The effect of soil water-table depth
on root-plate development and
stability of Sitka spruce
DUNCAN RAY and BRUCE C. NICOLL
Forestry Commission Research Agency, Northern Research Station, Roslin EH25 9SY, Scotland
Summary
Stability was tested on 46-year-old Sitka spruce trees (over 20 m tall) growing on gleyed soils.
Trees with a range of root depths on peaty gley and surface-water gley soils were selected by
studying their water regime. Trees were pulled over horizontally with a winch and the vertical
displacement of the soil-root plate was measured as the load was applied. The resistive turning
moment at the point of soil failure and the maximum resistive turning moment were measured.
Thin plates, which developed over a shallow water table, had a greater surface area than thick
plates that developed over deeper water tables. However, they also had a smaller soil mass and
tended to shear in soil material of lower shear strength and were particularly flexible. Thicker
plates sheared in stronger soil and were more rigid. Soil-root plate rigidity was shown to be a
major factor affecting the contribution of soil resistance to overturning. Regression analysis
revealed that the more rigid the plate the greater the applied turning moment at soil failure.
Trees that develop rigid soil-root plates through adaptive growth of their structural roots will
therefore also have improved stability. The results indicate that intensive drainage of peaty gleys
will increase rooting depth and the resistance to overturning.
Introduction
Sitka spruce (Picea sitchensis (Bong.) Carr.) has
been extensively planted in the uplands of
Britain, particularly in the north and west.
However, high rainfall in combination with
soils of low hydraulic conductivity leads to frequent waterlogging except for short periods in
the summer (Pyatt and Smith, 1983) when evapotranspiration is higher. Waterlogging causes
anaerobic soil conditions (Boggie, 1977; King et
al., 1986) which are unfavourable for the survival of growing Sitka spruce roots (Coutts and
O littritute of Qurtercd Foretterx, 199S
Philipson, 1978). This leads to shallow root
plates which are particularly unstable in the
windy British climate (Quine et al., 1995). Any
silvicultural treatments that improve rooting
depth, and hence stability, would lead to longer
rotations with the benefits of increased production as well as improved forest biodiversity
(Peterken et al., 1992; Ratcliffe, 1993).
In this paper the stability and root development of trees in relation to water-table depth,
using undrained and drained areas of a plantation on two types of soil are investigated. A
Fomrry, Vot 71, No. 2 , 1998
170
FORESTRY
previous study on the same site (Ray et al., 1992)
showed that intensive drainage (10 m spacing) of
peaty gley soil significantly lowered the water
table by 0.07-0.15 m compared with the currently
recommended wider drain spacing of 40 m (Pyatt,
1990). In this study the water table below trees
was measured in two winter periods before the
trees were pulled over and the soil-root plate
characteristics measured. This allowed an assessment of the effect of drainage on water-table
depth, and of the relationship between watertable depth and tree morphology and stability.
Drains were installed on this site when the
trees were 17 years old. Although the structural
root systems of forest trees commonly develop
from roots that have grown to be largest within
the first 8-15 years (Deans, 1981; Courts, 1983a)
of the tree's life, the root system remains highly
responsive to changes in the soil environment.
Courts (1983a) found small roots increasing in
size as late as 18 years to become important
structural roots, and Wagg (1967) found large
changes in the root system architecture of white
spruce in response to changing water tables and
soil surface levels until the trees were over 70
years old. The authors therefore expected root
systems on this site to have exploited, and developed into, progressively deeper soil as a result of
a deeper water table following drainage.
Coutts (1983b) suggested that shear would
begin at the edge of a rigid soil-root plate and
move rapidly towards the centre as a tree started
to overturn. In such a system the calculated
turning moment to fracture the soil-root plate
was six times greater than the observed turning
moment of flexible plates. Measurements of soil
displacement while a turning moment was
applied to a thin, flexible soil-root plate showed
that soil sheared first under the plate close to the
centre of the stem on the windward side, spreading quickly outwards towards the edge of the
soil-root plate (Coutts, 1983b and 1986).
There have been a number of studies demonstrating the effect of various silvicultural techniques on root development and tree stability.
Ground preparation techniques have a major
influence on the radial symmetry of a root system. For example Savill (1976) reported that
shallow furrows 0.3 m deep could restrict the
lateral growth of Sitka spruce planted on surface-water gleys. Restricted root spread, e.g. by
spaced furrow ploughing, has been shown to
reduce the stability of conifers, particularly
when root depth is also restricted (Pyatt and
Booth, 1973; Savill, 1976; Hendrick, 1989).
Quine et al. (1991) found marked asymmetry in
the distribution -if spruce roots when planted
next to or close to a stump, whereas the roots
of trees planted more than 0.7 m from a stump
had better radial symmetry.
In plantations on cold wet sites, some ground
preparation is essential for successful tree establishment, and a mound provides a warmer, drier,
less compact, well aerated, weed-free planting
site in which soil nutrients are more accessible
than in uncultivated sites. These factors combine
to produce good establishment success (Binns,
1962; Taylor, 1970; Tabbush and Ray, 1989). In
particular, the higher soil temperature gained by
a mound early in the growing season (Ray and
Anderson, 1990) will help initiate and stimulate
root growth (Coutts and Philipson, 1987) at a
time that is critical to the development and establishment of the structural root system (Deans,
1981; Coutts, 1983a; Coutts, 1987). Unlike continuous plough ridges with furrows at 4-m spacing, mounds are less restrictive to lateral root
growth, producing trees with better radial root
symmetry (Quine et al., 1991).
In this study the stability of large, 46-year-old
trees planted on spread turfs was examined. The
'spread turP (or 'Belgian turf method') was used
extensively in the 1940s and 1950s, and involved
excavating a shallow (approx. 0.3 m) 'turf
drain', manually or with a plough, to provide
'cut turfs' which were then spread manually into
two rows, providing planting positions at
1.8—2.0 m spacing. The authors believe this
study is applicable to new planting sites cultivated using 'continuous acting' mounding
machines as well as drain mounding and hinge
mounding techniques, which are both currently
used on wet peaty gley and surface-water gley
restock sites (Forestry Commission, 1993).
Experimental details
Site
The experiment was located at Crookburn in
Kershope forest, north-east Cumbria, UK
WATER-TABLE EFFECT ON STABILITY OF SITKA SPRUCE
(200-245 m altitude, latitude 55°66'N, longitude
2°47rW). Exposure is moderate with a southeasterly aspect, with windthrow hazard class
(WHC) (Quine and White, 1993) varies between
4 and 5. WHC increases with windspeed and
exposure on a scale of 1 to 6.
Soils
Soils have developed a 3-m thick layer of glacial
till, and vary from peaty gleys (humic stagnoorthic gley; Avery, 1990) on gentle slopes to surface-water gleys (stagno-orthic gley) on steeper
slopes. The soils are of the Carter Association
(Muir, 1956) and have a slowly permeable subsoil of clay-loam texture, and a saturated
hydraulic conductivity ranging from 0.6 to 60
mm day"1 (Ray et al., 1992). Soils equivalent to
those in the Carter Association are described by
the Soil Survey of England and Wales as the
Wilcox 1 Association (Jarvis et al., 1984).
Silviculture
Sitka spruce was planted in 1948 on spread turfs
at a spacing of 1.8 m. The turfs were cut from
a plough ridge turned out of shallow (~0.3 m
deep) furrows at a spacing of approximately
3.6 m. The spread turf method provided a planting position similar to a mound, but with a shallow furrow between every two rows of trees.
In 1965, a drainage experiment was installed
on the site (Pyatt et al., 1985), with three drain
spacing treatments of 10 m, 20 m and 40 m.
Half of the experiment was felled between 1983
and 1985, and replanted with a mixtures experiment in 1986 (Ray et al, 1992). However, the
tree pulling study was located in a 20-ha area
within the unfelled half of the drainage experiment, and in an adjacent undrained stand of
trees. The area of forest had not been thinned,
apart from cutting racks to enable drainage in
1965.
Selection of sample trees
In 1991, 100 dominant and co-dominant trees
were selected on a stratified-random basis, by
dividing the area into approximately 1-ha
squares and randomly choosing five of the
largest trees in each square. This ensured that
171
sample trees were selected in undrained, moderately drained and intensively drained stands,
growing on both peaty-gley and surface-water
gley soils. A tree was rejected and a replacement
selected: if it was next to a drain, where it did
not have an adjacent furrow. Trees were also
rejected if they were 'pumping'. A tree is
described as 'pumping' when its soil-root plate
has sheared from the sub-soil, allowing the plate
to rock freely in the hollow. This rocking causes
soil water under the tree to mix with the fine
mineral material in the soil, forming a 'soupy'
suspension that is pumped out of the hollow as
the soil-root plate rocks. As the soil-root plates
of 'pumping' trees have already sheared from
the sub-soil, they are unsuitable for a study of
plate bending during soil failure.
Coutts (1983b and 1986) chose dominants in
his tree pulling studies at Kershope forest, and
the same methodology was followed in this
study. The dominant trees may be the most vulnerable to wind damage, since they are taller
with larger crowns than sub-dominant trees.
Water-table study
A borehole lined with 0.05-m diameter, 1-m
long perforated plastic pipe, was installed in a
position 1 m from the geometric centre of each
tree on the side away from the furrow. The
depth to the water table was measured each
week for 8 weeks in November and December
1991. From the initial 100 trees, 50 were selected
on the basis of the depth to the borehole water
level below the ground surface, i.e. with a range
of water-table depths, which were expected to
produce a similar range of tree rooting depth. A
further three boreholes were installed around
each of these 50 study trees. Three were
installed vertically, 1 m from the geometric centre of the tree, and a fourth was installed at an
angle of about 60° close to the stem on the furrow side of the tree, to enable measurements of
the water table directly under the tree stump.
Although the water level in the three vertical
boreholes was measured directly, it was necessary to calculate the water level in the angled
borehole.
The borehole water level in the four positions
under each of the 50 study trees was measured
for two, 10-week winter periods: period 1, 12
172
FORESTRY
February-15 April 1992; period 2, 21
October-23 December 1992. Two trees were
blown over by winter gales early in period 2,
and were removed from the assessment.
Felling
The site was clearfelled during the period
October 1993 to June 1994 by a mechanized
harvester. The tops of the study trees were
removed leaving a 3-m high stem. The machines
on the site were excluded from the area within
4 m of each study tree to avoid damage to soilroot plates. During the felling operation a further nine trees were blown over and had to be
removed from the study. The blown trees were
close to a harvester 'drift' and so were critically
exposed along a newly created east facing edge
during an easterly gale.
Tree pulling
The force required to pull over horizontally
each of the 39 remaining trees was measured by
a 10 tonne load cell connected to a data logger.
A load was applied to each tree, across the load
cell, by attaching one end of a steel cable to the
tree at 2 m height and the other to a hydraulic,
tractor-mounted winch. The load and time were
recorded at 1.5 s intervals by a data logger during the pull.
Every soil-root flate was pulled in a consistent manner by positioning the tractor 25 m
away, anchoring the cable to the tree 2 m above
the ground, returning the cable on to the drum
of the winch at the same slow rate (0.02 m s~'),
and pulling each soil-root plate away from the
furrow. The angle of the cable from horizontal
was also recorded. The side of the tree towards
the pull direction, equivalent to the leeward side
of a windthrown tree, is here described as the
'pull side', and the side away from the pull
direction, equivalent to the windward side, is
here termed the 'furrow side'.
from the centre of the tree on the furrow side
and the last 1.2 m on the pull side. Each sensor
consisted of a displacement transducer mounted
on a stand, which was made from a 0.3-m long
piece of 0.18-m diameter plastic pipe (Figure 1).
A 1-m long mild steel rod was located within a
0.04-m diameter hole, augered to a depth of 0.5
m through the top soil. The rod was then hammered into the subsoil, to a depth of approximately 0.95 m, leaving 0.05 m protruding. This
ensured that the rod did not move when the
soil-root plate was pulled. The cord of a displacement transducer was attached and the
movement sensor was then placed over the steel
rod. Two spikes (0.2 m long) fixed to the base
Direction of pull
Elevation
Tree
n/n nln n
nn
Furrow
Movement sensor/
o o d o cj> o o o o
i\ i m
I\
I
^ ^ F Direction of pull
ooQpqoooo
Displacement
trmoiducer
Root plate movement and bending
Stedrod
Eighteen movement sensors were installed on
the soil-root plate at 0.40-m intervals in two
parallel transects, parallel to the line of pull.
The first movement sensor was positioned 2.0 m
!
Figure 1. Plan and elevation of movement sensor layout around a study tree, and detail of a movement
sensor.
WATER-TABLE EFFECT ON STABILITY OF SITKA SPRUCE
of the stand secured the movement sensor firmly
to the soil-root plate when pressed into position
(Figure 1). Sensors were connected to a data logger which recorded the rime and displacement at
1.5-s intervals until the furrow side of the soilroot plate had been displaced by about 0.3 m.
This limit ensured that transducers were not
damaged by pulling the cord beyond the furthest
point of travel.
So/7 and soil-root plate measurements
Having measured the load applied to the soilroot plate beyond the maximum value, movement sensors were disconnected and removed,
and the plate was pulled through 90° and
secured to allow safe working beneath it. Root
plate measurements (Nicoll and Ray, 1996) were
made of the width and depth along eight cardinal radii from the centre of the stem.
The soil was described and sampled from a
soil profile pit excavated next to each plate. The
depth of each soil horizon was measured, and
three soil-cores of known volume were taken
from each of the mineral soil horizons around
the edge of the plate, and bulked together. In
peat, the core sampler caused too much compression. Instead, undisturbed blocks of peat
(about 0.2 X 0.2 X 0.2 m in size) were cut from
the soil pit, and sealed in plastic bags. The volume of each peat block was determined by a
fluid displacement method (Pyatt and John,
1989). All samples were weighed fresh, before
drying to constant weight at a temperature of
80°C. It was therefore possible to calculate the
water content and the wet bulk density of each
soil horizon at the rime of tree-pulling. Soil-root
plates were too large and heavy to weigh
directly; instead we estimated the mass of each
plate by multiplying the volume of soil material
in each horizon by its wet bulk density.
A Pilcon Hand Vane Tester (English Drilling
Equipment Co. Ltd, Huddersfield, UK) was used
to measure the soil shear strength within the
horizon exposed at the shear plane, and in the
other soil horizons exposed by the soil-root
plate. Six replicate readings were made in each
horizon exposed, and exceptionally high readings, caused by the vane touching a stone or
root, were discarded.
Soil was then cleaned from each plate using a
173
jet of compressed air from an 'air-knife' (Briggs
Technology Inc., Pittsburgh, US). The root systems were removed from the site and transported to the Northern Research Station for
root architecture studies (Nicoll and Ray, 1996).
The woody root systems were also too heavy to
weigh fresh, and because there was a 2-3 week
gap between tree pulling and root cleaning, it
was necessary to calculate fresh root weight.
Samples of fresh root from two plates were
weighed immediately following tree pulling.
The samples were dried at 80°C to constant
weight, from which the water content and density of the wood were calculated. Later in the
study the air-dry, cleaned roots were weighed,
and again samples of root were taken and
weighed, dried to constant weight at 80°C, and
re-weighed as before. From the differences in
the water content between the fresh roots, air
dry roots, and the oven dried samples, the fresh
weight of each root system could be calculated.
Statistical analysis
Data presented in Tables 1-3 were analysed
using analysis of variance. Other relationships
were explored
using linear regression.
Throughout this paper, differences are described
as significant at the P < 0.05 level and as highly
significant at the P < 0.01 level.
Results
Rainfall and water-table depth
The mean annual rainfall of the site is 1400 mm
(Anderson et al., 1990) giving a weekly rainfall
of approximately 30 mm. Both assessment periods were slightly wetter than average, as was
expected during the winter, with 319 mm of rain
falling over the 10-week period (12 February-15
April 1992), and 371 mm of rain falling in the
second 10-week period (21 October-23
December 1992).
There was a large range in the mean winter
borehole water level between the 50 trees
(0.25-0.60 m). Table 1 shows the effect that an
intensive drainage system had on the mean winter water-table depth, and how the drainage
effect varied between soil types. Sample trees in
174
FORESTRY
Table 1: Mean winter water-table depth (m) under sample trees growing on surface-water gley (peat thickness
20.10 m), shallow phase peaty gley (peat thickness >0.10 and 20.25 m) and deep phase peaty gley (peat thickness >0.25 and 20.45 m), and the effect of drainage on the water table within each soil type
Soil type
All trees
Low intensity drains
High intensity drains
Surface-water gley
Shallow phase peaty gley
Deep phase peaty gley
46.7
42.8
35.9
45.2
35.7
33.0
48.0
48.0
46.0
Significant differences
*, differences significant at P £ 0.05; n.s., not significant.
a stand with 10 m or 20 m spaced drains are
described as having a high intensity drainage
system, whereas sample trees in stands with no
drainage or with drains at 40 m spacing are
described as having a low intensity drainage system. On the surface-water gley soils, the depth
of the mean winter water table was not significantly different between trees in areas of high
intensity and low intensity drainage. However,
on the peaty gley soils the water table was significantly deeper with high intensity drainage
than with low intensity drainage. The mean
winter water table was deeper on surface-water
gley soils than on peaty gley soils, differences
approached significance (P = 0.07).
1.00 0.K 040 0.70 040 0.10 0.40 0J0
0-10
0.10 0.00
0-10
Tree dimensions in relation to site
There was a slight, but not significant, inverse
trend between increased above ground biomass
and deeper winter water tables. Both crown
width and crown depth increased with tree
mass. Root:shoot ratio, i.e. fresh root weight
divided by stem and canopy weight was significantly (negatively) related to mean winter
water-table depth (Figure 2). Although the data
show considerable scatter (r2 = 0.3), the regression coefficient was highly significant.
Tree pulling
An example of soil-root plate displacement,
while a turning moment was applied about the
base of a tree, is shown in Figure 3a. The horizontal positions represented on the jc-axis as
negative numbers identify movement sensors
positioned on the furrow side, whereas positive
values identify movement sensors on the pull
side. Individual lines on the graph represent the
0J6
040
044
0.40
0.41 0M
I n n wlnttr wittr UbW dipt* (m)
041
040
Figure 2. The relationship between root: shoot ratio
and the mean winter water-table depth for all study
trees ( • , linear regression
).
position of the soil-root plate at 1.5-s time intervals as the load was applied, causing the plate
to move. On the pull side, the compressive
forces bearing on the soil caused soil failure and
compaction at 0.8 m from the stem. On the
furrow side, the plate lifted causing bending
followed by shearing from the underlying soil,
initially at a point between -0.4 m and -0.8 m
from the centre of the tree. Figure 3b shows the
applied turning moment, and the change in the
applied turning moment for the same tree plotted as a time series. In previous studies (Courts,
1986; O'Sullivan and Richie, 1993), the applied
turning moment (M) was plotted against displacement (D). In this study, the load was
applied slowly and continuously through time
allowing M to be plotted against time (f) The
WATER-TABLE EFFECT ON STABILITY OF SITKA SPRUCE
175
failed, and the position of the root plate and its
curvature
were measured, at the time of failure.
^ tkK root taDor*
Trees in the present study were large and
showed considerable buttressing of the structural roots close to the stem (Nicoll and Ray,
1996) which resulted in very little bending of the
root plate closer than 0.5 m from the centre of
the stem.
Curvature was calculated as the reciprocal of
the radius of a circle, whose arc fitted the coordinates of the movement sensors positioned at
-0.4 m, -0.8 m and -1.2 m from the centre of
the stem, at the time of soil failure. The authors
were interested in the relationship between soilroot plate curvature and the resistive turning
moment, and therefore to remove the effect of
tree size (increased stem and root plate mass) on
turning moment (Fraser and Gardiner, 1967) the
resistive turning moment was divided by stem
mass. Figure 4 shows a significant negative relationship between the normalized turning
moment at soil failure plotted against curvature,
although points had a large scatter about the
regression line (r2 = 0.20). This regression shows
that the more rigid the soil-root plate, the
greater the applied turning moment at soil failFigure 3. (a) Soil-root plate displacement during tree- ure. Curvature was not significantly correlated
pulling on one example tree, lines represent the plate
with soil-root plate thickness, or rooting depth
position at 1.5 s intervals ( •
•)
(P = 0.03). The mean turning moment at soil
(b) The turning ( •
• ) moment and change in
failure
for all of the test trees was 35 kNm (45.3
the turning moment (dM/df) ( •
A) applied to
Nm kg"1 normalized by dividing by stem
the same tree.
assumption is made in this study, that the peaks
in dM/dt represent in sequence: soil failure, failure of the roots on the furrow side (windward
roots), and hinge resistance.
Tree pulling measurements were completed
on 34 of the soil-root plates (five more trees
were removed due to technical problems with
the movement sensors). Thirteen of the study
trees showed signs of pumping and were
removed from the curvature analysis described
below as it would not be possible to examine
their curvature at the time of soil failure.
From the data it was possible to find the
applied turning moment and displacement of
the plate at the point of soil failure represented
by the first peak in dM/df (Figure 3b). For each
tree calculations were made of when the soil
—
to •
B
i '
8
-a
B
0.02 0.0J 0.04 0.05 0M 0.07 0JM OJJ»
8otl-root ptata cunratar* (1An)
Figure 4. Normalized turning (TM/stem weight) at
soil failure against soil-root plate curvature at the
moment of soil failure ( • , linear regression
).
176
FORESTRY
—1
0.2!
1
0.J0
1
1
1
I
049
0.40
0.49
0.90
MMti wlntar wtt«r tab!* diplh (m)
I
1
0.99
0.80
Figure 5. Normalized turning moment (TM/stem weight) at soil failure against mean winter water-table depth
( • , linear regression
).
weight). This was about half of the mean maximum turning moment of 70 kNm for all of the
test trees (88 Nm kg"1 normalized).
Figure 5 shows the relationship between the
normalized turning moment at soil failure, and
mean winter water-table depth, for the group of
pulled trees that had not been pumping. Again
there was considerable scatter about the regression line (r2 = 0.21), but Figure 5 does show a
significant trend for trees growing in drier soil
above a deeper water table to be more resistant
to soil failure than trees growing in more waterlogged conditions.
The relationship between the maximum turning moment, normalized by dividing by stem
weight, and water-table depth was tested using
the complete data set of 34 trees. The data
exhibited a large amount of scatter about the
regression line (r2 = 0.2), which was significant,
showing that as water-table depth increased
from 0.25 m to 0.55 m, the average normalized
turning moment increased from approximately
60 Nm kg"1 to 90 Nm kg"1. Figure 6 shows
regression lines fitted separately according to
soil type. There was no significant difference
between the deep and shallow phase peaty gley
regressions, and when combined (as in Figure 6)
the coefficient of regression was significant,
showing that resistance to overturning increased
significantly with water-table depth, from about
60 Nm kg"1 to 90 Nm kg"1 with a lowering of
the water table from 0.3 m to 0.6 m. The relationship between resistive turning moment and
water-table depth on surface-water gley soil
appeared to show a good relationship, but the
coefficient of regression was not significant, pos-
i
O20
OJB
i
i
i
i
i
0J0
OJS
0.40
0.48
0.50
Mean wfcrtor water tabto depth (m)
r
i
0.66
000
Figure 6. Maximum normalized turning moment
(max TM/stem weight) against mean winter watertable depth on peaty gley (if, linear regression line ) and surface-water gley ( • , linear regression ) soils.
WATER-TABLE EFFECT ON STABILITY OF SITKA SPRUCE
177
040 —
0.79 0.70 049 £
040 -
?
040 -
£
c
0.49 -
*
0.40 -
•
. ••'1:1 lint
ojg OJO ojg OJO
r
i
OJI
OJO
i
i
i
i
049
0.40
0.49
040
M M D wlntar water U b k depth ( • )
i
i
049
040
Figure 7. Rooting depth plotted against mean winter water-table depth ( • ) , showing the linear regression line
(
), one-to-one line (
), and estimated relationship of soil-root plate depth
).
sibly because of the small sample size (seven
trees).
Soil-root plate characteristics
The centrally weighted rooting depth (mean
thickness of soil-root plate along eight cardinal
radii, measured at 0.5 m and 1.0 m from centre
of the stem) was significantly correlated (Figure
7) with the mean winter water-table depth (P <
0.001). The linear regression line was above the
1:1 line, indicating that roots show a degree of
tolerance to waterlogging in the winter months.
Table 2 shows a summary of the data,
arranged into three categories of peat thickness:
surface water gleys—peat thickness less than
0.10 m, shallow phase peaty gleys—peat thickness 0.10-0.25 m and deep phase peaty gleys—
peat thickness 0.25-0.45 m, and the least
significant difference between means. Soil type
varied with site slope angle, surface-water gleys
occurred on the steeper slopes and deeper phase
peaty gleys on gentle slopes and flat sites. The
maximum resistive turning moment increased
with plate thickness, and was almost twice as
large for plates on surface-water gley soils (85
kNm) compared with plates on deeper phase
peaty gleys (48.4 kNm), however, this difference
was partly due to larger trees (stem mass—936
kg) with heavier soil-root plates (5.77 tonne) on
the surface-water gley, compared with the deep
phase peaty gley (stem mass—662 kg, soil-root
plate mass—3.53 tonne). After normalizing the
maximum resistive turning moment by dividing
by stem weight, differences between soil types
were not significant.
Surface-water gley soil-root plates were
thicker because of the deeper winter water table,
and were heavier because they contained a
greater volume of mineral soil with a larger wet
bulk density than plates on peaty gley soils.
Trees grew significantly taller on the surfacewater gley and the shallow phase peaty gley
than on the deeper phase peaty gley (Table 2).
Soil strength
Mean soil strength, from the hand vane measurements, increased significantly in successively
deeper soil horizons (Table 3). The mean
strength of soil on the B horizon (51 kPa) was
almost twice the strength of the A horizon
material (27.2 kPa) and the peat (18.3 kPa).
Discussion
The relationship between the resistive turning
moment and water-table depth, and between the
resistive turning moment at soil failure and
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Table 2: Summary of tree and root plate data sorted by soil type
Factor/Soil type
All trees
Trees on deep
phase peaty
gley. Peat
thickness
>0.25<0.45 m
(a)
Trees on
shallow
phase peaty
gley. Peat
thickness
>0.10<0.25 m
Trees on
surface-water
gley. Peat
thickness
<0.10 m
LSD , P £ 0.05
(1) (a)-(b)
(2) (bHO
(3) (aHc)
(c)
(b)
62.4
48.4
64.6
85.0
Max turning moment/
stem mass (Nm kg"1)
81.0
76.4
80.7
90.8
Water-table depth (m)
0.43
0.395
0.432
0.467
Site slope (°)
4.2
2.9
4.9
5.3
Rooting depth (m)
0.55
0.48
0.58
0.63
Surface area (m2)
6.13
6.53
5.93
5.84
Soil volume (m3)
3.35
3.10
3.42
3.71
Soil mass (tonne)
4.48
3.53
4.75
5.77
Hinge distance (m)
0.922
0.991
0.905
0.823
Tree height (m)
24.23
22.79
25.18
24.77
Diameter at breast
height (m)
0.32
0.30
0.32
0.35
Crown width (m)
6.87
6.48
7.00
7.33
Crown depth (m)
10.29
10.24
10.77
9.6
Stem mass (kg)
770
634
801
936
M a x turning moment
(kNm)
*, differences significant at P £ 0.05; n.s., not significant.
16.0*
20.0*
20.6*
17.4 n.s.
21.7 n.s.
22.4 n.s.
0.08 n.s.
0.09 n.s.
0.10 n.s.
0.1 n.s.
1.1 n.s.
1.2*
0.07*
0.10 n.s.
0.10*
1.38 n.s.
1.73 n.s.
1.77 n.s.
0.89 n.s.
1.12 n.s.
1.14 n.s.
1.36 n.s.
1.69 n.s.
1.75*
0.15 n.s.
0.19 n.s.
0.19 n.s.
1.48*
1.83 n.s.
1.89*
0.03 n.s.
0.05 n.s.
0.05 n.s.
1.0 n.s.
(2) 1.22 n.s.
(3) 1.26 n.s.
(1) 1.48 n.s.
(2) 1.85 n.s.
(3) 1.25 n.s.
(1) 179*
(2) 223 n.s.
(3) 230*
(1)
(2)
(3)
(1)
(2)
(3)
(1)
(2)
(3)
(1)
(2)
(3)
(1)
(2)
(3)
(1)
(2)
(3)
(1)
(2)
(3)
(1)
(2)
(3)
(1)
(2)
(3)
(1)
(2)
(3)
(1)
(2)
(3)
(1)
WATER-TABLE EFFECT ON STABILITY OF SITKA SPRUCE
water-table depth, is fundamentally important
to the stability of Sitka spruce planted on wet
soils. Courts (1986) found that soil failure
occurred after 0.002-0.005 m displacement at a
mean turning moment of 6.3 kNm in plates
pulled monotonically, whereas during cyclic
loading, O'Sullivan and Ritchie (1993) found the
soil-root plate was displaced on average 0.007 m
at soil failure. Their displacement results were
slightly less than those found in the present
study, which showed a mean displacement of
0.010 m at soil failure.
The estimated mean turning moment at soil
failure in the present study was considerably
larger than reported by Coutts (1986), and there
are several possible reasons for this difference.
First, there were differences in soil wetness
between the studies; whereas the soil was quite
dry at the time of pulling in this study, soil
conditions were wet in Coutts' (1986) study.
Second, in this study soil-root plates were
larger; the trees were 10 years older than those
in Coutts' study. Third, at the point of soil failure the elastic limit of the soil was exceeded.
Coutts found a failure sequence beginning at a
point close to the base of the tree, and spreading in the form of cracks mainly towards the
windward side of the tree. There is likely to be
some lag in the response of the soil-root plate to
an applied turning moment, because the energy
in the applied load is transmitted and dissipated
first through the wood tissue of the stem and
roots and then through the soil. In the present
study soil-root plates resisted overturning in a
dynamic way; the resistive turning moment
responded to the continuous application of a
load and, unlike the root plates in Coutts' (1986)
study, were not given time to equilibrate with
the applied turning moment while it was held
constant in order to read instruments.
Consequently, because the method in this study
differed from earlier 'monotonic' tree pulling
studies, the instruments measured a bigger load
at soil failure than reported in earlier studies.
All of the soil-root plates in the study sheared
close to the maximum depth of the root system,
some fine roots protruded less than 0.04 m from
the base of the soil-root plate and all roots with
secondary thickening were contained above the
shear plane (Nicoll and Ray, 1996). Rooting
depth corresponded with, but was slightly
deeper than, the winter water table. It is likely
that there is some minimum threshold for root
plate thickness irrespective of winter water-table
depth. If so the data might be distributed parallel to the 1:1 line for deeper water tables but
would bend towards a threshold root plate
thickness as shown in Figure 7. This study confirmed the assumption that root systems would
develop into and fully exploit soil as the water
table dropped after drains were installed on the
site. Some plates sheared within the peat, others
within the mineral soil; Anderson et al. (1989)
also reported a similar soil-root plate shear zone
pattern for Sitka spruce root plates. Many of the
thinner plates on the deeper phase peaty gley
soil sheared within the Eg horizon. This is an
eluviated horizon from which the fine colloidal
clay particles have been removed during some
stage in the soil formation process, leaving a
material of more sandy texture than the deeper
clayey mineral horizons. Shear strength is commonly measured in place of soil tensile strength
and although soil wetness was not taken into
consideration, the data show that the Eg was
somewhat weaker than the underlying Bg horizon (Table 3). In contrast, Anderson et al.
(1989), as well as reporting smaller values of soil
shear strength at this site (10-25 kPa) than in
this study (18-51 kPa), found no difference in
Table 3: Mean soil shear strength (kPa) measured with a hand vane tester and least
significant difference (LSD) at P < 0.05.
Soil horizon
Soil shear strength (kPa)
LSD (between value and one below)
Peat
A
18.3
27.2
42.6
51.0
5.6*
5.1*
7.7*
*, difference significant at P < 0.05.
179
180
FORESTRY
the soil shear strength between peat and mineral
horizons. However, they did report an increase
in the normal stress of a soil-root plate as the
proportion of mineral-peat soil increased in the
shear zone. From Anderson et al. (1989) and
from the present data, it appears that an
improvement in stability should result from
deeper winter water tables, and hence deeper
root plates, because the base of the plate will
contain soil of higher shear strength, and
because a larger volume of soil of higher bulk
density will be incorporated within the plate.
The effect of soil wetness on soil strength must
also be considered in predicting stability, since
soil strength is inversely related to soil water
content. This will have the effect of causing the
soil strength component of tree stability to
change seasonally, with trees most vulnerable
during winter.
Soil failure is an important threshold event
after which some trees begin to 'pump', before
overturning in subsequent high winds. Certainly
soil failure reduces the bending moment that a
tree can resist (O'Sullivan and Ritchie, 1993),
and Courts (1986) suggested that after soil failure a small space will form below a plate
because it will not return completely to its
former position once the force is relaxed. In wet
soils, such gaps will quickly fill with water, and
soil surrounding these saturated areas is probably more likely to liquefy, because tree sway
creates an oscillating pressure which will be
effectively transmitted through the increased
volume of water-filled pores and cavities. Such
conditions appear to be required for liquefaction failure of the soil (Rodgers et al., 1995) and
the onset of pumping. Some trees may however
recover from this destabilization if warmer and
drier conditions, which stimulate active root
growth, follow the period of soil disturbance.
The repeated rocking of the plate in its cavity,
represents a considerably less stable condition
for the tree; wind energy is dissipated in the
rocking motion rather than in stem bending,
and the repeated stress of rocking may progressively weaken windward and leeward roots,
thus further reducing anchorage (O'Sullivan and
Ritchie, 1993). These trees are therefore vulnerable to windthrow at lower wind speeds with a
shorter return period (Quine, 1994).
The hypothesis that soil-root plate stiffness is
an important contributor to tree stability
(Coutts, 1983b; Deans and Ford, 1983), is confirmed by the significant negative relationship
between soil-root plate curvature and the turning moment at soil failure (Figure 4). It may be
expected that soil-root plate thickness would be
the most important factor affecting plate stiffness. However, the regression of soil-root plate
curvature at soil failure on soil-root plate thickness was not significant, possibly due to the
complicating additional effect of structural root
development. There was a significant negative
relationship between root:shoot ratio and water
table depth (Figure 2). This indicates that trees
with thin soil-root plates growing on shallow
water-tables either invest more biomass in
woody structural root systems than deeper
rooted trees, or that shallow rooted trees have a
lower nutrient budget, are less productive and
therefore support smaller crowns. In a followon study, root systems of trees in this study were
found to have developed cross sectional shapes
that would resist bending, with the shallowest
roots having the greatest 'adaptive' development
(Nicoll and Ray, 1996). Roots had developed
I-beam and T-beam shapes (as described by Rigg
and Harrar, 1931), that would have enhanced
the rigidity of the shallowest soil-root plates
(Nicoll and Ray, 1996).
The large variation between tree characteristics and turning moments in this study (seen as
a wide scatter around regression lines) results
both from differences in soil conditions, even
between neighbouring trees, as well as genetic
variation in tree morphology. Nicoll et al.,
(1995) described substantial variation in
root:shoot ratio and structural root system
architecture between shallowly rooted Sitka
spruce clones, and suggested that some clones
would be particularly vulnerable to windthrow.
In an adjacent experiment, Ray et al. (1992)
showed that drains spaced closer than 40 m had
no effect in lowering the water table on steeper
sloping surface-water gley soils, but on the gently sloping peaty gley soil drains spaced at 10 m
produced a small (0.07-0.15 m), but significant,
lowering of the water table compared with
drains at 40 m. The data presented here also
show that drainage can promote deeper water
tables and allow deeper soil-root plates to
develop, even though drains were excavated
WATER-TABLE EFFECT ON STABILITY OF SITKA SPRUCE
when the crop was 17 years old. A conservative
estimate of the improvement in stability resulting from deeper water tables from Figure 6 is 1.5
Nm kg"1 increase in the resistive turning
moment for every 0.01 m lowering of the water
table. Therefore, for a 47-year-old Sitka spruce
• tree growing on a peaty gley soil, 24 m high
with a diameter at breast height of 0.31 m and
a mean stem weight of 750 kg, intensive
drainage will improve the rooting depth by
about 0.10 m, increasing the resistive turning
moment (the tree stability) by 20 per cent from
50 kNm to 60 kNm.
The authors aim to continue this analysis to
investigate if the increase in the resistive turning
moment of trees in drained stands will significantly improve the rotation length of the crop.
This will permit a cost-benefit analysis to examine the economic benefits of intensive drainage
on peaty gleys.
181
Coutts, M.P. 1983a Development of the structural
root system of Sitka spruce. Forestry 56, 1-16.
Coutts, M.P. 1983b Root architecture and tree stability. Plant Soil 71, 171-188.
Coutts, M.P. 1986 Components of tree stability in Sitka
spruce on a peaty gley soil. Forestry 59, 173-197.
Coutts, M.P. 1987 Developmental processes in tree
root systems. Can. ]. For. Res. 17, 761-767.
Coutts, M.P. and Philipson, J.J. 1978 Tolerance of
tree roots to waterlogging. I. Survival of Sitka
spruce and lodgepole pine. New Phytol. 80, 63-69.
Coutts, M.P. and Philipson, J.J. 1987 Structure and
physiology of Sitka spruce roots. Proc. Royal Soc.
Ed. 93B, 131-144.
Deans, J.D. 1981 Dynamics of coarse root production
in a young plantation of Picea sitchensis. Forestry
54, 139-155.
Deans, J.D. and Ford, E.D. 1983 Modelling root
structure and stability. Plant Soil 71, 189-195.
Forestry Commission 1993 Forests and Water
Guidelines, 3rd edn. HMSO, London.
Fraser, A.I. and Gardiner, J.B.H. 1967 Rooting and
stability in Sitka spruce. Forestry Commission
Bulletin No 40. HMSO, London.
Acknowledgements
Hendrick, E. 1989 The effect of cultivation method
on the growth and root anchorage of Sitka spruce.
We wish to thank the following for considerable help
Ir. For. 46, 19-28.
during the fieldwork phase: David Clark, staff at the
Jarvis,
R.A., Bendelow, V.C., Bradley, R.I., Carroll,
Kielder TSU Field-station, the NRS Workshop, Forest
D.M., Furness, R.R., Kilgour, I.N.L. and King, S.J.
Enterprise at Kielder Forest District, the Forest
1984 Soils and their use in northern England. Soil
Enterprise Mechanical Engineers at Kielder and
Survey Bull. No 10. Soil Survey of England and
Dumfries, and the Bush TSU Field-station. In addiWales, Harpenden.
tion, thanks to the six students involved in this study:
Simon Parkin, Andrew Cameron, Elaine Washington, King, J.A., Smith, K.A. and Pyatt, D.G. 1986 Water
and oxygen regimes under conifer plantations and
Tobias Kerzenmacher, Mairi Nichol and Alice
native vegetation on upland peaty gley soil and
Hague. Thanks also to Alvin Milner for statistical
deep peat soils. ; . Soil Set. 37, 485-497.
advice, and to Barry Gardiner for his critical
Muir, J.W. 1956 The soils of the country round
appraisal of an earlier draft.
Jcdburgh and Morebattle—Sheets 17 and 18.
Memoirs of the Soil Survey of Great Britain.
Department of Agriculture for Scotland, HMSO,
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Received 8 October 1996