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EROSION AND SITE PRODUCTIVITY
IN WESTERN-MONTANE FOREST
ECOSYSTEMS
Walter F. Megahan
Washington) established a policy dealing with soil
productivity and erosion:
ABSTRACT
Soil loss from erosion affects site productivity by reducing the nutrient pool and water-holding capacity of the
soil and by direct damage to vegetation. The effects of
erosion depend on the type of erosion processes because
of differences in the depth and areal extent of soil loss, the
downslope rate of soil movement, and the probability of
redeposition of eroded material. The most severe and longlived site productivity losses occur from debris avalanches
and gullying. Forest management practices can increase
erosion rates, but wildfires have the greatest potential to
accelerate erosion. Erosion increases following fire are
directly proportional to fire intensity. Debris landslides
and gullying cause serious and long-term reductions in
site productivity, but the areas affected are small. Surface
erosion occurs over much larger areas and does tend to
reduce site productivity, but the magnitude of the reduction is poorly defined because of the compounding effects
of compaction on logged areas and water repellency on
burned areas. Methods to better assess the erosional
effects of forest management on site productivity require
a combination of controlled bioassay studies and growthsimulation models.
... to plan and conduct land management activities so
that soil loss from accelerated surface erosion and mass
wasting caused by these activities will not result in an
unacceptable reduction in soil productivity and water
quality (Howes 1988).
Swanson and others (1989) provide an overview of the
effects of erosion on long-term site productivity in the
Pacific Northwest. However, their work stresses the
results of studies in the Coast and Cascade Ranges in
Washington and Oregon. The present discussion concentrates on erosion processes and resulting impacts on site
productivity in the interior West, described in this symposium as the western-montane zone. The purpose of this
paper is to: describe the potential effects of erosion on
productivity, consider how the different erosion processes
occurring on forest lands relate to these effects, and
summarize the few available published reports documenting the magnitude of the reductions in site productivity
associated with erosion in the western-montane region.
EROSION EFFECTS ON
PRODUCTIVITY
INTRODUCTION
The effects of erosion on site productivity result from a
change in the total depth of soil material at a site or direct
damage to vegetation. Changes in soil depth can affect
productivity by changing the total nutrient pool and by
changing the water storage capacity. Direct damage to
vegetation is manifest by changes in mechanical support,
changes in the availability of propagules, and direct
damage to trees. I use the term "change" in describing
the factors influencing site productivity because the reductions in productivity that occur at the site of erosion
may be accompanied by increases in productivity at downslope deposition sites. This is especially true in forested
settings where slope irregularities and large volumes of
surface debris may cause deposition of eroded material
within short distances downslope. For example, within
the first 4 years after construction, over 95 percent of
the material eroded from roadfills is deposited within
20 meters downslope from the bottom of the roadfill
on granitic slopes in Idaho (Megahan 1984). Thus, it is
important to recognize that the actual effects of erosion
on productivity are represented by the net differences
in productivity that occur at both erosion and deposition
sites. The relative amount of soil loss and deposition
depends on the type of erosion processes acting in the
area and will be discussed in more detail later.
Erosion is a geomorphic process that is a natural
component of any forest ecosystem. However, erosion
rates can be accelerated by both natural and human
disturbances. Wildfire is the most common cause of
accelerated erosion in the "natural" forest. Forest
management activities, especially timber harvest and
road construction, have been shown to increase erosion
rates on forest lands. Megahan (1981) summarized the
results of 30 studies documenting the effects of fire and
forest management practices on erosion rates in the
western-montane region.
To date, the largest concern with accelerated erosion
in relation to forest management has been directed at the
resulting downstream sedimentation and accompanying
damage to beneficial uses of water. However, concern
about the onsite impacts of erosion on forest lands is
increasing. The Pacific Northwest Region of the Forest
Service, U.S. Department of Agriculture (Oregon and
Paper presented at the Symposium on Management and Productivity
of Western-Montane Forest Soils, Boise, ID, April 10-12, 1990.
Walter F. Megahan is Research Hydrologist, Intermountain Research
Station, Forest Service, U.S. Department of Agriculture, Boise, ID 83702.
146
EROSION PROCESSES
Table 1-Mean horizon thickness, plant-available water, and
nutrient content for four granitic soils in the Idaho Batholith
(Clayton 1990)
0
Parameter
Thickness (cm)
Available water (percent by volume)
Range in K, Ca, Mg, N, P, S (percent of total)
Average K, Ca, Mg, N, P, S (percent of total)
3
82-93
88
The actual impacts of different erosion processes on site
productivity are influenced by: (1) the depth of erosion
(determines the amounts of soil components that are lost
from the site), (2) the areal extent of the erosion (determines the area over which the losses occur), (3) the downslope rate of movement of eroded material, and (4) the
probability of redeposition of the eroded material at downslope locations. Relative rankings of these four factors for
each of the erosion processes described later are given in
table 2. Evaluation of the total effects of erosion on site
productivity must consider the net effects of all these
factors.
Horizon
A
C
25
11.3
4-11
9
46
4.1
1-7
3
The data in table 1 summarize the average total nutrient pool and plant-available water for four granitic soils
in the Payette River drainage of Idaho. Note the large
decreases in available water and nutrients with depth.
The depth of erosion is important in terms of what is
limiting at a site. If nutrients are limiting, loss of only
1. 7 cm of surface organic matter will remove 50 percent
of the total nutrients on the site. If water is limiting, as
may often be the case in the western-montane area with
hot, dry summers, all of the organic horizon and 17 cm
of the A horizon must be eroded to remove 50 percent of
the site's water-holding capacity.
Direct damage to trees caused by erosion occurs frequently in the interior West. Erosion may be severe
enough to expose roots, reducing growth rates or reducing
mechanical support to the point that the tree falls or is
blown over by wind. Such damage can occur from erosion
of surface soils (Carrara and Carroll 1979) but is especially prevalent on slopes adjacent to roadcuts due to
rapid erosion of the steep roadcut surfaces (Megahan and
others 1983). Tree fall caused by lateral erosion of roadcuts is a major concern for road engineers responsible for
the maintenance of roads on steep slopes. Additional
direct damage to vegetation is attributed to loss of propagules as surface soils are eroded. Seeds, root stock, and
sometimes entire plants may be damaged or displaced by
erosional processes. Finally, direct damage to large trees
may occur from tilting, splitting, or abrasion of the trunks
or by burying the lower portions of the trunk.
Surface Erosion
Surface erosion is defined as the movement ofindividual soil particles by a force. Major factors regulating
surface erosion include: soil cohesion, slope gradient, slope
length, rainfall intensity, soil infiltration rate, and the
amount of ground cover protecting the soil surface. Four
different types of surface erosion processes are recognized
and all are common in the Intermountain West. They are:
splash, ravel, rills, and gullies. Splash erosion is caused
by the impact of raindrops and occurs anywhere mineral
soils are exposed. Splash erosion is most important on
noncohesive soils. Ravel (sometimes called dry ravel or
dry creep) occurs on steep slopes, generally over 60 percent,
where gravity forces exceed the cohesive forces holding
individual soil particles in place. It occurs during dry
periods, primarily under the influence of wind, and is
most common on noncohesive soils on bare roadfills and
roadcuts and on natural slopes where logging, fire, or
both, have exposed mineral soils. Rills and gullies (usually defined as rills more than 30 em deep) are caused by
channelized overland flow. Such flow is relatively rare
on forest soils, even when bare, except where infiltration
rates have been reduced by compaction, such as on skid
trails or roads, or in the case of severe soil damage and
the formation of water repellency as occurs on intensely
burned areas.
Table 2-Properties of the different types of erosion processes
Erosion process
Depth of
erosion
Areal
extent
Rate of
movement
Probability
of slope
storage
Surface
erosion
Splash
Ravel
Rilling
Gullying
mm-cm
mm-cm
cm
cm-m
widespread
localized
localized
concentrated
m/yr
m/yr-m/sec
m/day-m/sec
m/day-m/sec
high
high
moderate
low
Mass
erosion
Creep
Earthflow
Slump
Debris slide
Debris flow
soil depth
m-m x 10
m-m x 10
cm-m
cm-m
widespread
localized
localized
concentrated
concentrated
mm/yr
cm/yr-m/yr
m/yr-m/day
m/sec
m/sec
high
moderately high
moderate
moderately low
low
147
Debris types offailures include debris slides and flows
and involve the surface soil mantle sliding over the
underlying bedrock or parent material. Lengths are long
relative to their depth (by a factor of 20 times or more)
and widths are generally small (meters to tens of meters).
Debris failures occur on steep slopes usually greater than
60 percent. Slope depressions that serve as water accumulation zones are the most common sites for the initiation of debris failures. The release of the failures is sudden
and movement rates are rapid with velocities of meters
per second. Downslope delivery is relatively high, especially for debris flows, because of the high water content
of the slide material. The primary factors affecting debris
failures are: soil strength, vegetation roots, slope gradient, groundwater depth, and soil depth. Of these, vegetation roots, groundwater depths, and slope gradients (in
the case of forest roads) are sensitive to forest management practices.
Debris failures have a severe impact on productivity
at the site offailure because of the sudden, total loss
of the soil mantle. Additional erosion sometimes occurs
as a result of scour as the rapidly moving mass of eroded
material moves downslope and from subsequent surface
erosion in the slide site. However, the area affected is
small because of the concentrated nature of the failures.
Megahan and others (1978) collected data on 629 landslides in the Clearwater National Forest in Idaho. For
a 3-year study period, the total area affected by landslides
amounted to 16.5 ha, about 0.003 percent of the total
non wilderness area of the forest.
Wildfire can increase the number of debris failures per
unit area of forest land. Jensen and Cole (1965) reported
a total of 34,000 cubic meters of soil loss from the 400-ha
Poverty Burn on steep slopes in the South Fork of the
Salmon River as a result of debris failures. Assuming
an average soil depth of 0.6 meters (reasonable for steep
slopes in this area), this volume of soil loss would indicate
that about 2 percent of the burn area was affected by
debris failures. Although three orders of magnitude
greater than the percentage of land affected by mass
failures on the Clearwater National Forest, the loss of
productivity occurred only on 2 percent of the Poverty
Burn, a relatively small area.
Expressing productivity loss on the basis of area
affected by debris avalanches can be misleading. This
is because failure sites are usually located in slope depressions that serve as both soil and water accumulation
zones. Because of greater soil depth and increased water
availability, such sites also tend to be some of the better
sites for tree production. Thus, total site productivity loss
may greatly exceed the percentage of area affected by
debris failures.
In general, surface erosion rates are greatly influenced
by the amount of vegetative cover and forest litter that
are available for protection of the soil surface. Road construction and wildfires generally cause the greatest reductions in vegetative cover protection and thus commonly
result in the greatest increase in erosion rates. However,
even on roads and burned areas, erosion rates may decrease rapidly over time as revegetation occurs. Megahan
(1974) found road erosion rates on granitic soils decreased
about 90 percent by the second year after construction.
Similar recovery was recorded about 2 years following a
wildfire on a clearcut north slope in granitic soil (Megahan
and Molitor 1975) but not following a controlled burn on
a clearcut south slope in the same vicinity. In the latter
case, considerable active erosion was still occurring 10
years after disturbance (Megahan 1990). Wildfire can
cause greatly accelerated surface erosion. Connaughton
(1935) evaluated the degree of accelerated surface erosion
on an 18,000-ha wildfire in southern Idaho. Accelerated
erosion was found on 42 percent of the area on cutover
lands and on 28 percent of virgin forest land. In addition
to the effects oflogging on erosion severity, there were
large increases in the severity of erosion with increasing
hillslope gradient and burn intensity.
Of all the erosion processes, splash erosion is most
widespread since it can occur anywhere bare soil is subjected to raindrop impact. However, the average depth
of soil loss and the downslope rate of movement of eroded
material are small. Thus, even though large volumes of
soil may be moved by splash erosion, the total effect on
forest site productivity is low. In contrast, gullying results in the rapid removal of considerable depths of
material and transports that material long distances,
usually to the nearest stream channel. In this case, productivity is greatly reduced by gully formation. Gullies
normally occupy a very small area so the net reduction
in productivity for the forest site is again small.
Mass Erosion
Mass erosion is defined as the movement of many soil
particles en masse, primarily under the influence of
gravity, and occurs when shear stresses exceed shear
strength. Unlike surface erosion, which progresses from
the surface downward, mass erosion usually includes the
entire soil mantle and often part of the underlying parent
material as well.
The five major kinds of mass erosion include: creep,
earthflow, slump, debris slide, and debris flow (table 2).
Creep involves imperceptibly slow (mmlyr) downslope
movement of the soil mantle under the sustained influence of gravity on steep slopes. Effects on productivity
are essentially nonexistent. Earthflow and oftentimes
slumps tend to be deep-seated types of slope failures with
the movement plane usually well beneath the soil in the
underlying parent material or bedrock. Earthflows and
slumps move meters to tens of meters per year and can
cause direct damage to trees by tilting and splitting.
Aside from road construction, effects of forest management activities on slumps and earthflows are not well
defined.
STUDIES DOCUMENTING EROSION
EFFECTS
The effects of debris types of landslides where the entire
soil mantle is lost all at once are relatively clear. Severe
reductions in productive capacity occur until new soils
accumulate at the slide sites. Smith and others (1986)
reported a 70 percent reduction in conifer productivity
148
,.,:,:.'
The latest version of the model (FORCYTE-ll) appears
to have promise for evaluating the effects ofa number of
components of the site-productivity question including the
effects of erosion (Kimmins and others 1988). However,
HB studies specifically designed to evaluate the effects
of soil erosion alone are still needed to validate the model.
Klock (1982) developed an interesting alternative
to the historical bioassay approach. He used a greenhouse bioassay technique to assess the effects of various
amounts of erosion on four different forest soils in central
Washington. Soil samples were collected from depths of
0-30 cm, 3-30 em, 7.5-30 cm, and 15-30 cm to simulate
respective erosion amounts of 0,3, 7.5, and 15 cm.
Growth of ponderosa pine, Douglas-fir, lodgepole pine,
and orchard grass (Dactylis glomerata L.) in pots was
used to index the effects of erosion. At the end of varying
lengths of time, the vegetation was clipped at the soil
level and ovendried and weighed. Erosion effects were
evaluated by comparing the percentage of vegetation
weight for the various eroded soils to that for the uneroded soil. Productivity losses ranged from none to as high
as about 85 percent depending on the type of soil, the
amount of erosion, and the type of vegetation. Klock
(1982) concluded that, although the procedure does not
provide a true measure of productivity loss from erosion,
it does provide a means to compare the relative effects
of different sites and erosion rates and to evaluate the
sensitivity of different types of vegetation.
on slide sites in the first 60 years and about 50 percent
at the end of 80 years on slides occurring on the Queen
Charlotte Islands, British Columbia. A gradual increase
in productivity is expected for subsequent rotations. No
such data have been collected for slides occurring in the
interior West. However, personal observations of slide
scars in the region suggest that the magnitude and duration of productivity losses are at least as great as those
reported by Smith and others (1986).
Aside from the formation of gullies, which would appear
to have effects on productivity similar to those of debris
failures, the effects of surface erosion processes on productivity are much more difficult to evaluate. On logged
areas, other factors often associated with or causing erosion, including soil displacement and compaction by timber
harvest equipment (Froehlich 1988) and the formation of
water-repellent soil layers on burned areas (DeBano and
others 1970), can also adversely affect productivity. Thus,
studies of effects of timber harvest or fire on productivity
have not clearly isolated the effects of surface erosion
alone. Clayton and others (1987) showed that the degree
oflateral soil displacement was associated with decreased
tree diameter and height following logging oflodgepole
pine (Pinus contorta Engelm.) and ponderosa pine (Pinus
ponderosa Laws.) in central Idaho. Tree diameter (breast
high) was reduced an average of 21 percent going from
slight to high soil displacement; tree height was reduced
an average of 24 percent. In this case, one might assume
that the displacement effect would be a good indicator
of erosion effects even though the displacement was not
entirely caused by erosion. However, they also reported
that tree diameter and height in the same area decreased
an average of 19 and 18 percent, respectively, with increasing penetrometer resistance, an index of increasing
compaction. Thus, it is impossible to discriminate between the negative effects of compaction and soil loss.
Smith and Wass (1980) reported reductions in the height
growth of Douglas-fir (Pseudotsuga menziesii [Mirb.]
Franco) on logging skidroads on sensitive sites in interior
British Columbia. But again, it was impossible to isolate
the effects of erosion alone.
Because of confounding effects of other factors affecting
productivity, it appears unlikely that it will be possible to
accurately assess the effects of surface erosion on productivity based on field observations of tree growth in timber
sale areas. Such studies are referred to as historical bioassay (HB) studies (Kimmins and others 1988). Also,
HB approaches are based on observations for the growth
conditions that existed during the life of the vegetation.
Using the results of such studies for prediction purposes
requires the assumption that the soil and atmospheric
resources remain static. Such an assumption is open to
question, especially at present when concerns for global
climate change are widespread. Process models of vegetation growth processes offer an alternative for evaluating
effects of erosion but have limitations primarily because
of intensive input data requirements (Kimmins and
others 1988). A hybrid simulation model called FORCYTE combines the best features of both HB and process
model approaches while minimizing some of the problems.
CONCLUSIONS
Erosion can cause large decreases in forest productivity
at the site of soil loss. Reductions in productivity are
directly, but not linearly, proportional to the depth of soil
lost. The greatest and longest duration (decades) impacts
are caused by debris landslides and gullies. Both landslides and to a lesser extent gullies are cOllcentrated in
small areas that tend to be soil and water accumulation
zones. Such areas also tend to be relatively high in site
productivity, so the percentage of loss of productivity
exceeds the small percentage of the area affected by the
erosion. Surface erosion removes much less total depth
of soil than mass erosion, but may have a greater shortterm impact on productivity because larger areas are
affected. However, surface erosion rates tend to decrease
rapidly over time (a few years) so the long-term effects are
limited.
Erosion rates are minimal in the undisturbed forest but
can be greatly accelerated by natural or man-caused disturbances. Of the various types of disturbances, wildfire
has the largest potential for reducing productivity because
of large soil losses over broad areas. Except for landslides
and gullies, the erosional consequences of forest management activities are difficult to evaluate because of the
confounding effects of other types of site disturbances.
Considerable research, including both bioassay and process
modeling techniques, is needed to better quantify the
effects of erosion on site productivity. Studies need to
consider the net effects of both on-site soil loss and downslope deposition of eroded material.
149
REFERENCES
Spec. Publ. 45. Madison, WI: American Society of
Agronomy: 53-66.
Megahan, W. F. 1974. Erosion over time on severely
disturbed granitic soils: a model. Res. Pap. INT-156.
Ogden, UT: U.S. Department of Agriculture, Forest
Service, Intermountain Forest and Range Experiment
Station. 14 p.
Megahan, W. F. 1981. Effects of silvicultural practices on
erosion and sedimentation in the interior West--a case
for sediment budgeting. In: Baumgartner, D. M., ed.
interior West watershed management: Proceedings
of the symposium; 1980 April 8-10; Spokane, WA.
Pullman, WA: Washington State University, Cooperative Extension: 169-181.
Megahan, W. F. 1984. Data on file at: U.S. Department
of Agriculture, Forest Service, Intermountain Research
Station, Boise, ID.
Megahan, W. F. 1990. Data on file at: U.S. Department
of Agriculture, Forest Service, Intermountain Research
Station, Boise, ID.
Megahan, W. F.; Day, N. F.; Bliss, T. M. 1978. Landslide
occurrence in the western and central northern Rocky
Mountain physiographic province in Idaho. In:
Youngberg, C., ed. Forest soils and land use: Proceedings, North American forest soils conference; 1978
August; Fort Collins, CO. Fort Collins, CO: Colorado
State University: 116-139.
Megahan, W. F.; Molitor, D. C. 1975. Erosional effects
of wildfire and logging in Idaho. In: Watershed management symposium; 1975 August 11-13; Logan, UT:
423-444.
Megahan, W. F.; Seyedbagheri, K. A.; Dodson, P. C. 1983.
Long-term erosion on granitic roadcuts based on exposed tree roots. Earth Surface Processes and Landforms. 8: 19-28.
Smith, R. B.; Wass, E. F. 1980. Tree growth on skidroads:
on steep slopes logged after wildfires in central and
southeastern British Columbia. BC-R-6. Victoria, BC:
Environment Canada, Canadian Forestry Service,
Pacific Forest Research Centre. 28 p.
Smith, R. B.; Commandeur, P. R.; Ryan, M. W. 1986.
Soils, vegetation, and forest growth on landslides and
surrounding logged and old-growth areas on the Queen
Charlotte Islands. FishIForestry Interaction Program.
Land Manage. Rep. 41. Victoria, BC: British Columbia
Ministry of Forests. 95 p.
Carrara, P. E.; Carroll, T. R. 1979. The determination of
erosion rates from exposed tree roots in the Piceance
Basin, Colorado. Earth Surface Processes. 4: 307-317.
Clayton, J. L. 1990. Data on file at: U.S. Department of
Agriculture, Forest Service, Intermountain Research
Station, Boise, ID.
Clayton, J. L.; Kellogg, G.; Forrester, N. 1987. Soil
disturbance-tree growth relations in central Idaho
clearcuts. Res. Note INT-372. Ogden, UT: U.S. Department of Agriculture, Forest Service, Intermountain
Research Station. 6 p.
Connaughton, C. A. 1935. Forest fires and accelerated
erosion. Journal of Forestry. 33: 751-752.
DeBano, L. F.; Mann, L. D.; Hamilton, D. A. 1970. Translocation of hydrophobic substances into soil by burning
organic litter. Soil Science Society of America Proceedings. 34: 130-133.
Froehlich, H. A. 1988. Causes and effects of soil degradation due to timber harvesting. In: Lousier, J. D.; Still
J. W., eds. Degradation of forest land: "Forest soils at
risk": Proceedings, 10th British Columbia soil science
workshop; 1986 February; Land Manage. Rep. 56.
Victoria, BC: British Columbia Ministry of Forests:
3-12.
Howes, S. W. 1988. Consideration of soil productivity
during forest management activities: the USDA Forest
Service approach in the Pacific Northwest. In: Lousier,
J. D.; Still, J. W., eds. Degradation of forest land:
"Forest soils at risk": Proceedings, 10th British
Columbia soil science workshop; 1986 February;
Land Manage. Rep. 56. Victoria, BC: British Columbia
Ministry of Forests: 185-190.
Jensen, F.; Cole, G. 1965. South Fork Salmon River storm
and flood report. Unpublished report on file at: U.S.
Department of Agriculture, Forest Service, Payette
National Forest, McCall, ID.
Kimmins, J. P.; Scoullar, K. A.; Chatarpaul, L. 1988. Long
term impacts of forest management on soil fertility and
forest productivity: a systems analysis approach using
FORCYTE. In: Lousier, J. D.; Still, J. W., eds. Degradation of forest land: "Forest soils at risk": Proceedings of
the 10th British Columbia soil science workshop; 1986
February; Land Manage. Rep. 56. Victoria, BC: British
Columbia Ministry of Forests: 116-129.
Klock, G. O. 1982. Some soil erosion effects on forest soil
productivity. In: Determinants of soil loss tolerance.
150