Ribbon Forest Dynamics and Snow Conditions
in Glacier National Park, Montana
Matthew F. Bekker, Dept. of Geography, Brigham Young University
& Daniel B. Fagre, USGS-Northern Rocky Mountain Science Center
Introduction
Results & Discussion
Linear patterns of subalpine forests termed ribbon forest have been
described in several locations in the Rocky Mountains. The origin and
maintenance of these patterns have been attributed to endogenous windsnowdrift interactions and underlying microtopography, but the dynamics
of ribbon forests have not been well studied and are poorly understood.
Here we describe the results of a study of ribbon forest dynamics in
relation to snow conditions in Preston Park, a glacial valley in Glacier
National Park, MT (Fig. 1). The edges of most ribbons in Preston Park
are characterized by successively smaller trees, suggesting
advancement of trees into adjacent meadows (Fig. 1 inset). We used
dendrochronology and snow depth measurements in late winter-spring to
test the hypothesis that recent and continuous advancement is taking
place and that it is associated with areas of earliest snowmelt.
Tree establishment was
episodic rather than recent and
continuous, peaking between
1947 and 1976 (Fig. 3) which
coincides with a strong
negative Pacific Decadal
Oscillation (PDO) phase, and
dropping abruptly thereafter. A
negative PDO phase is
associated with increased
snowpack in Glacier.
Maximum tree establishment
occurred primarily at the
extreme edge of the June
snowpack along the windward
and sun-exposed edge of
ribbons (Fig. 4, Fig. 5). These
data suggest that there is a
trade-off in the positive and
negative effects of snow on
tree establishment and
survival, including a short
growing season, protection
from wind damage and low
temperatures in winter, and
increased moisture in summer.
Figure 2. Sampling trees (left) and snow depth (above)
in Preston Park.
Figure 3. 5-yr running mean of tree
establishment among all tramsects since
1800. Peak tree establishment occurred
between 1947 and 1976, during a strong
negative PDO cycle (red bars).
Figure 5. Dense seedling establishment at
the edge of the June snowpack and along a
windward and sun-exposed ribbon edge
(right side of photo).
Conclusions
Ribbon forest dynamics in Preston Park are controlled by a
narrow range of snow conditions suitable for seedling
establishment and survival in adjacent meadows. Ribbonmeadow boundaries may be sensitive to future changes in
snowpack, but the gradual decline in establishment during the
1947-1976 period and steep decline thereafter suggest that
they are currently stable. Future work will include analysis of
hemispherical photos to assess the effects of solar radiation,
and analysis of temperature data.
Figure 1. The study site, Preston Park, Glacier National Park, MT. Inset: cross-section of a
meadow illustrating tree advancement along edges
Methods
We established belt transects ranging from 1-2 m wide across eight
meadows, and sampled trees in 1x1 or 1x2-m quadrats. Transect
locations were recorded using a global positioning system. In each
quadrat we recorded the species, height, and DBH of all stems >30 cm
in height. We cored trees and cut seedlings and saplings that were
representative of different size classes. Samples were prepared and
crossdated using standard dendrochronological procedures. Snow
depth measurements were taken at 5-m intervals along the same
transect lines on April 1, May 30, and June 19, 2007 (Fig. 2).
Figure 4. Tree establishment and winter-spring snow depth measurements between ribbons in Preston Park.
Transects extend from N-S or E-W across meadows from ribbon to ribbon. Most establishment is occurring on
the windward and sun-exposed sides of ribbons (left side of graphs). Small symbols on snow depth lines
indicate linear interpolations between measured values.
Acknowledgements: We thank Lisa McKeon, Kim Corette, Chris
Miller, Bryan Tikalsky, Emilee Tikalsky, Brayden Bekker, Jeremy
Bekker, George Malanson, and Joseph Malanson for invaluable
assistance in the field and Steve Bills in The laboratory. This
research was supported by Brigham Young University.