The Impact of Forest on Snowpack Stability

A presentation by Miles Daly3/1/08

Winter Ecology Field Course

Mountain Research Station, University of Colorado, Boulder

A Fine Line

Forests as mechanisms of avalanche prevention

orDo trees lead to snowpack

instability

What effects do forest disturbances such as fire and logging have on avalanche activity?

Are mature forests the preventative factor on otherwise avalanche prone slopes?

Decrease snow driftingDecrease in overall catchIncrease surface roughness

The impact of individual trees on local snowpack Increase surface roughness

Prevents wind depositionIncreases slope stability (?)

Increasing local T.G.’sBiological activity as heat sourceTrees as conductive transport of solar

radiation and ground heatDecrease in snow depth

Possible Implications

Backcountry enthusiastsTrees as snowpack anchors or

hazard zones

Avalanche MitigationCan Vegetation be used as

avalanche prevention?

Avalanche FactorsFactors needed for Avalanches

Shear stress must overcome shear strength

Shear stress = (mass)*(gravity)*(sinθ)

Forests effects on snow depositionPrevents added mass

Canopy catches snowDecreases wind depostion

*Lowers shear stress value*

Study of Gaspe Peninsula

Looks at avalanche activity at sites of fire and logging disturbances

Tree-ring reconstruction methods Determine time and severity of

avalanches

Relate avalanches to time of disturbance

Site T-91938 Fire

Large-scale avalanches occurred for 15-20 years

Coincides with recruitment time of Balsam firs

Allowed for greater accumulation of wind deposited snow

1988 Large Scale AvalancheYear of extremely high snow fall

and winds Evidence of continued large scale

avalanches until 1995

Site T-10

1988 Logging ActivityExtends onto starting slopes

of 44 degrees.Allows for large

accumulations of wind deposited snow on steep aspects

Five years of large scale avalanche activity

•Same methods used as in Gaspe Peninsula study

D.M. McClung study: Logging effects on Avalanche Activity in British Columbia

McClung Study Findings

•In British Columbia clear cut logging has led to significant environmental damage due to avalanche activity

•10,000 sites affected by avalanches

•Types of environmental damage:

•Prevents regeneration of seedlings

•Threat to infrastructure

•Stripping away soil cover

•Deposition of debris into streams and lakes

Field Work MethodsSnow trench extending out

from subalpine fir Measured Snowpack profiles next

to tree and 1 m away from treeTemperature gradientHardness profileSnow grain types

Idea is to look for differences created by proximity to tree

Temperature Profile: Away from Tree

0

10

20

30

40

50

60

70

-5 -4 -3 -2 -1 0

Temperature C

Dep

th c

m

Temperature vs Depth

Temperature Profile: Next to Tree

0

10

20

30

40

50

60

70

-4 -3 -2 -1 0

Temperature C

Depth

cm

Temperature vs Depth

•Hardness Profile Next to Tree:

•4 fingers from surface to 10 cm

•Fist below 10 cm

•Hardness Profile Away from Tree:

•4 fingers from surface to 40 cm

•Fist below 40 cm

•Snow Grain Profile Next to Tree:

•Evidence of faceting below 5 cm depth

•Depth hoar a ground 3-4 mm!

•Snow Grain Profile Away from Tree:

•Rounding to 30 cm depth

•Faceting below 30 cm

•Depth hoar at ground ~ 2 mm

Data Findings: Hardness and Snow Grain Type

Discussion of Field Work

•Overall, more research should be conducted

•From my data:

•Snowpack depth will often be shallower next to trees

•Due to tree wells and extending tree trunks

•Can lead to stronger TG and faceted grains

•Future Research

•Horizontal temperature gradient extending out from trees

•Effects of low structural strength snow grains surrounding trees on overall snowpack stability

Summary of Main Points Large scale forests prevent snow

accumulation and avalanches on otherwise avalanche prone terrain.

This should be taken into account when determining where to place infrastructure and where to conduct logging operations.

More research is necessary to determine the impact of individual trees on snowpack stability

Literature cited

McClung, D.M. 2001. Characteristics of terrain, snow supply and forest cover for avalanche initiation caused by logging. Annals of Glaciology, 32: 223–229.

Germain Daniel, Filion Louise, Hetu Bernard. 2005. Snow avalanche activity after fire and logging disturbances, northern Gaspe Peninsula, Quebec, Canada. Canadian Journal of Earth Sciences. 42, 2103-2116.

Mark Williams of INSTAAR, snow hydrology class website