EARTH SURFACE PROCESSES AND LANDFORMS
Earth Surf. Process. Landforms 34, 1255–1269 (2009)
Copyright © 2009 John Wiley & Sons, Ltd.
Published online 22 May 2009 in Wiley InterScience
(www.interscience.wiley.com) DOI: 10.1002/esp.1813
Sediment transport due to tree root throw:
integrating tree population dynamics, wildfire
and geomorphic response
Chichester,
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Sediment transport due to tree root throw
J. M. Gallaway,1,2 Y. E. Martin1,2* and E. A. Johnson2,3
Department of Geography, University of Calgary, Calgary, Alberta, Canada
2
Biogeoscience Institute, University of Calgary, Calgary, Alberta, Canada
3
Department of Biological Sciences, University of Calgary, Calgary, Alberta, Canada
1
Received 23 September 2008; Revised 16 February 2009; Accepted 23 February 2009
* Correspondence to: Y. E. Martin, Department of Geography, University of Calgary, Calgary, AB T2N 1N4, Canada. E-mail: [email protected]
ABSTRACT: A field study was conducted to analyze root throw and associated sediment transport in Hawk Creek Watershed,
Canadian Rockies. A large crown fire in 2003 allowed the opportunity to study pre-fire and post-fire root throw. Based on field
data, a significant relation was found between gradient and root plate volume, as well as individual root plate dimensions. Given
that tree diameters increase as trees age and that a relation in the field data was found between tree diameter and root plate
volumes, sediment transport due to root throw is expected to change in response to forest disturbance and stand age. Sediment
disturbance, which is the amount of sediment upheaved during tree topple and does not take into account transport distance,
shows higher values on steeper gradients. Sediment transport was notable for the steepest plots, with pre-fire values of
0·016 cm3 cm–1 a–1 and post-fire values of 0·18 cm3 cm–1 a–1. A tree population dynamics model is then integrated with a root
throw transport model calibrated for the Canadian Rockies to examine the temporal dynamics of sediment transport. Fire is
incorporated as a disturbance that initiates development of a new forest, with the model cycling through generations of forest.
Trees fall according to an exponential rate that is based on time since death, resulting in a time lag between tree mortality and
sediment transport. When values of time-since-previous-fire are short, trees are generally <13 cm, and minimal sediment is
upheaved during toppling. If trees reach a critical diameter at breast height (dbh) at time of fire, a pulse of sediment occurs in the
immediate post-fire years due to falling of killed trees, with tree fall rates decreasing exponentially with time-since-fire. A second
pulse of root throw begins at about 50 years after the previous fire, once new recruits reach a critical dbh and with initiation of
competition-induced mortality. Copyright © 2009 John Wiley & Sons, Ltd.
KEYWORDS:
tree root throw; sediment transport; wildfire; tree topple
Introduction
Tree topple, which may involve stem breakage or uprooting,
plays a critical role in forest population dynamics (for review
see Quine and Gardiner, 2007), with events ranging in size
from severe storm tree blow downs of kilometers in length to
individual tree death due to competition, insects, or disease
and subsequent tree topple due to decay of the supporting
root structures or the stem. Root throw is defined as tree
uprooting when the root plate is upheaved along with any
attached sediment. Root throw is recognized as an important
near-surface process affecting infiltration, air capacity, and
remixing of organic material (e.g. Lutz, 1940; Meyers and
McSweeney, 1995; Clinton and Baker, 2000) and is also an
important sediment transporting agent on forested hillslopes
(e.g. Dietrich et al., 1982; Swanson et al., 1982; Roering
et al., 2002; Gabet et al., 2003; Osterkamp et al., 2006).
Root throw results in vertical and horizontal displacement
of sediment attached to the roots (called the root plate). The
disturbed sediment often remains attached to the root plate
for a period of time after root throw. Subsequent root plate
disintegration due to weathering and decay of the roots leads
to vertical fall of sediment, which may remain in situ or move
horizontally and/or vertically due to gravity and inertia. A pitmound pair is the resulting geomorphological feature from
this process (Stephens, 1956). Mound degeneration may
occur through weathering and transport processes, such as
rainsplash or diffusive creep. Subsequent mound disintegration
is not considered as part of the root throw process herein.
The volume of soil disturbed during a root throw event
depends on factors including: tree species and age; whether
the tree was alive or dead at time of fall; soil texture; rooting
structure and depth; and moisture content of soil at time of
the event (Norman et al., 1995). Root plate volumes are smaller
for trees that have been dead for some time prior to falling
(Cremeans and Kalisz, 1988; Norman et al., 1995; Ulanova,
2000), due to decay of fine roots and reduced cohesion between
the root structure and the soil (Swanson et al., 1982). The
nature of how the tree topples determines the location of the
root plate relative to the pit, which in turn affects the fall
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EARTH SURFACE PROCESSES AND LANDFORMS
location of disintegrating sediment (e.g. Beatty and Stone,
1986). The root plate may be situated either upslope or downslope of the originating pit (with some lateral component also
possible), or may be situated directly above the pit. Hillslope
gradient, angle of tree fall relative to contour lines, and final
resting location of the root plate are key factors affecting the
volume of sediment that ultimately falls either outside or
within the pit (Norman et al., 1995). Sediment that falls from
the root plate may contribute to downslope sediment transport,
but in some cases sediment may be deposited upslope of the
pit, or it may be deposited in the pit itself. Finally, the rate of
sediment disintegration affects the final transport rate (assuming
the transport process is only completed once sediment deposits
on the ground surface), with the disintegration rate being
influenced by factors such as root plate dimension, sediment
particle size, and weathering intensity.
How important is root throw relative to other hillslope geomorphic processes? Gabet et al. (2003) estimated sediment
transport rates for root throw using a model calibrated with
field data, and obtained values of the order 10–3 m3 m–1 a–1.
Roering et al. (2002) estimated sediment transport rates due
to biogenic processes (e.g. root growth, root throw) of the order
10–2 to 10–3 m3 m–1 a–1. For comparison, these values are close
to or somewhat higher than typical soil creep rates reported
in other studies [see Martin (2000) for compilation of published
creep rates], and lower than mass wasting rates due to shallow
landsliding in coastal British Columbia (Martin, 2000; Martin
et al., 2002).
To the best of our knowledge no published studies have
explicitly connected the timing and rates of sediment transport
due to root throw to tree population dynamics driven by wildfire
disturbance. Furthermore, root throw studies have not been
undertaken in the forests of the Canadian Rocky Mountains.
The primary objective of this study is to develop a model that
integrates tree population dynamics (i.e. tree recruitment,
growth, mortality, toppling) with sediment transport due to
root throw for this region. To achieve this goal, a field program
was undertaken to document root throw occurrences and
characteristics, and associated sediment transport, for both
pre-fire and post-fire scenarios in the Canadian Rockies. In
particular, a knowledge of root plate characteristics in these
forests based on field evidence is essential to the calibration
Figure 1.
of our integrated forest population dynamics/sediment transport
model. To study the temporal dynamics of root throw over
scales of 103 years, the model cycles through generations of
forests and associated root throw as conditioned by crown
fire disturbances which cause death of the tree population. Using
this integrated model, we strive for improved understanding
of how tree population dynamics driven by wildfire disturbance
influences the temporal dynamics of sediment transport due
to root throw in the Canadian Rockies.
Study Area
The field program was undertaken in Hawk Creek, Kootenay
National Park, south-eastern British Columbia (Figure 1). The
region is underlain by folded and faulted sedimentary rocks,
which were uplifted during the Cretaceous-Tertiary with elevations ranging from 800 m to 3400 m. The major valleys have
been glaciated, resulting in u-shaped main valley floors and
hanging valleys for many tributaries. Hawk Creek drainage
basin is approximately 24 km2 in area and is a fourth order
tributary of the major Vermillion River. Elevation is 1330 m
at the confluence with the Vermillion River, and 3086 m in
the upper slopes. Hillslope gradients are moderate (generally
<30º) in the lower third of the basin, where the study plots
are situated, and become steeper (up to 45°–50°) with increasing elevation. Slopes immediately adjacent to Hawk Creek
have steep gradients (25°–40°) for much of the main stem
length and often constrain development of a floodplain and
riparian zone. The lower portion of the basin contains morainal
material overlying bedrock and includes bedrock outcrops
and small areas of colluvium. The soils are unconsolidated
and unsorted, with a significant number of cobbles and boulders
within the matrix. The matrix is a silty-sand, with clay content
<2% or absent.
The temperatures in Kootenay National Park are influenced
by cold continental air masses from the north or maritime winds
from the west. The continental divide at the northeast boundary
allows for greater influence by maritime weather patterns,
resulting in a somewhat milder and moister climate than east
of the divide. Winter precipitation is largely in the form of
snow, averaging about 170 cm annually; summer rainfall is
Hawk Creek, Kootenay National Park, British Columbia, Canada. Extent of the 2003 burn is shown in the right-hand figure.
Copyright © 2009 John Wiley & Sons, Ltd.
Earth Surf. Process. Landforms 34, 1255–1269 (2009)
DOI: 10.1002/esp
SEDIMENT TRANSPORT DUE TO TREE ROOT THROW
frequently delivered by convectional thunderstorms, with an
average annual rainfall of about 340 mm. Precipitation increases
with elevation throughout the region (Environment Canada,
2005).
The vegetation for the Vermilion basin consists of subalpine
forest and alpine tundra. The lower subalpine forest is composed
primarily of lodgepole pine (Pinus contorta Loudon var. latifolia
Engelm.) and Engelmann spruce (Picea engelmannii Parry ex.
Engelm.), and the upper subalpine forest of Engelmann spruce
and subalpine fir (Abies lasiocarpa [Hook.] Nutt.).
The fire season is from May to September, with peak
lightning activity in July and August (Masters, 1990). Crown
fires are the dominant type of fire. Masters (1990) calculated
a return interval for the study area of 75 years for pre-1768
data, and 267 years for post-1768 data. The change in fire
interval during the 1700s is related to the Little Ice Age, as
has been found for other Rocky Mountain locations (Johnson
and Larsen, 1991). These fire return intervals are less than the
potential lifespan of the canopy trees (250–300 years, possibly
up to 375 years), making wildfire an important determinant of
tree population dynamics. The duff layer (F and H organic layers
of the soil) in unburned forests of this region is continuous
and up to 15 cm thick. As in other Canadian Rocky Mountain
locations, crown fires consume large amounts of the duff layer.
In late July 2003, lightning ignited two fires in the Vermillion Valley. These fires merged to burn approximately 17 000
hectares (see Figure 1). About 80% of the forested area in
Hawk Creek basin was burned by high intensity (crown)
wildfire, including much of the riparian zone. Only the very
upper part of the drainage basin did not burn. The previous
burn date at Hawk Creek was in 1835 (Masters, 1990).
Field Methods
To estimate rates of root throw and associated sediment
volumes, three plots, having areas of 2·5, 4·1, and 3·2 ha
(Plots 1, 2, and 3, respectively), were delineated in the lower
reaches of the Hawk Creek drainage basin. The plots were
located within 150 m of each other, had a southwest aspect
and hillslope gradients of 3°, 15° and 28°, but maintained
reasonable consistency in most other physical attributes. All
field measurements were made after the fire. The collection
of pre-fire data for root throw was based on information
derived from fallen trees and root plates, which were inferred
to have been in place at the time of fire based on certain
observed characteristics. Root throw frequency and characteristics were monitored in the first two years following the
1257
fire. Topples caused by a break or snap in the bole were not
considered as no sediment upheaval is associated with this
type of tree fall event; this study focuses exclusively on root
throw events (i.e. tree topples that upheave sediment through
an uprooting event). Root plates for fallen trees were surveyed
and flagged to allow identification of new topples that occurred
between surveys and to monitor root plate disintegration.
Root plates were included in the survey if sediment was still
attached to the root structure.
Four root plate age classes, modified from Brown et al.
(1998), were derived for the current study (see Table I). Based
on the characteristics listed in Table I, root plates were assigned
to one of three pre-fire age classes, or to the post-fire category.
In the first-year of field work (2004), post-fire topples were
identified by unburned roots at the base of the root plate. All
root plates, including those which existed before the fire and
those that came into existence due to post-fire toppling, were
flagged to allow identification of new topples in 2005. Postfire root throw frequency was monitored by counts of tree
falls in the three plots. Forest density defines the number of
trees available for toppling, and was estimated by tree counts
of standing trees with diameter at breast height (dbh) ≥10 cm
in six sample tracts (each ~150 m2) within each plot.
Surveys of existing root plates in each plot were performed
in the summers of 2004 and 2005. Detailed data were
collected for root plates within Age Classes 1–3. Root plates
in Age Class 4 were generally more deteriorated, reducing
measurement accuracy for bole, root plate, and disintegration
rates; therefore, they were not included in detailed field
measurements of sediment volumes or root plate disintegration, but were only included in bulk volume estimates.
The detailed analysis is as follows. Fall directions for root
plate bole and angle of fall relative to local contour were
recorded. Fall angles were recorded in Survey Plots 2 and 3,
where hillslope gradient was sufficient to allow identification
of direction of steepest ascent. The two areal dimensions for a
root plate are designated as width and height. Measurements
of root plate width (w) were made parallel to the ground
surface, and height (h) measurements were taken orthogonal
to width. Measurements were normally taken on the underside
of the root plate (i.e. the rounded side with newly exposed
soil), but in some cases, pit infilling or pit-plate configuration
forced measurements to be made on the ground surface side
of the root plate. It is important to distinguish between
the dimension that is parallel and that which is normal to the
ground surface for later sediment transport calculations. The
procedure to determine depth (d) of the root plate required
two measurements. The first measurement was made by placing
Table I. Root throw age classes based on bole description; modified from Brown et al. (1998) to simplify application in a
post-fire environment
Age
class
Short
description
1
New
2
Recent
3
Deteriorating
4
Old
Age estimates at time
of first measurement
Characteristics
Roots at bottom of root plate not burned
so tree fell after the fire
Fully barked (>80%); bole solid; no
checkerboard burn pattern on bole, indicating
tree was not dead long prior to burning
Bark 0–80%; some sapwood decay but bole
generally whole; checkerboard burn pattern
on some or all of bole
Sapwood flaking, easily removed; settling of
stem or flattening of circumference;
checkerboard burn pattern on all of bole
Post-fire
0–2 years before fire
2–30 years before fire
30–90 years before fire
Note: Bole description includes amount of bark on the bole, burn pattern, and amount of soft or rotting bole.
Copyright © 2009 John Wiley & Sons, Ltd.
Earth Surf. Process. Landforms 34, 1255–1269 (2009)
DOI: 10.1002/esp
1258
EARTH SURFACE PROCESSES AND LANDFORMS
Figure 2. (A) Area dimensions associated with a root plate. View is shown looking at underside of root plate. (B) Left-hand photograph shows
area dimensions for a root plate. Right-hand photograph shows depth of a root plate. Photograph by J. Gallaway, Kootenay National Park, British
Columbia, June 2004.
a rod at right angles to the bole where the ground surface
intersected the trunk, and measuring from this rod to the plane
estimated to pass through the outside edge of the roots. A second
depth measurement was made from the rod to the maximum
depth of the root plate. The difference between these two measurements defines depth for the half ellipsoid. This approach
excludes sediment that may exist in the root plate directly beneath
the bole, which may partially compensate for unrealistic smoothness of the half ellipsoid surface that may overstate the volume
in the lower portion of the root plate. For asymmetrical root
plates with considerable differences in depth across the plate,
multiple depth measurements were taken and averaged.
The half-ellipsoid model of Denny and Goodlett (1956) and
Norman et al. (1995) was used to calculate the root plate
volume (Figure 2):
VRP =
2 ⎛w h d ⎞
π
× ×
3 ⎜⎝ 2 2 2 ⎟⎠
(1)
where VRP is volume of root plate (in m3), w is width (in
meters), h is height (in meters), and d is depth (in meters).
Visual estimation was made of the percentage (0, 25%, 50%,
Copyright © 2009 John Wiley & Sons, Ltd.
75%, 100%) of root plate sediment still attached to the root
structure and the percentage (0, 25%, 50%, 75%, 100%) of
root plate sediment that would eventually fall outside of the pit.
Estimates were limited to these values to facilitate consistency
and repeatability, and were performed by multiple personnel
in the field. Older root plates were only included in total
volume estimates (i.e. volume estimates summed for all root
plates), and were classified based on width of root structure
(small: <75 cm, medium: 75–150 cm, large: >150 cm).
In addition to the collection of root plate data, a meteorological station recorded precipitation, wind speed, and wind
direction over the study period. Additional data recorded for
each tree topple included: (i) position on hillslope within the
plot (bottom third, middle third, upper third); (ii) bole dbh
(taken at 1·4 m above ground); and (iii) tree species.
Field Results and Analysis
Root throw frequency and direction
The detailed survey included 166 root plates, with a further 143
older root plates classified as small/medium/large. No notable
Earth Surf. Process. Landforms 34, 1255–1269 (2009)
DOI: 10.1002/esp
SEDIMENT TRANSPORT DUE TO TREE ROOT THROW
1259
Figure 3. (A) Rose diagram showing tree fall direction in degrees. (B) Histogram showing frequency of uphill and downhill topples. A value of 0º
represents an uphill fall direction (perpendicular to hillslope contour lines) while a value of 180º represents a downhill fall direction, with other
values falling in between.
trends are found in the frequency rate per hectare of root throw
amongst the three gradient classes (Table II). When tree density
is taken into consideration, there are still no obvious patterns
in the data amongst the gradient classes. The data are then
split up and analyzed according to pre-fire and post-fire root
throw events (Table III). Data for pre-fire events show no notable
increase in rates with average gradient, with values ranging
from 0·29 to 0·37 events annually per hectare. The post-fire
data indicate increasing root throw frequency with gradient.
The disparity between pre-fire and post-fire rates increases
with increasing gradient, showing notable differences for the
steepest gradient class of 28º.
Root throw data show a mean fall direction of 29º (Figure
3A). The wind direction recorded during the study period was
most frequently between 20° and 30°, a range that includes
the mean direction of tree fall. This wind direction across
local topography is predominantly uphill, and the effect is
apparent in the distribution of tree fall angles relative to
contour lines (Figure 3B). Approximately 65% of the tree falls
were uphill relative to local contour lines.
Root plates
When all root plates are considered, the relation between
dbh and root plate volume is (Figure 4):
Table II.
Topple
survey plot
1
2
3
Figure 4. Relation between diameter at breast height (dbh) and root
plate volume. Data are shown for Age Classes 1–3 (32-year period).
VRP = (−5·74 × 10−4 × dbh) + (5·82 × 10−3 × dbh2) R2 = 0·45 (2)
where VRP is root plate volume (in m3) and dbh is diameter at
breast height (in cm). The mean value of root plate volume
increases from a value of 0·2 m3 to 0·71 m3 (p < 0·01) as
gradient becomes steeper (Table IV).
Width and height of the root plate were defined earlier as
width being parallel to the ground surface and height being
Root plate counts for the three survey plots
Average
gradient (deg)
Plot
area (ha)
Tree density
(# ha–1)
Root plate
count
Root plates per
hectare (# ha–1)
Frequency
(% ha–1)
3
15
28
2·5
4·1
3·2
1665
1882
2200
85
112
112
34·0
27·3
35·0
2·0
1·5
1·6
Note: Root plate counts relative to the tree density for each plot are shown in the last column.
Table III.
Topple
survey plot
1
2
3
Annual topple rates by gradient
Average
gradient (deg)
Plot
area (ha)
Pre-fire topple
rate (# ha–1 yr–1)
Pre-fire topple
rate (% yr–1)
Post-fire topple
rate (# ha–1 yr–1)
Post-fire topple
rate (% yr–1)
3
15
28
2·5
4·1
3·2
0·37
0·29
0·35
0·022
0·015
0·016
0·40
0·60
1·7
0·024
0·032
0·078
Note: Time span for pre-fire root plates is 90 years; time span for post-fire root plates is two years. Rates are presented per hectare, and as percent
of standing tree density.
Copyright © 2009 John Wiley & Sons, Ltd.
Earth Surf. Process. Landforms 34, 1255–1269 (2009)
DOI: 10.1002/esp
1260
Table IV.
EARTH SURFACE PROCESSES AND LANDFORMS
Mean values of root plate dimensions for the three survey plots
Gradient class
1 (n = 45)
2 (n = 61)
3 (n = 60)
Root plate
volume (m3)
Width (cm)
Length (cm)
Depth (cm)
Ratio of length to width
0·22 (0·11)
SE = 0·044
0·35 (0·18)
SE = 0·058
0·71 (0·62)
SE = 0·072
91·2 (80)
SE = 6·0
115·5 (106)
SE = 6·9
150·4 (155)
SE = 5·6
117·1 (110)
SE = 6·8
160·0 (145)
SE = 10·7
219·5 (223)
SE = 9·3
26·9 (25)
SE = 1·9
29·7 (28)
SE = 1·9
39·3 (40)
SE = 2·1
1·33 (1·24)
SE = 0·043
1·42 (1·30)
SE = 0·0505
1·49 (1·43)
SE = 0·0488
Note: Median values are given in brackets, and the standard error (SE) is indicated.
normal to the ground surface. However, in that definition
either width or height could be the larger of the two dimensions,
likely meaning that empirical analysis of relations for changes
in a particular dimension with increasing slope gradient
would be obscured. Therefore, for this particular analysis and
table (Table IV, columns three to five), the shorter dimension
of the two is referred to as ‘width’ and the longer dimension
as ‘length’; the depth dimension remains the same as defined
earlier. Increases in magnitude with slope gradient for each
individual dimension (width, length and depth) were found to
be significant at p < 0·01. On slopes having a gradient >0°,
mechanical stress is expected to be unevenly distributed
around the tree diameter and the root system. Root growth
may respond by increasing the density of the root system or
by arranging roots in an asymmetric manner in the direction
of maximum stress, both of which serve to provide increased
anchorage for the tree and greater stability (Soethe et al.,
2006; Coutts et al., 1999). Unfortunately, our data do not
allow us to determine the direction in which our root plates
were oriented when originally in the ground. Nonetheless,
it is still of interest to assess changes in the ratio of length
to width for root plates measured in the field. A significant
increase is found for the ratio of length to width with increasing
slope gradient (Table IV, column six), lending some support to
the idea that asymmetry of the root plate will become more
pronounced as gradient increases.
Disintegration rates of root plates were determined by
estimating percentages of volume removed from the root
plates in 2004 and 2005 (Table V) for the three age categories
(1–2 years post-fire, two years pre-fire and 30 years pre-fire;
refer back to Table I). As of 2004, the youngest (most recent)
root plates had similar percentages of volume removed from
the root plate as the older category. Furthermore, in 2005 it
was these youngest root plates that had lost the greatest
amount of volume in the intervening period. For both pre-fire
and post-fire scenarios, the steepest-gradient plots showed the
greatest rates of sediment disintegration. A greater proportion
of sediment associated with root plates falls outside the
Table VI.
Table V.
Root plate disintegration averages by age class
Age
class
Age maximum
1
2
3
1–2 years post-fire
2 years pre-fire
30 years pre-fire
Average volume
off 2004 (%)
Average volume
off 2005 (%)
48·3
33·3
46·7
70·8
47·7
54·0
Note: Data are for detailed root plates only.
originating pit on steeper gradient slopes relative to gentler
slopes, possibly due to a greater occurrence of rotational falls
on steeper slopes (Table VI). For both pre-fire and post-fire
root plates the average percent falling outside the pit increases
from negligible values for relatively flat land surfaces, through
to 20–30% for the mid-slope category and up to about 50%
for the steepest gradients.
Areal sediment disturbance and soil turnover
The total area of ground surface disturbed by tree upheaval
(i.e. total area disturbed by all events occurring during a
specified time period) was based indirectly on the root plate
measurements and covers all four age categories, or a period
of approximately 92 years (Table I). The area subject to tree
root upheaval increases with slope gradient, with annual
percentages of disturbed land surface ranging from 0·003%
to 0·006% (Table VII).
Sediment disturbance and transport
Amounts of sediment disturbance include the volume of all
sediment that is uprooted, whether it is eventually returned
to the pit or not. Sediment returned to the pit during root
plate disintegration contributes to weathering and breakdown
Root plate disintegration data for the three survey plots
Topple
survey plot
Root
plate counts
Average percent
falling outside pit
Average percent
off root plate 2004
Average percent
off root plate 2005
Pre-fire root plates
1
2
3
43
56
49
5·0
30
52
46
46
33
54
53
53
Post-fire root plates
1
2
3
2
5
11
0·0
20
50
38
68
55
38
80
73
Note: Pre-fire and post-fire root plates are analyzed separately.
Copyright © 2009 John Wiley & Sons, Ltd.
Earth Surf. Process. Landforms 34, 1255–1269 (2009)
DOI: 10.1002/esp
SEDIMENT TRANSPORT DUE TO TREE ROOT THROW
Table VII.
Survey
plot
1
2
3
Total pit area disturbed and annual rate of pit formation
Average
gradient (deg)
Total pit area
(m2 ha–1)
Annual total
pit area (%)
3
15
28
28·7
36·0
59·6
0·003
0·004
0·006
of the soil layer, while sediment that disintegrates and lands
outside the pit contributes to sediment transport. The transport
distance of sediment after uprooting becomes important in
our calculations of sediment transport (see Equation 3) and is
not a consideration when estimating sediment disturbance
volumes. Sediment disturbance rates over a 32-year period
(time period for root plates of Age Classes 1–3) are evaluated
to assess if there is a gradient dependency on the total amounts
of uprooted sediment over this period. Results do not show a
notable difference in total volume uprooted per square meter
of hillslope for the two lowest-gradient study plots, with values
of 1·25 × 10–5 and 1·63 × 10–5 m3 m–2 a–1 respectively (Table VIII).
However, the steepest plot, Plot 3, does show a notably higher
value of 4·15 × 10–5 m3 m–2 a–1 compared to the other plots, in
large part due to the higher root plate volumes associated
with toppled trees at steeper gradients as discussed earlier.
We now consider sediment transport rates due to root throw.
It is assumed that sediment falling into the pit undergoes no
net transport, and thus only sediment that is involved in
forming mounds is considered in this analysis. A diffusive
approach to sediment transport may be appropriate if root
throw is considered to be a relatively slow, quasi-continuous
process with a dependency on hillslope gradient (Norman
et al., 1995), and with the potential to operate across the entire
forested portion of the landscape. Adopting such an approach
allows for comparison with transport rates for other processes
involved in medium-term drainage basin evolution (note that
the temporal dynamics of root throw transport will be explored
in more detail in the modeling section of this paper).
The equation used to calculate the sediment transport rate
for a plot of a given gradient is (adapted from Martin and
Church, 1997):
Σ(Vmd )
qs =
(3)
Apt
where qs is sediment transport rate (in m3 m–1 a–1), Vm is volume
of sediment landing outside the pits and which form mounds
(in m3), d is distance (in meters) that sediment is ‘transported’
along the ground (the net ground-parallel distance travel after
the sediment has been upheaved and falls to the ground), Ap
is area of survey plot (in m2) and t is the estimated number of
years that the root plates have existed.
Volume is calculated as the product of original root plate
volume, the percentage that had fallen off the root plate, and
the percentage falling outside the pit (this value must be
calculated for uphill topples and is assumed to be 100% for
Table VIII.
Survey
plot
1
2
3
1261
downhill topples; see explanation later). Sediment transport
was calculated for each survey plot, providing a transport rate
for three different gradients. This analysis is completed for
root plates of Age Classes 1–3, and thus t is 32 years.
Sediment transport distance was calculated using geometric
models based on field measurements, similar to the approach
of Gabet et al. (2003), but with two major differences. First, the
current study does not consider the entire root plate volume
in the transport calculations, but excludes the volume of root
plate sediment that will fall back into the pit. Second, this study
examines uphill and downhill sediment transport separately,
rather than calculating a single net transport value. The model
involves two components contributing to transport distance:
(i) slope-parallel transport distance during upheaval; (ii) slopeparallel transport distance during root plate disintegration.
Two assumptions are required for this model: (i) root plates
are assumed to be circular, with the dimension of the root
plate perpendicular to the ground representing diameter; and
(ii) the root plate comes to rest with the center of mass at the
edge of the pit (Figure 5A).
The ground-parallel transport distance for downhill topples
is now considered. The net downslope distance for upheaval
associated with downhill topples is the distance between two
‘contour lines’ situated across the slope, one which crosses
through the center of the pit and the other crossing through
the center of the root plate mass. This plan view distance is
converted into true distance by making an adjustment for
slope gradient. During the disintegration phase, all sediment
for downhill topples is assumed to fall outside the pit, with
simple geometric calculations used to calculate the net
downslope transport distance for this stage of the root throw
process. The net ground-parallel transport distance used in
calculations is shown in Figure 5(B).
Similar geometrical considerations are applied to the model
for uphill topples, but it cannot be assumed that all sediment
falls outside the pit. In addition, there is now both an uphill
and downhill component to the transport process, as the
upheaval moves sediment uphill, while disintegration moves
sediment downhill; both must be accounted for to determine
if there is net uphill or downhill transport. Once again, geometric
models are applied to determine the amount of sediment
associated with the root plate that will fall outside the pit. A
line in the direction of steepest gradient is imposed tangentially
to the rim of the pit. The intersection of this line with the root
plate half-ellipsoid (sitting at some angle above the pit)
determines the proportion of sediment that will ultimately
fall outside the pit; only this sediment is used in transport
calculations (sediment landing in the pit is not considered to
be part of the transport process).
In Survey Plot 1, where gradient was too low to accurately
determine contour line direction, only the disintegration
transport distance was used, resulting in understated transport
distances. The impact of this is expected to be small, as very
little sediment is transported by root throw (i.e. very little
sediment falls outside the originating pits) on low gradient
slopes.
Sediment disturbance values
Average
gradient (deg)
Area of
plot (ha)
Volume uprooted
over 32 years (m3)
Annual volume
disturbed (m3 m–2 a–1)
Average depth
(mm a–1)
3
15
28
2·5
4·1
3·2
10·01
21·45
42·49
1·25 × 10–5
1·63 × 10–5
4·15 × 10–5
0·013
0·016
0·041
Note: Sediment disturbance includes estimates of all sediment that was originally uprooted, although some of it may have since disintegrated and
formed part of a mound.
Copyright © 2009 John Wiley & Sons, Ltd.
Earth Surf. Process. Landforms 34, 1255–1269 (2009)
DOI: 10.1002/esp
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EARTH SURFACE PROCESSES AND LANDFORMS
Figure 5. (A) Circular pit and root plate, with center of mass of root plate situated at edge of pit. (B) In the model, the first stage of sediment
transport involves the movement of sediment from what becomes the pit to the new location of the root plate after tree topple. The second stage
of transport involves the vertical disintegration of sediment to the ground, with the distance for this stage defined as the distance between the root
plate centroid to the location where vertically falling sediment reaches the land surface (shown by the bold arrow). The double-headed arrow
shows the ground-parallel net transport distance associated with root throw transport.
Uphill and downhill sediment transport rates during the
pre-fire period are shown for root plates categorized in Age
Classes 2 and 3 (Figure 6A); sediment is considered as having
been transported once it falls to the ground from the root
plate. Sediment transport rates are also calculated for the two
post-fire years (Figure 6B); these data include trees that
toppled during both the post-fire and pre-fire periods (Age
Classes 1–3), but for which sediment actually fell to the
ground in the post-fire period. We first discuss the downslope
transport rates. Negligible sediment transport occurred on the
lowest gradient plot for the pre-fire period, as sediment was
usually returned directly to the pit. Moreover, transport rates
remained negligible for the mid-gradient plot, and it is only
for our upper-gradient plot, having a slope of 28°, that
notable increases in sediment transport rates were observed.
There appears to be a non-linear form to the plot; however,
with so few data points, broad conclusions should not be
made. The post-fire results show the same pattern in the data,
with the major difference being an approximately oneorder-of-magnitude increase in sediment transport for the
post-fire scenario versus the pre-fire scenario. Upslope transport
rates for both periods show negligible values on the low-gradient
field plot, with somewhat higher values for the steepergradient field plots.
Figure 6. Downslope and upslope sediment transport plots. (A) Transport results for the pre-fire period. (B) Transport results for the two post-fire
years. Note scale differences on the two graphs.
Copyright © 2009 John Wiley & Sons, Ltd.
Earth Surf. Process. Landforms 34, 1255–1269 (2009)
DOI: 10.1002/esp
SEDIMENT TRANSPORT DUE TO TREE ROOT THROW
Model Outline
1263
for fire intervals expresses the probability of having fires with
inter-fire intervals of length t (Johnson and Van Wagner, 1985):
Model overview
While the field results provide an indication of sediment
disturbance and transport rates due to root throw, the temporal
dynamics of root throw-driven transport is directly related to
the population dynamics of forests. To obtain greater insights
into these temporal dynamics, we develop a combined tree
population dynamics/sediment transport model that is calibrated
for the central Canadian Rockies. In simple terms, this model
germinates and grows trees, kills trees, and has these trees fall
to the ground. A portion of these fallen trees generates root
throw events. Fire events occur as a disturbance that kills all
trees and results in a new forest. Thus, the model cycles
through forest generations with life spans determined by the
variability of fire event recurrence. Calculation of the sediment
transport by root throw is organized as follows: first, between
wildfires the population dynamics of the trees is modeled
using the algorithms for recruitment, mortality, and growth in
diameter. Next the timing of mortality of all the trees in the
population from crown fire is determined based on information about fire return intervals. The tree falling (topple) rate
and chance of uprooting versus breakage of the dead trees,
either from fire or from inter-fire mortality, are determined.
Finally, the amounts of sediment associated with root plates
of toppled trees are determined and the transport rates are
calculated. A list of model parameters is given in Table IX.
Fire frequency
The lifespan of forest trees in the study region is largely determined by fire return intervals and type of fire. Mean fire
return interval in a region is estimated as the expected number
of years between fires. The Weibull probability density function
Table IX.
Input parameters for model simulations
Number of years of simulation
Plot area
Hillslope gradient
Average fire return interval
(scale parameter) (Equation 4)
Shape parameter for fire (Equation 4)
Duration of the fire cohorta
Between-fire falling rates (F in Equation 5)b
Fire cohort
Understory cohort
Post-fire falling rates for all fire killed trees
Fraction uprooted versus broken boles
Parameters to assign dbh distributions
for trees of certain age
Scale parameter
Shape parameter
dbh threshold for uprooting mass
1000 years
100 m2
20°
110 years
1·0
10 years
0·058c
0·058
0·084d
0·8
Mean dbh values
(Figure 8)
1·8
13 cm
a
The user input for span of fire cohort will determine when the canopy
is formed and new recruits are going into the understory cohort rather
than the fire cohort.
b
Fire ‘cohort’ is defined as those trees that germinate after fire and
form the canopy. Understory ‘cohort’ is defined as those trees that
germinate after the canopy is formed.
c
The value of 0·058 is based on data for Johnson and Greene (1991)
and Johnson (unpublished data) for the Kananaskis Valley.
d
The value of 0·084 is based on Lyon (1977). Fire killed trees may
generate root throw until next fire.
Copyright © 2009 John Wiley & Sons, Ltd.
γ ⎛t ⎞
f (t ) = ⎜ ⎟
α ⎝α ⎠
γ −1
⎛t ⎞
−⎜ ⎟
e ⎝α ⎠
γ
(4)
where t is time interval between two fires (in years), α is a
scale parameter (expected fire return interval in years) and γ is
a shape parameter (dimensionless). For this modeling exercise,
the shape parameter γ is set to one, resulting in a negative
exponential distribution, which studies in the area have shown
to provide a good fit to the data (Masters, 1990; Johnson and
Larsen, 1991; Reed et al., 1998). A value of 110 years is
chosen for the variable α. Random numbers are selected from
the Weibull distribution, which represent inter-fire time intervals
for the model run.
Tree population dynamics
In the subalpine forests of lodgepole pine (Pinus contorta
Loudon var. latifolia Engelm.) and Engelmann Spruce (Picea
engelmannii Parry ex. Engelm.) found in the region under
study (Johnson and Fryer, 1989; Johnson et al., 1994; Johnson
et al., 2003), two groups of trees contribute to tree toppling
and sediment transport. The first group consists of trees either
killed by the last fire or dead standing before the last fire, and
the other group consists of trees that are recruited after the
last fire. The first group represents a cross-section of the live
trees and standing dead trees at the time of the last fire,
showing a range of diameters. The second group considers
the recruitment and mortality of trees in two kinds of cohort.
A cohort is a group of trees recruited at the same time and
following a similar mortality schedule throughout their lives.
The fire cohort is defined as those trees recruited in the fiveto 10-year period after the fire in which the canopy was killed
(Johnson et al., 2003) and the forest floor was removed by
smoldering combustion (Charron and Greene, 2002; Miyanishi
and Johnson, 2002). The understory cohorts are those that begin
to grow under the fire cohort’s canopy and on a forest floor
occupied by groundcover.
Both the recruitment (Figure 7A) and mortality rates (Figure 7B)
used in our simulations are from Johnson and Fryer (1989)
and Johnson et al. (2003). The duration of the fire cohort is
the period of time for canopy trees to become established,
and in our model its duration is assigned a value of 10 years.
Afterwards, all new recruits are considered to be part of the
understory cohort and, as seen in our recruitment curve, it
generally has a lower recruitment rate. Mortality rates are
specified separately for each cohort (Figure 7B). The understory
cohort has a higher mortality rate than the fire cohort. Mortality
rates are related to age of trees, and within one time interval
different mortality rates are applied to trees of different ages
and different cohorts.
Root throw events
Two groups of trees contribute to root throw and transport:
(i) trees either killed by fire or dead standing before fire; and
(ii) trees recruited after fire with different mortality rates for
the two cohorts. Field data (Johnson, 1986, unpublished data)
show that topples occurring in the first several years after fire
of fire-killed trees or dead standing trees at the time of the fire
result from having their root support weakened by removal of
Earth Surf. Process. Landforms 34, 1255–1269 (2009)
DOI: 10.1002/esp
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EARTH SURFACE PROCESSES AND LANDFORMS
Figure 7. (A) Recruitment curve used in model runs. Trees recruited within the first 10 years after a fire are part of fire cohort and any trees
recruited thereafter are part of understory cohort. (B) Mortality schedule for understory and fire cohorts. Based on Johnson and Fryer (1989) and
Johnson et al. (2003).
organic matter around their bases (Miyanishi and Johnson,
2002) and burning of roots during the fire. Subsequent postfire tree topple is due to loss of root strength (Martin and
Johnson, 2004–2008, unpublished data). Dead trees will fall
by uprooting or breakage.
Toppling of standing dead trees occurs according to an
exponential model (e.g. Lyon, 1977, who used this model for
post-fire topples), whereby a constant fraction of the remaining
standing dead boles falls in any given year. The fraction of
dead trees that topple in a given time interval is obtained
from Equation 5:
FT = 1 − exp(−F ∗ dt)
(5)
where F is a parameterization based on field studies of fraction
dead standing trees that topple and dt is the time interval (in
years) used in our model runs. In our model, parameterization
was based on falling rates taken from studies by Johnson and
Greene (1991), Johnson (1986, unpublished data) and Lyon
(1977) (see Table IX for further details). The calculated fraction
is applied to the remaining standing dead boles for each time
interval to determine the number of trees that topple in that
period.
Empirical surveys show that in our study area about 80% of
trees uproot as opposed to breaking, and thus constitute root
throw events (Johnson, unpublished data); this value is incorporated into the model to obtain the actual number of root
throw events after the number of tree topples has been
calculated. Tree fall directions are random in the model runs
reported herein.
Root plate volume, necessary to calculate sediment disturbance and transport, is dependent on tree size, which in turn
is related to tree age when the root throw event occurred
(recall that a tree topple is only considered a root throw event
when sediment upheaval is involved). The distribution range
for dbh is influenced by tree density in the forest (Harper,
1977); higher density forests have smaller dbh values. Tree
density is dependent on factors such as soil conditions, tree
species, topography, climate, and probably other factors. The
dbh distribution in the present model is based on a density of
2000 trees per hectare as found in our field plots [refer to our
field data and also Smithers (1961)], and requires adjustment
if applying the model to different locations. For an event to be
included in our sediment disturbance and transport calculations, the relevant tree must have a diameter >13 cm. This
value provides an important condition that must be met in
the model for notable sediment upheaval to occur when a
tree topples (Johnson, unpublished data).
Copyright © 2009 John Wiley & Sons, Ltd.
To assign dbh in the model, mean tree diameter is first determined for a tree having a certain age in the model (Figure 8).
To ensure a more realistic distribution of diameters for trees of
a certain age, a two-parameter Weibull distribution is used to
assign actual diameters in the model, with the scale parameter
defined as the mean diameter for trees of that age and the
shape parameter being assigned a value of 1·8 (tree diameter
data are based on unpublished data of Johnson for the
Kananaskis Valley, Alberta).
Root plate volumes
Two pieces of data provide the basis to assign root plate
characteristics for each root throw event in the model: the
age of the tree when it died and its dbh (see earlier), and
the year of fall. Root plate volume is based on dbh using the
regression equation obtained from field data for Hawk Creek
(Gallaway, 2006) (see Equation 2).
Sediment transport
In the model, disintegration distributes the transfer of sediment
from the root plate to the ground over 100 years and is
calculated based on an empirical best fit to our field data
(Gallaway, 2006):
Figure 8. Mean diameters (dbh) for trees of a particular age. To
ensure a realistic distribution of diameters for each age of tree in the
model, a Weibull distribution is used in conjunction with the mean
diameter to assign a range of diameters. Based on unpublished data
by Johnson for the Kananaskis Valley, Alberta.
Earth Surf. Process. Landforms 34, 1255–1269 (2009)
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SEDIMENT TRANSPORT DUE TO TREE ROOT THROW
PV = −0·1029 + 46·19e−0·719t + 56·35e−0·02102t
(6)
where PV is the percent volume remaining and t is time in
years. The exponential form of the equation suggests that the
disintegration rate may stay constant as the volume decreases
over time. The volume falling off a root plate in a time interval
is the difference in the percent remaining as calculated for
two points in time. The volume of interest for transport is the
volume of sediment that falls outside the originating pit; this
is dependent on the fall angle of the tree (i.e. if a tree falls
directly upslope, it is assumed all sediment will return to the
pit). Conversely, if a tree falls directly downslope, all sediment
will fall outside the pit. The proportion of sediment falling
outside the pit is calculated as:
PS = FA/180
wRP = 0·65 + 4·65dbh R2 = 0·62
(8A)
hRP = 29·24 + 0·54wRP R2 = 0·62
(8B)
where dbh is diameter at breast height (in cm), wRP is root
plate width (in cm), and hRP is root plate height (in cm).
Transport for a particular time interval is calculated for the
sum of sediment volume falling off all existing root plates in
that time interval in conjunction with its transport distance.
n
∑ (VRP *distRP )
i =1
At
(9)
where qs is sediment transport rate (in m3 m–1 a–1), VRP is
volume of sediment disturbed for a root plate during a
particular event (in m3), distRP is travel distance associated
with that event (in meters), and At is the area of the plot being
modeled (in m2).
Model Results
Results for an example millennial-scale model run are presented
to highlight trends in temporal variations in tree populations
and associated tree mortality, tree toppling, root throw, sediment
upheaval, and disintegration. Values of the parameters used
in the model run are given in Table IX.
Fire-killed or standing dead trees at time of fire
Fires are the critical disturbance process in our modeled forest,
as fire events control the temporal dynamics of tree mortality,
tree toppling, and root-throw sediment processes. Random
Copyright © 2009 John Wiley & Sons, Ltd.
Times of fire in example model runa
Time of fires (years since start of model run)
50
130
235
280
580
790
805
870
920
a
Model assumes a new forest begins to grow at time 0. No fire-killed
trees exist at this time as no previous forest exists.
(7)
where PS is the proportion of sediment falling outside the pit
and FA is the falling angle of the tree. Transport distance is
the net slope-parallel distance after upheaval and disintegration.
Direction of this net distance may be: (i) downslope; (ii) upslope,
if upslope upheaval distance exceeds downslope disintegration
distance; or (iii) back into the pit. Ground-parallel transport
distance for sediment falling off a root plate is based on a
series of geometric calculations similar to those described for
the field data, and which requires a knowledge of root plate
height (the pit is assumed to be circular with a diameter
having this same value). The height dimension needed to
calculate transport distance is obtained from the following
equations:
qs =
Table X.
1265
fire distributions (see Equation 4) for this model run show a
range of fire return intervals (time period between successive
fires), with intervals as short as 15 years and up to 300 years
(Table X).
Figure 9 illustrates examples of tree age distributions at the
time of fire for several fires with different lengths of time since
the previous fire. The numbers and sizes of trees at the time
of fire are a significant determinant of post-fire sediment
disturbance and transport. The duration of time since the
previous fire is the time period available for tree growth,
and the trees are then subjected to fire-driven mortality and
subsequent toppling. The number of new root throw events
represents trees that not only have died and toppled, but that
also have upheaved sediment (Figure 10). Important to note is
that trees do not immediately topple after they die, but rather
they follow an exponential toppling rate based on time since
death. Therefore, we expect a time lag between tree mortality,
toppling, and associated sediment disturbance and sediment
transport processes.
When the time interval since the previous fire is short, very
large numbers of trees will be standing at the time of fire, but
their dbh values will be small due to their young age and
the dbh of most trees is below the threshold of 13 cm for
sediment upheaval (refer to Figure 9A). Hence, the mortality
and eventual toppling of these fire-killed (or dead standing)
trees involve very small amounts (or no) sediment and they
are not included as root throw events. For example, the fire
at Year 805 represents a disturbance having a short value of timesince-previous-fire, and no root throw events for fire-killed
trees occur immediately following the fire (refer to Figure 10).
When the fire return interval exceeds the time needed for
trees to reach the critical dbh, the fire-killed trees contribute
to root throw events and to sediment disturbance (refer to
Figure 9E). An example of this is the fire occurring at Year
580, which shows a notable post-fire increase in root throw
events (refer to Figure 10). In such cases, both the number of
trees above the critical dbh and their sizes are important in
determining the total amounts of sediment upheaved.
Once the time interval since the previous fire exceeds
approximately 125 years, the time when tree mortality of fire
cohorts decreases to very low levels in our model simulation,
the actual number of trees in the plot remains approximately
constant (refer to Figures 9D and 9E). However, these trees
continue to grow in size, and the amount of sediment
involved in the toppling of the trees will likewise increase.
Sediment disturbance is directly tied to the number of new
root throw events and, to a large extent, should follow the
patterns of new root throw events for fire-killed or dead
standing trees at the time of fire (Figure 11), as well as
patterns of mortality and toppling as the forest establishes
Earth Surf. Process. Landforms 34, 1255–1269 (2009)
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EARTH SURFACE PROCESSES AND LANDFORMS
Figure 9. Tree age distributions at time of fire. The distributions are shown for fires at the following times. (A) 805 years (previous fire interval of
15 years); (B) 870 years (previous fire interval 65 years); (C) 235 years (previous fire interval of 105 years); (D) 790 years (previous fire interval of
210 years); (e) 580 years (previous fire interval of 300 years).
Figure 10.
New root throw events for model run. Counts are for five-year bins. Years of fire events are shown by triangular symbols.
Figure 11.
year bins.
Volume of sediment disturbance for model run. Years of fire events are shown by triangular symbols. Results are grouped into five-
itself in between fire events. Sediment is only considered as
having been ‘transported’ once it disintegrates, with disintegration not being instantaneous but rather a process that is
extended in time. Therefore, while we expect a broadly similar
Copyright © 2009 John Wiley & Sons, Ltd.
pattern for sediment transport as for sediment disturbance,
the disintegration process results in a slight dampening and
extending out of the temporal patterns that are found for
sediment disturbance (Figure 12). Annual rates of sediment
Earth Surf. Process. Landforms 34, 1255–1269 (2009)
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SEDIMENT TRANSPORT DUE TO TREE ROOT THROW
Figure 12.
year bins.
1267
Annual sediment transport rates for model run. Years of fire events are shown by triangular symbols. Results are grouped into five-
Table XI. Frequency table of sediment transport rates for model run
of duration 103 years
Annual sediment transport
rate (m3 m–1 a–1)
0–0·0005
0·0005–0·0010
0·0010–0·0015
0·0015–0·0020
0·0020–0·0025
0·0025–0·0030
0·0030–0·0035
0·0035–0·0040
0·0040–0·0045
0·0045–0·0050
0·0050–0·0055
>0·0055
Frequency
128
15
18
9
9
5
7
2
2
2
1
3
transport show marked variability that changes in response
to the tree population dynamics. The frequency distribution
of annual transport rates for our millennial-scale model run
is shown in Table XI. The mean value for all annual transport
rates is 0·0012 m3 m–1 a–1, with a standard deviation of
0·0047 m3 m–1 a–1. Values range from 0 m3 m–1 a–1 to a
maximum of 0·059 m3 m–1 a–1.
Understory and fire cohorts
After fire occurrence, fire-killed or dead standing trees begin
to topple. During this same period, new recruits begin to
populate the stand in large numbers, and mortality of these
trees also occurs in relatively large numbers. After about 50
to 60 years, a second pulse of root throw events begins to
occur for several reasons. Firstly, the mortality rate of trees in
the fire cohort increases significantly after about 60 years (see
Figure 7B). Furthermore, trees that have reached this age may
begin to have dbh values that are >13 cm (critical dbh for
sediment upheaval). Once dbh values reach 13 cm (which
occurs on average after 80 years, although the random component for tree dbh in the model allows some trees to reach
this size several decades sooner), they begin to upheave notable
amounts of sediment as they topple. An example can be
observed in Figures 10 and 11, with notable sediment
disturbance beginning at about 50 years and increasing for
the next several decades after the fire event at Year 805. Prior
to these conditions being met, trees in the two cohorts that
are subject to mortality have little impact on sediment disturbance or transport as they are too small to upheave notable
amounts of sediment. Therefore, in addition to a pulse of
sediment upheaval due to fire-killed or dead standing trees in
the immediate post-fire years, at about 50 years onwards one
Copyright © 2009 John Wiley & Sons, Ltd.
may begin to observe a second pulse of sediment upheaval
and associated transport (Figures 11 and 12), with this pulse
expected to last several decades, after which time tree
mortality decreases to background levels.
Discussion and Conclusions
Sediment transport rates by root throw have been estimated in
some early studies on this topic (Denny and Goodlett, 1956;
Reid, 1981; Mills, 1984). However, the connection between
root throw and forest population dynamics, with the latter
driving the former by the process of tree toppling and root
upheaval, was not fully explored in these studies. Gabet et al.
(2003) expanded on earlier work by developing a model to
predict medium-term rates of root throw transport as a function
of hillslope gradient. Their model accounts for tree topple rates,
with the assumption that uprooting rates in the forest are
temporally constant at a rate of four trees per hectare per
annum. Their model utilizes results of previous field studies
(e.g. Norman et al., 1995) to obtain values for root plate
dimensions, and uses this information in conjunction with a
series of geometrical relationships pertaining to tree fall to
estimate transport rates. Some additional assumptions were
made in their model, such as values of dbh do not change
over time as trees age, and therefore root plate volumes also
do not change with time. The results of Gabet et al. (2003)
provide first-order approximations of medium-term sediment
transport rates due to root throw and mark notable progress in
our understanding of this process.
Without an explicit connection to forest disturbance and
tree population dynamics, the temporal dynamics of the root
throw process cannot be fully realized. To our knowledge our
study represents the first attempt to explicitly combine a
model of root throw with a detailed rendering of tree population dynamics to explain how ecological forcing drives the
temporal aspect of sediment transport by root throw. Because
there were no existing regional field data/observations of root
throw on which to base model calibrations and help make
inferences for our work in the Canadian Rockies, it was
necessary to include a field component in our study.
Tree topple rates may increase in the immediate post-fire
years as root support is weakened by the removal of organic
matter around the tree and due to the burning of roots during
the fire. The marked post-fire increase in toppling for the
steep gradient class relative to the less steep gradient classes
found in our field data may occur because the structural stability
of trees is lower on steeper slopes (Quine and Gardiner, 2007).
Our field data identified the relation between dbh, which
increases as trees age, and root plate volumes in our region;
such data is critical to integrate sediment transport with tree
stand ages in our model. The field data demonstrate a significant dependency between gradient and root plate volume and
Earth Surf. Process. Landforms 34, 1255–1269 (2009)
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EARTH SURFACE PROCESSES AND LANDFORMS
all individual dimensions (width, length, depth). An increase
in the ratio of length to width was found at steeper gradients
relative to less steep slopes (significance level of p < 0·05). It
may be the case that, as hillslopes become steeper, tree roots
spread perpendicular to contour lines to impart greater
stability to the tree (Coutts et al., 1999). Further investigation
of this idea is suggested, as such information would allow for
improved understanding of the resistance offered by trees to
mortality-induced toppling, wind events or landsliding.
The pit-mound features formed by tree uprooting and root
plate disintegration result in a unique microtopography that
contributes small-scale roughness elements to the landscape.
These roughness elements may affect depression storage and
ponding of water during large storm events, which may affect
the timing of hydrological responses in the watershed (Martin
et al., 2008). The mound provides an exposed, unconsolidated
supply of sediment for rainsplash or other water-driven processes
prior to colonization of significant vegetation cover on the
feature, while soil creep may act to diffuse the mound both
prior to and after vegetation recolonization. It is recommended
that future studies explore sediment transport processes operating
on pit-mound features to better understand their longevity
and contribution to microtopography in forests. Osterkamp
et al. (2006) found an average disturbance rate of the soil by
root throw of 0·0095% of the surface per year, which is
within an order of magnitude of our values. Our data suggest
that a complete turnover of the soil surface has not likely
occurred in our study area since post-Pleistocene reforestation
which began <10 000 years ago. Other studies, summarized
in Schaetzl et al. (1989), report disturbance cycles from 220
to 3174 years in eastern North American forests.
To allow for comparison with annual depths of sediment
disturbance due to root throw for other processes reported in
the literature, such as soil creep or landsliding, the volume
of sediment disturbed per meters squared of land surface is
converted into an annual depth of disturbance by dividing by
32 years, the approximate time period of the root plate upheaval
for Age Classes 1–3. Annual disturbance values are of the order
1·0 × 10–2 mm a–1. This result is only one-order of magnitude
lower than typical annual disturbance depths reported for
shallow landsliding in coastal British Columbia, with a value
of about 1·0 × 10–1 mm a–1 (Martin et al., 2002). This suggests
that upheaval of sediment due to tree topple is a notable factor
operating on forested hillslopes. When considering sediment
transport (incorporating both upheaval and disintegration) a
notable percentage of disturbed sediment is often returned to
the pit, particularly for our lowest gradient plots, and in such
situations does not contribute to net transport of sediment.
There is approximately a one-order of magnitude increase in
sediment transport for the post-fire versus the pre-fire rates for
all gradient classes. It is only for our steepest-gradient plot that
notable values of sediment transport were observed. Although
considerable volumes of sediment are involved in root throw
events (resulting in notable values of sediment disturbance),
the associated transport distances are relatively small (5 cm to
150 cm), leading to relatively low transport rates. Given the
limited time window of our field study, the millennial-scale
transport estimates derived in our model, which take into
consideration the temporal dynamics of sediment transport,
may be preferred.
In general, vegetation properties are treated very simply in
many geomorphic studies. The complex effects of vegetation
on sediment transport are often encapsulated in various
parameters within equations, rather than being explicitly
considered. However, within our model, the details of tree
population dynamics (recruitment and mortality) play a very
direct role in determining the timing and pulsing of root
Copyright © 2009 John Wiley & Sons, Ltd.
throw sediment transport. In particular, two notable pulses
of transport due to root throw are evident in our results: (i) if
trees are large enough to have reached a critical dbh at time
of fire, then a pulse of sediment occurs in post-fire years,
which decreases exponentially with time since fire; and
(ii) once new recruits have reached a critical dbh and with
competition mortality (thinning), then a second pulse of root
throw begins at about 50 to 60 years after the previous fire. If
the period of time since the previous fire is not sufficient for
enough standing trees to have reached a critical dbh, then
trees may topple but they will not upheave notable amounts
of sediment. It is only when the time since previous fire
exceeds the time needed for critical dbh to be reached that
notable sediment disturbance and transport begins to occur.
After about a century the number of trees does not decrease
very much and dbh continues to grow, increasing root plate
volumes for the trees, which eventually leads to increased
sediment disturbance and transport.
Transport rates for our millennial-scale model run are now
compared to published results in the literature. The model of
Gabet et al. (2003) estimates an annual transport rate due to
root throw of 1·6 × 10–3 m3 m–1 a–1 for a 20° slope. Roering et al.
(2002) estimated millennial-scale transport rates for biogenic
processes (e.g. root growth, root throw) based on vertical profiles
of tephra concentration and topographic derivatives. The
resulting K value (i.e. the diffusion coefficient, which represents
the transport rate at unit gradient or 45°) is 1·2 × 10–2 m3 m–1 a–1.
Since the form of this transport equation is linear, then the
transport rate for a slope gradient of 20° (the slope gradient
used in our model runs) can be inferred and has a value of
4·4 × 10–3 m3 m–1 a–1. Walther et al. (submitted for publication)
estimated the diffusion coefficient for soil movement by root
growth, bioturbation and tree throw based on the distribution
of tephra grains in soils found in southeast Washington State.
They obtained a diffusivity value of 4·8 × 10–3 m3 m–1 a–1,
which translates into a transport rate of 1·7 × 10–3 m3 m–1 a–1
for a slope gradient of 20°. Results for our millennialscale model run provide a mean annual transport rate of
1·2 × 10–3 m3 m–1 a–1, which compares favorably with the
longer term values obtained for the above-mentioned studies
(all values have the same order of magnitude). For further
comparison, Martin and Church (1997) reported an average
linear diffusion coefficient based on a large number of soil
creep rates reported in the literature of 2 × 10–4 m3 m–1 a–1
(7·3 × 10–5 m3 m–1 a–1 for a 20° slope), and reported a
diffusion coefficient for debris slides of 1 × 10–1 m3 m–1 a–1
(3·6 × 10–2 m3 m–1 a–1 for a 20° slope); our transport rates for
root throw are between these two values.
This study provides an example of how direct the links
may be between ecology and geomorphology. In our field
and modeling studies, tree age and size as determined by the
forest population dynamics was shown to be a key factor
affecting the timing of root throw and the amount of sediment
associated with these events. It is suggested that future
geomorphological studies attempt to strike a balance between
incorporating vegetation in a more realistic manner than has
often been the case in past geomorphic studies, while at the
same time keeping our understanding of the joint processes
tractable.
Acknowledgements—We acknowledge the financial support of the
Biogeoscience Institute and NSERC Discovery Grants to EAJ and
YEM. We also thank Kootenay National Park for the support provided
for the field component of this project. Numerous field assistants
provided excellent assistance in the field and contributed to the
success of this project. The manuscript benefited from the perceptive
comments of two anonymous reviewers.
Earth Surf. Process. Landforms 34, 1255–1269 (2009)
DOI: 10.1002/esp
SEDIMENT TRANSPORT DUE TO TREE ROOT THROW
References
Beatty SW, Stone EL. 1986. The variety of soil microsites created by
tree falls. Canadian Journal of Forest Research 16: 539–548.
Brown PM, Shepperd WD, Mata SA, McClain DL. 1998. Longevity
of windthrown logs in a subalpine forest of central Colorado.
Canadian Journal of Forest Research 28: 932–936.
Charron I, Greene DF. 2002. Post-fire seedbeds and tree
establishment in the southern mixedwood boreal forest. Canadian
Journal of Forest Research 32: 1607–1615.
Clinton BD, Baker CR. 2000. Catastrophic windthrow in the southern
Appalachians: characteristics of pits and mounds and initial
vegetation responses. Forest Ecology and Management 126: 51–
60.
Coutts MP, Nielson CCN, Nicoll BC. 1999. The development of
symmetry, rigidity and anchorage in the structural root system of
conifers. Plant and Soil 217: 1–15.
Cremeans DW, Kalisz PJ. 1988. Distribution and characteristics
of windthrow microtopography on the Cumberland Plateau of
Kentucky. Soil Science Society of America Journal 52: 816–821.
Denny CS, Goodlett JC. 1956. Microrelief resulting from fallen trees.
In Surficial Geology and Geomorphology of Potter County,
Pennsylvania, Denny CS (ed.), US Geological Survey Professional
Publication 288. US Geological Survey: Reston, VA; 59–66.
Dietrich WE, Dunne T, Humphrey NF, Reid LM. 1982. Construction
of sediment budgets for drainage basins. In Sediment Budgets and
Routing in Forested Drainage Basins, Swanson FJ, Janda RJ, Dunne
T, Swanston DN (eds). USDA Forest Service General Technical
Report PNW-141. Pacific Northwest Forest and Range Experiment
Station: Portland, OR; 5–23.
Environment Canada. 2005. National Climate Archives, Government
of Canada. http://www.climate.weatheroffice.ec.gc.ca/climateData.
Gabet EJ, Reichman OJ, Seabloom EW. 2003. The effects of
bioturbation on soil processes and sediment transport. Annual
Review of Earth and Planetary Sciences 31: 249–273.
Gallaway JM. 2006. Impact of wildfire on root throw and hillslope
erosion in a Canadian Rocky Mountain forest, Unpublished MSc
thesis, Department of Geography, University of Calgary; 168 pp.
Harper JL. 1977. Population Biology of Plants. Academic Press:
London; 892 pp.
Johnson EA, Fryer GI. 1989. Population dynamics in Lodgepole pine –
Engelmann spruce forests. Ecology 70: 1335–1345.
Johnson EA, Greene DF. 1991. A method for studying dead bole
dynamics in Pinus contorta var. latifolia – Picea engelmannii forests.
Journal of Vegetation Science 2: 523–530.
Johnson EA, Larsen CPS. 1991. Climatically induced change in fire
frequency in the Southern Canadian Rockies. Ecology 72: 194–201.
Johnson EA, Van Wagner CE. 1985. The theory and use of two fire
history models. Canadian Journal of Forest Research 15: 214–220.
Johnson EA, Miyanishi K, Kleb H. 1994. The hazards of interpretation
of static age structures as shown by stand reconstructions in a
Pinus contorta – Picea engelmannii forest. Journal of Ecology 82:
923–931.
Johnson EA, Morin H, Miyanishi K, Gagnon R, Greene DF. 2003. A
process approach to understanding disturbance and forest dynamics
for sustainable forestry. In Towards Sustainable Management of the
Boreal Forest, Burton PJ, Messier C, Smith DW, Adamowicz WL
(eds). NRC Research Press: Ottawa; 261–306.
Lutz HJ. 1940. Disturbance of Forest Soil Resulting from the
Uprooting of Trees, Yale University, School of Forestry Bulletin
No. 45. Yale University: New Haven, CT; 1–37.
Lyon LJ. 1977. Attrition of Lodgepole Snags on the Sleeping Child
Burn, Montana, INT-219. USDA Forest Service, Intermountain
Forest and Range Experiment Station: Ogden, UT.
Copyright © 2009 John Wiley & Sons, Ltd.
1269
Martin YM. 2000. Modelling hillslope evolution: linear and
nonlinear transport relations. Geomorphology 34: 1–21.
Martin YM, Church M. 1997. Diffusion in landscape development
models: on the nature of basic transport relations. Earth Surface
Processes and Landforms 22: 273–279.
Martin YM, Valeo C, Tait M. 2008. Centimetre-scale digital
representations of terrain and impacts on depression storage and
runoff. Catena 75: 223–233.
Martin YM, Rood K, Schwab J, Church M. 2002. Sediment transfer by
shallow landsliding in the Queen Charlotte Islands, B.C. Canadian
Journal of Earth Sciences 39: 289–205.
Masters A. 1990. Changes in forest fire frequency in Kootenay
National Park, Canadian Rockies. Canadian Journal of Botany 68:
1763–1767.
Meyers NL, McSweeney YK. 1995. Influence of treethrow on soil
properties in Northern Wisconsin. Soil Science Society of America
Journal 59: 871–876.
Mills HH. 1984. Effect of hillslope angle and substrate on tree tilt,
and denudation of hillslopes by tree fall. Physical Geography 5:
253–261.
Miyanishi K, Johnson EA. 2002. Process and patterns of duff
consumption in the mixedwood boreal forest. Canadian Journal of
Forest Research 32: 1285–1295.
Norman SA, Schaetzl RJ, Small TW. 1995. Effect of slope angle on
mass movement by tree uprooting. Geomorphology 14: 19–27.
Osterkamp WR, Toy TJ, Lenart MT. 2006. Development of partial
rock veneers by root throw in a subalpine setting. Earth Surface
Processes and Landforms 31: 1–14.
Quine CP, Gardiner BA. 2007. Understanding how the interaction of
wind and trees results in windthrow, stem breakage and canopy
gap formation. In Plant Disturbance Ecology: The Process and the
Response, Johnson E, Miyanishi K (eds). Elsevier Academic Press:
Amsterdam; 103–156.
Reed WJ, Larsen CPS, Johnson EA, MacDonald GM. 1998.
Estimation of temporal variations in fire frequency from time-sincefire map data. Forest Science 44: 465–475.
Reid LM. 1981. Sediment Production from Gravel-surfaced Forest
Roads, Clearwater Basin, Washington, University of Washington,
Fisheries Research Institute Publication FRI-UW-8108. University
of Washington: Seattle, WA; 247 pp.
Roering JJ, Almond P, Tonkin P, McKean J. 2002. Soil transport
driven by biological processes over millennial timescales. Geology
30: 1115–1118.
Schaetzl RJ, Johnson DL, Burns SF, Small TW. 1989. Tree uprooting:
review of terminology, process, and environmental implications.
Canadian Journal of Forest Research 19: 1–11.
Smithers LA. 1961. Lodgepole Pine in Alberta, Department of
Forestry, Government of Canada Bulletin 127. Government of
Canada: Ottawa; 153 pp.
Soethe N, Lehmann J, Engels, C. 2006. Root morphology and
anchorage of six native tree species from a tropical montane forest
and an elfin forest in Ecuador. Plant and Soil 279: 173–185.
Stephens EP. 1956. The uprooting of trees: a forest process. Soil
Science of America Proceedings 20: 113–116.
Swanson FJ, Fredriksen RL, McCorison FM. 1982. Material transfer in
a western Oregon forested watershed. In Analysis of Coniferous Forest
Ecosystems in the Western United States, Edmonds RL (ed.).
Hutchinson Ross Publishing Company: Stroudsburg, PA; 233–266.
Ulanova NG. 2000. The effects of windthrow on forests at different
spatial scales: a review. Forest Ecology and Management 135:
155–167.
Walther SC, Roering JJ, Almond PC, Hughes MW. Submitted for
publication. Long-term biogenic soil mixing and transport in a
hilly, loess-mantled landscape. Catena.
Earth Surf. Process. Landforms 34, 1255–1269 (2009)
DOI: 10.1002/esp