapered beams, while the thinning threshold is defined by plant size and density. In this study the breakage threshold gave a critical avalanche slope position of tan 0.48 (for average slope 24?) and tan 0.35 (for average slope 26?), and a return interval of 15 to 20 yr. On slope positions steeper than this critical position, or with time intervals shorter than the critical avalanche return interval, avalanche mor? tality is always more important to trees than thinning mortality. In this study the thinning threshold occurred at slope positions of about tan 0.4 (for average slope 24?) and tan 0.26 (for average slope 26?), and a return interval of 150 yr. On slope positions not as steep as

E. A. JOHNSON

52

this critical position, or with time intervals longer than the critical avalanche return interval, avalanches do not occur. That is, these critical values give the time and space scale at which avalanches cease to be a dis? turbance factor. For plants between the breakage and thinning thresholds, there are slope positions and avalanche return times where neither avalanche nor thinning mortality has a clear ascendancy in determining the canopy growth habit. An avalanche that destroys the forest enlarges the length of the path influenced by disturbance at the expense of thinning. In the years that follow an ava? lanche, the trees will regrow and slowly begin the thin? ning process, thus increasing the length of the path over which the vegetation is influenced by thinning. ACKNOWLEDGMENTS Several people helped in the fieldwork: L. Hogg, M. J. Hood, T. Leonard, K. Nordin, and C. Carlson. Critical and useful comments were ofFeredon the research and drafts of the paper by R. Perla, D. H. Knight, and two anonymous referees. The research was supported by the Natural Sciences and Engineering Research Council of Canada. The personnel of Kan? anaskis Provincial Park?J. Murphy, R. Chamney, and G. More?were always helpful. LlTERATURE ClTED Adamovich, L. L. 1975. Engineering characteristics of Canadian trees?centre of gravity and green weight components of four species in interior British Columbia. Canadian Forestry Service Forest Management Institute Information Report FMR-X-74. American Society for Testing and Materials. 1984. Volume 4.09:Wood. In Annual book of ASTM standards, section 4. American Society for Testing and Standards, Philadelphia, Pennsylvania, USA. Bisshopp, K. E., and D. C. Drucker. 1945. Large deflection of cantilever beams. Quarterly of Applied Mathematics 3: 272-275. Burrows, C. J., and V. L. Burrows. 1976. Procedures for the study of snow avalanche chronology using growth layers of woody plants. Institute of Arctic and Alpine Research, Uni? versity of Colorado, Occasional Paper 23. Butler, D. R. 1979. Snow avalanche path terrain and vege? tation, Glacier National Park, Montana. Arctic and Alpine Research 11:17-32. Carrara, P. E. 1979. The determination of snow avalanche frequency through tree-ring analysis and historical records at Ophir, Colorado. Geological Society of America Bulletin 90:773-780. Ferrill, E. B. 1958. Plotting experimental data on normal or log normal probability paper. Industrial Quality Control 15:12-15. Fitzharris, B. B. 1981. Frequency and climatology of major avalanches at Rogers Pass, 1909 to 1977. Division of Building Research Paper Number 956, National Research Council of Canada, Ottawa, Ontario, Canada. Fritts, H. C. 1976. Tree rings and climate. Academic Press, New York, New York, USA. Gardner, J. S. 1983. Observations on erosion by wet snow avalanches, Mount Rae area, Alberta, Canada. Arctic and Alpine Research 15:271-274. Gardner, J. S., D. J. Smith, and J. R. Deologes. 1983. The dynamic geomorphology of the Mt. Rae area: A high mountain region in southwestern Alberta. Number 19, Depart? ment of Geography Publication Series, University of Waterloo, Waterloo, Ontario, Canada.


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February 1987


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