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Vegetative Response to Landscape Disturbance: Is There a Correlation Between DebrisFlow Type Disturbances and Invasive Species in the Oregon Coast Range?
Abstract
The Willamette Basin of western Oregon is associated with a wide
variety of geomorphic and anthropogenic disturbances in riparian zones.
These processes include timber harvest, cultivation, flooding, and mass
wasting (landslides and debris flows). Riparian areas are particularly
vulnerable to habitat disturbance.
Because debris-flows cause transfer of large amounts of soil, rock
and organic material, extensive scour and deposit zones are formed in the
disturbed zone. These stream channels and riparian areas are well suited for
vegetative colonization from both native and invasive species. Investigation
of invasive plants suggests that while plant communities vary in their
susceptibility, invasion may be dependent on the type of soil disturbance, or
at least increase the likelihood that an invasive plant might occupy an area.
The migration and distribution of both native and invasive plant species have
important ecological implications for river restoration and regional land
management strategies.
Landslide Disturbance
1 – Landslides, also called debris flows, can rapidly change the landscape of
riparian zones.
2 – The sudden movement of debris and water can scour the upper slope of all
its soil, forming a heterogeneous deposit of organic debris, rocks, and
sediment (Swanson et al., 1987).
3 – These debris flows can be incredibly destructive, altering and shifting
stream channels, changing a streams gradient and destroying roads,
bridges and homes (Benda, 1990).
Prepared by:
Shawn Coreson
ES 473 Environmental Geology
Benefits of Debris Flow in the Local Ecosystem
Native vs. Invasive Species
•Plant growth and variation was highly influenced by the composition of the
deposit.
•Early colonizers also had a profound effect on local plant progression.
•Substrate variability created by debris flows increases plant diversity.
Allows for transport of seeds and rhizomes downstream.
• Colonization can occur quickly. In the debris flow deposit over 80% of the flora
had been documented by the second year.

Sediment and woody debris in stream channels affect aquatic habitat.
•Furthermore, over two thirds of those found in year two were still present at 7
years later.

They create an array of new surfaces for future plant colonization.

The developing plants affect stream and riparian ecosystems by providing
shade, increasing nutrient retention and input of organic material into
streams.

• Native species did well at colonization and competing as long as invasives
were not able to colonize before natives could become established (Pabst and
Spies, 2001).
A
A
B
Figure 3
Percent cover of plants by (A) life form and (B) origin, in the five areas of a
debris flow deposit (Pabst and Spies, 2001).
Conclusions
(1) Debris flows are a normal part of the landscape in the Oregon Coast range.
(2) While sometimes destructive, debris flows help to create beneficial terrestrial
and aquatic environments.
(3) Newly exposed substrates provide open light areas and exposed soils that
allow for increased plant diversity (Pabst and Spies, 2001).
(4) Initial colonizers and their perseverance were key in plant community
development.
(5) Invasive species in this study only became dominant if established before
native species. The largest benefit was to invasives with early growing
seasons which could become established before native plants.
B
References Cited
Benda, L., 1990, The Influence of Debris Flows on Channels and Valley Floors in the Oregon Coast
Range, U.S.A.
Earth Surface Processes and Landforms, v. 15, p 457-466
“ODA Plant Division, Noxious Weed Control”, Oregon.gov. 11 Feb 2008. 27 May 2008.
http://www.obce.state.or.us/ODA/PLANT/WEEDS/profile_himalayanblackberry.shtml
Pabst, R. J., T. A., Spies, 2001, Ten Years of Vegetation Succession on A Debris-Flow Deposit in
Oregon: Journal of the American Water Resources Association, v. 37, No 6, p. 1693-1708
Figure 1
Figure 2
A) A small debris flow downslope from a disturbed area in the Oregon coast
range.
A) An open exposed area with a variety of species, both native and
invasive.
B) Mean percent of ground cover 5 years after a debris flow deposit. Error bars
show standard error. Soil in this figure are particles 2mm or less (Pabst, R.
Spies,T., 2001).
B) Number of species increased and decreased on a per-year basis on
the debris flow deposit. Annual rate of turnover, using beta turnover
(Wilson and Shmida, 1984), is shown as a percentage below each bar
Swanson, F. J., L.E. Benda, S.H. Duncan, G.E. Grant W. F. Megahan, L. M. Reid, and R. R. Zeimer,
1987. Mass Failures and Other Processes of Sediment Production in the Pacific Northwest
Forest Landscapes. Streamside management: Forestry-Fishery Interactions, University of
Washington, Seattle, Washington, p 9-38
"Tree Damage, Timber Blowdown, and Forest Debris from Winter Storm 2007." Oregon.gov. 14 Apr.
2008. Oregon dept. of Forestry. 27 May 2008.
http://www.paroleboard.state.or.us/ODF/TimberBlowdown.shtml .