Journal of Experimental Botany, Vol. 48, No. 314, pp. 1703-1716, September 1997
Journal of
Experimental
Botany
The function of buttress roots: a comparative study of the
anchorage systems of buttressed (Aglaia and Nephelium
ramboutan species) and non-buttressed (Mallotus wrayi)
tropical trees
M.J. Crook1, A.R. Ennos and J.R. Banks
School of Biological Sciences, Stopford Building, University of Manchester, Oxford Road, Manchester M13 9PT,
UK
Received 18 December 1996; Accepted 15 March 1997
Abstract
The anchorage mechanics of mature buttressed trees
of Aglaia and Nephelium, and of non-buttressed
Mallotus wrayi have been investigated by combining a
study of the morphology of their root systems with a
series of anchorage tests.
Both types possessed tap roots, but only buttressed
trees possessed sinker roots, which branched from
the ends of the buttresses. The anchorage strength of
the buttressed trees was almost double (10.6 kNm)
that of the unbuttressed ones (4.9 kNm), and the maximum moment was generated at lower angles. In buttressed trees, the leeward buttresses were pushed
into the soil before bending and eventually breaking
towards their tip, whilst the windward buttresses
pulled out of the soil or delaminated if they possessed
sinker roots. The tap root rotated in the soil to windward. In contrast, during failure of unbuttressed trees
the tap root both moved and bent towards the leeward,
the windward roots were pulled out of the soil, and
the leeward laterals simply buckled. Strains along buttresses were much higher than along the laterals of
unbuttressed trees.
These results suggest that buttresses act in both
tension and compression and make a much larger
contribution to anchorage than the thin laterals of nonbuttressed trees. The relative contribution of the buttresses was determined by carrying out a further
series of anchorage tests in which both buttressed
and unbuttressed trees were pulled over after all their
1
laterals had been cut away. These trees were therefore
only anchored by their taproot. Failure of both types
was similar to intact unbuttressed trees, and they had
similar anchorage strengths to each other, 4 kNm,
around 80% of the value for intact non-buttressed
trees, but only 40% of the strength of intact buttressed
trees. Buttresses therefore contribute around 60% of
the anchorage of buttressed trees, producing around
six times more anchorage than the thin laterals of
unbuttressed trees.
Key words: Anchorage, root architecture, sinker roots, tap
roots, root bending strength, buttresses.
Introduction
Root buttresses, triangular flanges joining the roots and
lower trunk, are a characteristic feature of many tree
species of lowland tropical rainforest (Richards, 1952)
which have intrigued biologists for generations. This is
largely because they are very rare in temperate zones,
though in tropical rainforests they are far from universal.
There have been various theories proposed to the
function of buttresses. For example, Black and Harper
(1979) suggested that the buttresses serve to prevent vines
climbing up them. This hypothesis, was later discredited
by Boom and Mori (1982). The general consensus, however, is that buttresses, as their name suggests, have a
mainly mechanical role, helping to provide stability and
so preventing the tree from toppling over (Senn, 1923;
Richards, 1952). The evidence researchers brought to
To whom correspondence should be addressed at: Harper Adams Agricultural College, Newport, Shropshire TF10 8NB, UK. Fax: + 44 1951 825 340.
E-mail:[email protected]
Oxford University Press 1997
1704
Crook et al.
support this theory is, however, mostly indirect; most
workers have attempted to deduce their function by
correlating the development of the tree with the environmental conditions in which they grow (Richards, 1952;
Richter, 1984; Lewis, 1988).
It has also been stated that trees with well developed
tap roots rarely produce buttresses (Francis, 1924; Navez,
1930; Petch, 1930; Corner, 1988) and the development of
buttresses is also correlated with tree height; buttressing
is more common in emergent and canopy trees than in
those growing in the understorey (Richards, 1952; Smith,
1972). The extent of buttressing is also correlated with
soil texture and depth. Buttresses tend to be larger in
trees growing in weak silty soils (Richards, 1952),
in shallow waterlogged soils (Richards, 1952) or where
there is a shallow humus layer overlying rock or subsoil
(Richards, 1952; Navez, 1930; Petch, 1930). There is also
a correlation with wind direction; though buttresses are
randomly orientated around the trunk, the buttresses on
the side of the trunk which faces the prevailing wind tend
to extend further from the trunk (Warren et al., 1988;
Lewis, 1988).
Whilst these studies have shown that certain factors
affect buttress root development, only a few investigations
have attempted to explain how the buttresses actually
anchor the tree. Henwood (1973) suggested that buttresses act as tensile elements which increase the anchorage provided by the lateral roots on the windward side
of the trunk by lengthening the moment arm about which
they act. His conclusions were based on the observations
of Senn (1923), Navez (1930) and Baker (1973) who
noted that buttresses on the side of the trunk facing the
prevailing wind tend to extend further along the trunk.
However, this model of anchorage is unconvincing, as
the lateral roots are poorly orientated to transmit forces.
More recently, Mattheck has proposed a much more
plausible explanation for the function of buttresses
(Mattheck, 1991, 1993).
Though most of the roots of tropical trees are superficial (Stark and Jordan, 1978; Jordan, 1982) they also
possess sinker roots which branch from the laterals along
the trunk and penetrate down into the subsoil (Jenik,
1978; Baillie and Mamit, 1983). Mattheck suggested that
it is these roots which anchor the tree, those on the
leeward side resisting downward and those on the windward side upward forces. Being vertically orientated, these
roots are ideally placed to resist these forces. The buttresses, meanwhile, prevent the roots from splitting along
their length or snapping by bracing the roots to the trunk,
smoothly transmitting tension to the windward sinkers
and compression to the leeward sinkers (Fig. 1).
Whilst Mattheck's model of anchorage is mechanically
feasible, to date, his theory has not been tested. In this
paper, therefore, how the roots of mature subcanopy
buttressed tress (Aglaia and Nephelium species) anchor
Wind Force
Tapered trunk
Buttress
To sinker
From sinker
To sinker
Fig. 1. Mattheck's model for the function of buttresses If a tree is
pushed over by the wind the bending force is transmitted smoothly to
lateral sinker roots by the buttresses. Windward sinkers resist upward
forces, the buttresses being put into tension, while the leeward sinkers
resist downward forces, the buttresses being put into compression.
the tree has been investigated by carrying out simulated
windthrow tests, using techniques developed in earlier
studies (Coutts, 1983a, 1986; Crook and Ennos, 1996)
and examining their root morphology. To assess the
contribution in anchorage strength that buttresses make
to tree stability and to increase our understanding of why
some trees produce buttresses whilst others do not, trees
of the non-buttressed Mallotus wrayi were also winched
over and the results compared.
Materials and methods
Field site
All trees investigated in this study were growing in an area of
primary dipterocarp rainforests just outside the conservation
area at Danum Valley, Lahad Datu, Sabah, Malaysia.
The original intention was to compare two species of tree,
one buttressed and one non-buttressed. Aglaia affinis (family
Meliaceae) was chosen as the buttressed species because it
develops large buttresses whilst still small enough to be pulled
over with a hand winch and seemed to have characteristic fluted
trunks and red bark. However, it was impossible to identify the
species accurately in the field and therefore leaf samples were
taken from each tree and stored until they could be identified
later by an experienced rainforest taxonomist. This meant that
species were not identified properly until after all the tests were
performed. As a result, therefore, as well as nine mature A.
affirms examined, other tree species were also included in the
tests. These were additional Aglaia species {A. affinis Meur, A.
luzonensis (vid) Merr et Rolfe, A. elliptica Bl and A. crassinervia
Kurz ex Hiern) and Nephelium ramboutan-ake (Labill).
However, their morphological similarity resulted in sufficient
similarity in their mechanical behaviour to describe these trees
together as a single population of buttressed trees. All these
Anchorage mechanics of buttressed trees
species are of similar size; Aglaia species typically grow up to
25 m and N. ramboutan up to 35 m (Ng, 1989).
The non-buttressed M. wrayi (family Euphorbiaceae) was
used as the non-buttressed tree species for this study, not only
because of its ubiquity at Danum valley and its ease of accurate
identification in the field, but because large M. wrayi grow to
comparable sizes as the Aglaia and Nephelium species, growing
up to a height of 24 m (Whitmore, 1972). The trees which were
examined varied in girth at breast height (gbh) from 30-50 cm
for the buttressed species and 25-45 cm for the non-buttressed
M. wrayi. Although these size classes are slightly different this
was unavoidable due to restraints in finding enough trees
suitable for testing.
Preliminary tests: root movements during uprooting
Before any quantitative experiments were carried out the
movements of the root system of buttressed and non-buttressed
trees were examined qualitatively during simulated anchorage
failure.
Two buttress rooted trees, one Aglaia affinis and one
Nephelium ramboutan, of girth at breast height (gbh) 35 cm and
42 cm, respectively, and two M. wrayi of gbh 33 cm and 34 cm
were studied. Soil was dug away from one side of each tree,
parallel to the direction in which it would be pulled over,
making a trench 60 cm deep, 60 cm wide and extending 1 m
either side of the trunk. Any lateral root growing out from the
trunk and interfering with the digging of the trench was cut
away with an axe. The location and orientation of the main
lateral roots and any sinker roots were also noted.
The tree was then winched over at about 15° min"' whilst
paying particular attention to the movements of the roots, the
soil and the centre of rotation of the tree. Sounds of roots
breaking or being uprooted were also noted. Once the tree had
toppled the root system was examined closely to see whether
the roots had broken or for other signs that they had failed
mechanically.
Anchorage mechanics
(a) Preparation and soil shear strength: Twelve buttressed (three
Nephelium ramboutan-ake (Labill) Leech and nine Aglaia trees:
six A. affinis Meur, one A. luzonensis (vid) Merr et Rolfe, one
A. elliptica Bl, and one A. crassinervia Kurz ex Hiern) and
fourteen non-buttressed Mallotus wrayi King trees were prepared
for testing by cutting down the trunk at a height of 3 m above
the ground. This is essential because the nearby trees would
otherwise hinder the tests, preventing them from being
winched over.
Tests on the buttressed trees took place from November 1995
to January 1996 and on the non-buttressed trees during
February and March 1996. Prior to tests, however, the shear
strength of the soil around each test tree was measured using a
shearvane, three readings being taken for each tree just prior
to testing at a soil depth of 10 cm. These measurements were
necessary because the tests were carried out over an extended
period during which changes in moisture content of the soil
might have affected soil strength and hence the anchorage
strength of the trees (Spoor and Godwin, 1979; Ennos, 1990;
Crook and Ennos, 1993). If the shear strength of the soil was
higher than 100 kPa the tests were postponed until it had rained
sufficiently for the shear strength to fall.
A sling was looped around the trunk of the test tree at a
height of 2.2 m from the ground and connected to the winch
cable, and the winch (Haloagger RQ417/4), capable of
producing a maximum force of 0.9 kN was secured to the base
of another tree, typically 8 m away using a further sling. A
1705
force transducer capable of measuring forces up to 20 kN was
shackled in between the sling and the winch cable and its
output was recorded on an ADC 11 data logger (PicoLog,
Hardwick, Cambridgeshire). Any slack in the winch cable was
then taken up.
(b) Anchorage tests: strain measurements along the trunk and
roots-preparation: To gain further insight about how the roots
anchor the tree, strain gauges were attached along the trunk
and lateral roots, so 'mapping' the force flow along the trunk
and into the ground when a tree is pulled over.
All of the buttressed trees and five of the non-buttressed M.
wrayi trees were tested. After examining the overall root system
morphology only the straightest and most windward or leeward
roots (depending upon the direction in which the tree was to
be pulled over) were considered suitable for affixing strain
gauges. This was to ensure that the force flow along the trunk
would run straight along the length of the lateral root.
The trunk and lateral root were then prepared for the
attachment of strain gauges: first, a clean, smooth, woody
surface 20 mm long and 15 mm wide just beneath the surface
of the bark was made using a chisel at each of the locations at
which gauges were to be positioned. The 5 mm length gauges
(Kyowa KFG series, 5 mm gauge length, 120 ohm resistance,
supplied by Graham and White Instruments, St Albans,
Hertfordshire) were then glued in position with general purpose
cyanoacrylate (superglue) and allowed to set.
Up to eight gauges were glued along the trunk and the lateral
root at the following points:
(1) 1.0 m up the trunk on the leeward face.
(2) 0.1 m up the trunk on the leeward face ('base').
(3) At the 'join' of the leeward lateral with the trunk, along
the top edge.
(4) 0.5 m along from the 'join' between leeward lateral and
trunk, along the top edge.
(5) 1.0 m up the trunk on the windward face (directly opposite
gauge 1).
(6) 0.1m up the trunk on the windward face ('base').
(7) At the 'join' of the windward lateral with the trunk, along
the top edge.
(8) 0.5 m along from the 'join', away from the trunk, between
windward lateral and trunk, along the top edge.
Gauges were connected to a Kyowa strain meter (model
SM-60D) via a switching and balancing box (Kyowa SS-12R,
Graham and White Instruments) along with a common 'dummy'
gauge and zeroed. The dummy gauge reduces drift caused by
temperature variations.
Because the buttressed trees were also a part of a different
experiment (presented in another paper, Crook and Ennos, in
preparation) it was not possible to attach gauges to both the
leeward and windward sides. Instead, six of the trees had gauges
positioned on the leeward side only (gauges positions 1-4) and
six trees had gauges positioned on the windward side only
(gauges 5-8). All of the non-buttressed M. wrayi trees had
gauges positioned on both the lee and windward sides (gauge
positions 1-8).
(c) Anchorage tests-testing: The winch handle was then cranked
at a rate of one cycle every 5 s. After five complete cycles the
winching was stopped so that any root movement or damage
could be examined in greater detail. The process was repeated,
cranking the winch a further five cycles and recording the loads.
This procedure was repeated at 15, 20, 25, 30, 35, 40, 45, and
50 winches, approximating to a trunk lean of between 5° and
48° from the vertical. The strains along the trunk and root were
also measured at each incremental cycle.
1706
Crook et al.
Each cycle of the winch drew in 3.1 cm of cable, so it was
also possible, by geometry, to calculate the angle through which
the trunk had moved. The mechanism by which the tree failed
was also recorded.
Although measurements were made over an extended period
(about 1 h) the soil will creep only slightly (Crook, 1994), and
is therefore unlikely to influence the results.
(d) Root system morphology: In order to allow the roots to be
examined more easily after the tests, the trees were excavated
and manhandled clear of the soil. The morphology and
orientation of structural roots (defined as any root having a
width greater than 1 cm at a distance of 10 cm out from the
trunk) of each tree was then investigated. The number of
laterals (buttressed or non buttressed) was recorded along with
their height and width 10 cm from their join with the trunk.
The distance along the laterals from which the sinker roots
branched was measured. The tap roots had both their maximum
and minimum width measured at the join with the trunk
together with the length to a taper of 35 mm (in height) and
their maximum depth. A record of the overall form of the root
system was also made, noting the degree to which laterals
branched and the extent of branching of finer roots.
Measuring the morphology of all the laterals, tap roots and
sinkers was sometimes difficult because on occasion the roots
had snapped off and remained in the ground. These roots were
dug up and 'pieced' back into position so that the measurements
could be made.
The orientation of individual roots was recorded in one of
three categories: (1) windward root: any root emerging from
the trunk on the counter-winchward side of the tree, (2) leeward
root: any root emerging from the trunk on the winchward side
of the tree and (3) side root: any root that emerged from the
trunk on neither the windward or leeward sides was classified
in this category. Root morphology was considered evenly
distributed if the tree had both leeward and windward roots
and asymmetrically distributed if either leeward or windward
roots were absent.
Because models of anchorage mechanics are based only on
the overall form of root systems, this simple method of assessing
root orientation was sufficient for this study. Hence, a detailed
analysis of root architecture (Mardia, 1972; Henderson et al.,
1983) was not made.
(e) Damage to the trunk and root system: Damage to the trunk
and root system was recorded, noting the extent of any
delamination of the wood, together with the position along the
lateral of any break and the height and the width of the root
at the break.
Components of anchorage
To determine the extent to which the tap roots contribute to
the anchorage strength of the tree two further A. affinis, one
N. ramboutan and five M. wrayi were pulled over after all their
lateral roots had been cut away using a hand axe. The trees
were then winched over in the same way as the trees that had
their root systems left intact. To ensure a valid comparison, the
trees that were chosen were of the same gbh as trees that had
been pulled over with their lateral roots intact.
Four of the buttressed trees from the initial anchorage
mechanics tests proved too well anchored to be pulled over
with the winch. These trees also had their buttresses cut away
and retested, winching them over as before. Despite inaccuracies
caused by cutting the roots away and thereby altering the
anchorage mechanics of the system (Ennos et al., 1993) this
method is still useful because, although altered, the centre of
rotation of the system remained on the leeward side and in a
similar position to that of trees with an entire root system.
Results
Root system morphology
All the non-buttressed M. wrayi and 11 of the 15 buttressed trees had symmetrical root systems with laterals
emerging from all around the trunk. Of the remaining 4
trees with asymmetrically distributed roots 2 had no roots
on the leeside (counter-winchward) and 2 had none on
the windward (winchward) side.
Details of the root system morphology of buttressed
and non-buttressed trees are given in Table 1 which
includes only trees with gbh greater than 30 cm and less
than 40 cm. This was to ensure that the two tree populations were directly comparable. This meant that four of
the buttressed trees (over 40 cm gbh) and five of the nonbuttressed trees (under 30 cm gbh) were excluded from
the analysis. However, in order to increase sample size
wherever possible, all trees were included in the analysis
of the number of roots of a tree. This is valid because
the number of lateral roots is determined whilst the tree
is still small (Fayle, 1975; Coutts, 1983*).
Both buttressed and non-buttressed trees produced a
single tap root directly under the bole, but though the
two tree types were similar in this respect their lateral
root morphology was obviously very different: M. wrayi
produced more laterals than the buttressed trees and each
of these was circular in cross-section in contrast to the
lateral roots of the buttressed trees which had a rectangular cross-section. These differences are quantified in the
ratio of height of the root to its width, buttressed roots
are almost eight times as high as they were wide, while
in contrast, non-buttressed trees produced circular roots
with a ratio of height to width of around 1 (Table 1).
Only the buttressed trees produced sinker roots along
their laterals. Surprisingly, however, this was only true in
10 of the 15 buttressed trees, and only 28% of the total
number of roots examined. There was also a general
trend that the proportion of buttresses with sinker roots
increased with tree size. However, possibly because the
sample size was small, and root variation large, this
correlation was not significant (P = 0.096).
The lateral roots of both species branched little, with
the main roots remaining straight for over 2 m out from
the trunk. The sinker roots of buttressed trees tended to
branch at their tips into a mass of finer roots. Many of
the tap roots of M. wrayi also produced laterals about
half-way along their length. These roots, like the surface
laterals, remained growing horizontally rather than down
into the soil. Where these roots did exist, they were fewer
in number and thinner than the laterals.
Preliminary tests: buttressed trees
Excavation revealed that of the two buttressed trees
examined in the preliminary tests only the larger
Anchorage mechanics of buttressed trees
1707
Table 1. Root system morphology of mature buttressed trees (Aglaia and Nephileum species) and non-buttressed trees (M. wrayi)
(a) Overall root system morphology, (b) morphology of the tap roots, (c) morphology of the laterals, and (d) morphology of the sinker roots.
Means, SE of the mean and the number of samples (n) are given. Note: All trees excavated were analysed for (a) ranging in gbh 30-50 cm in the
buttressed trees and 25-40 cm in the non-buttressed trees, whilst only trees ranging in gbh of between 30-40 cm of both species were analysed in
(b) (c) and (d) to ensure comparable population classes.
Room system morphology
Buttressed trees
Mean
(a) Overall root system morphology
Number of tap roots
Number of lateral roots
Number of sinker roots
Percentage laterals with sinkers
(b) Tap root morphology
length of tap root to a taper of 35 mm
diameter (m)
Depth of tap root (m)
Maximum width at base: MaxWid (mm)
Minimum width at base: Min Wid (mm)
MaxWidjMinWid
(c) Lateral root morphology
Height at buttress join (mm)
Width at buttress jom (mm)
Height/width at buttress join
(d) Sinker root morphology
Distance along lateral (m)
Diameter (mm)
0.9
3.8
1.1
30.2
Non-buttressed trees
SE
n
Mean
SE
n
0.09
0.38
0.31
8.28
14
14
14
14
1.0
6.0
0.0
0.0
0.0
1.6
0.0
0.0
14
14
14
14
0.61
0.06
7
0.64
0.06
9
0.88
21.6
21.3
1.1
0.16
106
11.5
0.09
7
7
7
7
1.08
13.2
11.9
1.2
8.43
0.9
1.2
0.09
9
9
9
9
24.6
0.99
0.89
31
31
31
33.8
32.4
1.07
1.9
1.7
0.13
58
60
58
-
-
-
216.5
28.2
7.9
0.30
31.1
Nephelium had sinker roots growing down from its buttresses. However, despite this difference in root morphology the two trees failed in a similar way as the trees were
pulled over (Fig. 2). Significant movements in the roots
only occurred after the trunk had been displaced by c.
20° from the vertical. Both trees then began to rotate
about a point on the leeward side of the trunk, at a depth
of about 25 cm (Fig. 2-lb, 2b) and as the tree was pulled
over the leeward buttressed lateral (or more accurately,
the winchward lateral) was pushed into the soil. The
buttress, with its high bending rigidity bent imperceptively
as it was pushed into the ground. However, at a trunk
inclination of 30-35° from the vertical these buttressed
roots broke towards the end of the buttress (Fig. 2-2c).
In contrast, behaviour of the windward buttresses differed
between the two trees. In the case of the Aglaia affinis
the buttress, which did not possess sinker roots to anchor
it into the ground, was levered out of the ground
(Fig. 2-lb). Snapping of finer roots was heard when the
tree trunk was inclined by about 30° from the vertical.
The windward buttress of the Nephelium was held securely
in the ground by its sinker root (Fig. 2-2b).
Consequently, as the test proceeded very little root movement was detected on the windward side. At c. 20°,
however, the root began to delaminate, starting in the
middle of the buttress about 10 cm out from the join with
the trunk (Fig. 2-2b). Delamination propagated up the
buttress and 1 m up the windward side of the trunk as
the test proceeded (Fig. 2-2c).
In both trees the tap roots rotated about the centre of
0.049
4.25
8
8
rotation, coming up on the windward side after originally
being confined by the surrounding soil.
Non-buttressed trees
As the trees were pulled over, the two M. wrayi trees
failed in a similar manner to each other but in a dissimilar
way to the buttressed trees (Fig. 3a). Significant movements in the roots occurred at small trunk displacements
(about 10° from the vertical) and the trunks rotated
about a point close to the leeward side of the trunk, at
the greater depth of around 50 cm (Fig. 3b). The leeward
roots, with their low bending rigidity bent and buckled
rather than pushing far into the ground whilst the windward laterals either snapped at their base or remained
attached to the trunk and were uprooted. The behaviour
of the tap roots also differed from that of the buttressed
trees being pushed sideways into the soil on the leeward
side, leaving a cavity where they had previously been
(Fig. 3b). As the trees were pulled over the tap root
continued to be pushed into the ground and the leeward
roots buckled further (Fig. 3c).
Roots orientated at 90° to the direction of pull (growing
out the 'sides' of the tree) probably contribute little to
anchorage of either buttressed or non-buttressed trees
because any restoring force generated will act close to the
centre of rotation of the tree.
Anchorage mechanics: soil shear strength
Throughout the sampling period the clay soil remained
at or close to field capacity. This meant that the shear
1708
Crook et al
1) a)
2) a)
Fig. 2. Trunk and root movements during anchorage failure of buttressed and non-buttressed trees, (la) Buttressed tree without sinker roots (Aglaia
affinis), (2a) buttressed tree with sinker roots (N ramboutan). Note: sinker roots may be present or absent on both Aglaia and Nephelium species.
(la) Buttressed tree without sinker roots The tree is anchored into the ground by the thick buttressed lateral roots and the tap root. (lb) As the
tree is pulled over, the trunk rotates about a point just on the leeward side. Initially, the roots firmly anchor the tree in the ground, the leeward
laterals resisting bending being pushed into the ground and the tap root resisting uprooting. The windward buttress, held in the ground by fine
roots only, uproots easily. ( l c ) As the test proceeds the leeward buttress finally fails, breaking towards end of the buttressing The centre of rotation
changes so that the tree rotates about this leeward hinge and the tap root is levered out of the ground or breaks. (2a) Buttressed tree with sinker
roots. The tree is anchored into the ground by the thick buttressed laterals by their sinker roots and by the tap root. As the tree is pulled over
(2b), the tree rotates about a point just on the leeward side. Initially the roots firmly anchor the tree in the ground, the leeward laterals resisting
being pushed into the ground and the tap root resisting uprooting. The windward buttress, held in the ground securely by the sinker roots also
withstands uprooting and instead begins to delaminate. As the test proceeds (2c) the leeward buttress finally fails, breaking towards the end of the
buttressing. The centre of rotation changes so that the tree rotates about this leeward hinge and the windward root continues to delaminate.
a)
b)
c)
\
Crevice •
C.OB
Fig. 3. Trunk and root movements during anchorage failure of non-buttressed trees (M. wrayi). (a) Non-buttressed tree. The tree is anchored into
the ground by the tap root and to a lesser extent the lateral roots, (b) as the tree is pulled over, the tree rotates about a point just on the leeward
side of the tap root at a depth of c.0.5 m. The leeward laterals, are pushed only slightly into the soil and then buckle whilst the laterals on the
windward side resist being pulled up, acting in tension. The tap root pushes into the soil on the leeward side both bending slightly and rotating
above the centre of rotation and below this bends and moves slightly windward. A crevice is formed on the windward side as the tap root rotates.
As the test proceeds (c) these root movements continue, the leeward laterals buckling, the windward laterals uprooting and the tap root pushing
into the soil, increasing the size of the crevice.
Anchorage mechanics of buttressed trees
1709
trees failed in their roots rather than by the trunk
breaking.
In contrast to the buttressed trees, noticeable movement
of the roots began at the onset of the tests and the
maximum anchorage moment was reached at 27 ±9°,
Anchorage tests: buttressed trees
later than for the buttressed trees; by this time the tree
All the trees that were pulled over failed in the root
had already begun to uproot (Fig. 4b). After the windsystem rather than by the trunk breaking. Despite the
ward roots had come up, the anchorage resistance began
large number of different buttressed tree species used in
to fall only slowly because the resistance of the tap root
this study the mechanism of anchorage failure of all these
was still high.
trees was similar to those initially observed, with the
The shapes of the anchorage moment/trunk displacewindward buttressed laterals either pulling out of the
ment
graph (Fig. 4b) for all the trees was similar. The
ground or delaminating and the leeward buttresses pushmaximum
anchorage strength was 4.9 ±1.45 kNm.
ing into the ground and breaking at their end (Fig. 2-lb).
However,
comparing
similar sized buttressed and nonHalf of the trees tested failed by delamination along their
buttressed
trees,
the
anchorage
strength of M. wrayi was
windward buttress (all Aglaias) and half had their windjust
over
half
(56
+
5.7%)
that
of
buttressed trees.
ward buttresses uproot cleanly (one Aglaia species and
The
anchorage
strength
of
the
non-buttressed
trees also
two N. ramboutans). One of the N. ramboutans uprooted
correlated
with
tree
size
with
bigger
trees
being
better
even though its buttresses possessed sinker roots. Of the
anchored
(^
=
49.3%,
P<0.0\,
n
=
\A).
12 trees originally tested, four of them were too well
anchored to be pulled over with the hand winch. These
Damage to the root systems: buttressed trees
trees were retested after their buttresses had been cut
away (see trees without laterals, below).
Without exception, all the leeward buttresses were damThe mechanism of anchorage failure was also similar
aged during the anchorage tests, with their roots breaking
in trees that had asymmetric rooting patterns, with their
30-50 cm out from the tree, towards the end of their
buttresses on the leeward or windward side only, as were
buttressing. In contrast, fine roots excepted, the windward
the shape of the anchorage moment/displacement curves.
buttresses without sinkers pulled out of the ground whilst
However, the trees with tap roots and windward butthose that had sinkers delaminated. In 2 of the 8 trees
tresses rotated closer to the trunk, whilst the trees with
which pulled over the tap roots broke cleanly at their
leeward buttresses and tap roots only rotated about the
base, leaving the root embedded in the ground.
leeward hinge and earlier during testing. Two trees, one
Aglaia and one N. ramboutan lacked tap roots, but the
Non-buttressed trees
shape of the anchorage strength/displacement curve was
similar in shape to the other trees, probably because of
Unlike the leeward roots of the buttressed trees only a
their symmetrically distributed buttresses.
small proportion of the leeward laterals of non-buttressed
trees were damaged noticeably during the tests. Only four
The maximum anchorage strength of the 8 trees which
(10%) of the leeward roots broke; these roots were the
were successfully winched over was 10.6 + 3.19 kNm at a
largest
and leastflexibleand the damage was always close
trunk displacement of 18 + 4.8°. The shapes of the anchorto
the
join
with the trunk. The majority (90%) of the
age moment/trunk displacement graph (Fig. 4a) for all
leeward
laterals,
being thinner and moreflexible,buckled
the trees was also similar, with the anchorage strength
instead of breaking, and appeared to be undamaged. The
initially rising rapidly and then falling rapidly as the roots
windward roots either snapped at their base (one-fifth of
uproot or break. Figure 4a shows the relationship between
roots) or pulled out of the ground. Only three of the 14
anchorage strength and trunk displacement for individual
trees. The anchorage strength of the tree correlated
tap roots broke during uprooting, and where damage did
with tree size with bigger trees being better anchored
occur it was always close to the base.
strength of the soil did not differ significantly with time
or between locations. The average strength (and sd) was
62±18kPa.
Relative anchorage strength and components
Non-buttressed trees
Unlike the buttressed trees all 14 M. wrayi had an evenly
distributed lateral root system with roots emerging from
all around the trunk. The mechanism of anchorage failure
of all the non-buttressed M. wrayi trees was the same as
that initially observed, with the windward laterals
breaking or uprooting, the leeward laterals buckling and
the tap root pushing into the soil on the leeward side. All
To highlight the differences between buttressed and nonbuttressed trees in the relationship between their anchorage moment and the angle of inclination (Fig. 4) the
relative anchorage moment of each tree at each inclination
were calculated by dividing the raw figure by the anchorage moment at 18%, where buttressed trees usually show
the greatest moment. Although the buttressed trees
belonged to different species, this procedure shows that
1710
Crook et al.
b) Nonbuttressed
a) Buttressed
15
£
z
E
o
10
en
o
A luxonensis
A elliptica
A affinis
A affinis
A affinis
0
10
20
30
40
50 60
Trunk displacement from the vertical (°)
0
10
20
30
40
50 60
Trunk displacement from the vertical (")
Fig. 4. The anchorage moment generated as trees are pulled over for (a) buttressed trees (Aglaia species solid lines, N. ramboutan dashed lines, n =
8) and (b) non-buttressed trees (all M. wrayi, n= 14)
grouping them together is appropriate because the shapes
of the original curves were similar (Fig. 5).
Trees without laterals
All trees whose laterals had been removed failed in a
similar manner, with the tap root pushing into the ground
on the leeward side, just as in intact Mallotus. Figure 6
shows the relationship between the anchorage strength
and the displacement of the trunk. The shapes of the
curves of both tree types were also similar in shape to
the intact M. wrayi, with the anchorage strength plateauing rather than falling after the maximum anchorage
strength had been reached (Fig. 4b), and the anchorage
strength of the two types was similar.
Paired Mests revealed that for both tree types the
anchorage strength was significantly different from
the anchorage strength of trees with an intact root system
(/> = 0.04, n = 3 for the buttressed trees, P = 0.015, n = 5
for non-buttressed trees). However, the maximum anchorage strength of the cut buttressed trees was only 32 ± 12%
of intact trees; in contrast, cut non-buttressed trees had
75 ± 12% of the anchorage strength of intact trees. These
results suggest that tap roots make a far more important
contribution to anchorage in the non-buttressed than the
buttressed trees.
anchorage moment and the strains generated along the
trunk and roots of both the buttressed and non-buttressed
trees. However, assessing the strain curves of all trees
showed that general patterns of strain behaviour did
occur. Figure 7 shows the strain responses of individual
trees that best highlight these responses.
Buttressed trees
For buttressed trees the strains on both sides of the trunk
rose and fell with the moment applied by the winch; when
the overturning force was at its peak so was the strain
along the trunk. As the tree came out of the ground, and
the anchorage moment was reduced, so were the strains.
The maximum strains on the trunk differed considerably
between the trees, ranging from —1200 to over
— 2100 ^Strain on the compression side and between
2300-2820 ^Strain on the windward, tension, side. At the
same applied moment, strain 1 m up the trunk was
negatively correlated with trunk diameter (^ = 43%,
P<0.0\). The strains up the trunk were also higher than
those about the base of the tree. This indicated that the
force flow is concentrated along the buttresses rather than
down the trunk (Figs 7-lb; 8).
The strains along the windward buttress roots were
highest where the lateral joined on to the trunk
Anchorage tests: strain measurements along the trunk and (Fig. 7-lb) falling only slightly further along the root.
roots
When the roots were pulled out of the ground the strains
fell because the roots no longer anchored the tree and
Because of large variation in root morphologies and
had only to support their own weight.
orientation there was no 'typical' response between the
Anchorage mechanics of buttressed trees
a) Buttressed
1711
b) Nonbuttressed
10
c
E
o
0.8
CD
o
0.6
o
c
.>
0.4
en
0.2 •
0.0
10
20
40
50
60
10
20
30
40
50
60
Trunk displacement from the vertical (°)
Fig. 5. The mean relative anchorage moment of (a) buttressed (n = 8) and (b) non-buttressed trees (n= 14). Bars represent ± S E of the mean. Filled
symbols represent trees with an entire root system and open symbols, the component tests, where the trees had their laterals cut away, leaving only
the tap root to anchor the tree (n = 3 for buttressed trees and n = 5 for non-buttressed trees). Values are calculated relative to the anchorage moment
at 18° of each individual tree, except the relative anchorage strength of the component test trees which were calculated relative to the anchorage
strength of trees of the same species of similar size. Hence, the shapes of curves and contribution of the tap root can be compared between
buttressed and non-buttressed trees, but not their anchorage strength.
The pattern of strain development in the leeward
buttress was similar, with the strains along the trunk and
join following the shape of the anchorage strength curve
(Fig. 7-la, b). However, unlike in the windward buttressed laterals, the strain further out along the root
continued to rise after the anchorage moment began to
drop; this corresponded to the centre of rotation moving
further to the leeward (Fig. 2-1 c) and the catastrophic
bending at the ends of the leeward buttresses. Therefore,
at small trunk displacements the strains generated at the
join and along the root were similar on both the leeward
and the windward sides; however, at maximum anchorage
strength, strains on the leeward roots were up to seven
times greater than those on the windward roots.
Non-buttressed trees
Figure 7-2a shows the anchorage moment produced by
the root system of an individual non-buttressed M. wrayi
tree when pulled over, together with the strains generated
along a windward root and a leeward root (Fig. 7-2b).
Just as in the buttressed trees the strains on both sides
of the trunk were proportional to the anchorage moment
of the tree (Fig. 7). Again the maximum strains on the
trunk differed considerably between the trees, ranging
from —1500 to over — 3760/^.Strain on the compression
side and between 1760-2900 ^Strain on the windward
side. Unlike the buttressed trees, however, the strains
about the bases of the trees (both on the tension and
compression sides) were higher than they were up the
trunk (Fig. 8).
Figure 8 compares the ratio between the strains 1 m up
the trunk and the base of the trunk of both buttressed
trees and non-buttressed trees. Strains at the base of the
buttressed trees were lower than up the trunk whilst in
the non-buttressed M. wrayi the strains were greater at
the base than up the trunk, showing force flow is concentrated down the tap root rather than the laterals.
The strains along the windward laterals were highest
where the lateral joined on to the trunk (Fig. 7-2b) and
fell along the root, just as in the buttressed trees. However,
the difference was far more marked, the strain at the join
reaching over 30 000 ^Strain (and off the scale of the
strain amplifier). The pattern of strain development in
the leeward laterals was also similar to those of buttressed
1712
Crook et al.
b) Nonbuttressed
a) Buttressed
15
10
o
o
u
c
5 -
0
A affinis
A odoratisTma
A affinis
A affinis
A crassinervia
10
20
30
40
50
60
Trunk displacement from the vertical (°)
0
10
20
30
40
50
60
Trunk displacement from the vertical (°)
Fig. 6. The anchorage moment generated as the tree is pulled over after the lateral roots have been cut away (component tests), (a) buttressed trees
(Aglaia species solid lines, N. ramboutan dashed lines, n = 6) and (b) non-buttressed trees (all M. wrayi, n = 5).
trees and to the windward roots (Fig. 7-2b), with the
strain rising throughout the tests.
Statistical analysis of anchorage strength and root attributes
Stepwise multiple regression was used to determine which
roots contributed most to anchorage strength of the
buttressed and non-buttressed trees. The analysis correlated the total areas of windward, side, lee laterals and
tap roots 10 cm along their length with anchorage
strength. In the case of the buttressed trees none of these
components were correlated with the anchorage strength
of the tree. This was partly because of the small sample
size and large variation in root orientation; both the
leeward and windward roots are likely to contribute
substantially to anchorage and the analysis could not
separate these out. To overcome this difficulty a further
multiple regression examined the correlation between the
area of the tap root and the total area of all buttress
roots and anchorage strength and revealed that of the
two components only the area of the buttress roots were
significantly correlated with anchorage strength (^ = 0.46
P<,0.05, n = 8) and the regression equation is:
Anchorage strength (kNm) = 7.0 + 297 x area of the
buttress roots (m2)
The constant is high (P<,0.0\) indicating that the buttresses are not the only important factor and suggesting
that the tap root contributes to anchorage after all.
Whilst this analysis was of limited use for determining
importance of root type and orientation on the anchorage
strength of buttressed trees, the technique proved to be
far more successful in the case of the non-buttressed
species: only the area of the tap root (P<,0.00l) and the
area of the windward laterals (P<,0.05) were significantly
correlated with anchorage strength (^ = 0.82):
Anchorage strength (kNm)= —0.11 +319 x area of the
tap root (m2) + 742 x area of the windward sinkers (m2)
Therefore, using average values for the areas of the tap
root and the windward laterals (calculated from Table 1),
their relative contribution to anchorage around 72% and
28%, respectively, comparable to the value obtained from
the anchorage component tests.
Discussion
Although the study was limited by the number of suitable
buttressed trees available it was still possible to identify
how buttressing contributes to tree stability. Most of the
methods described worked well, though one criticism of
the initial trenching tests could be that, because the root
system was damaged and less confined by the soil, the
root movements may have differed from untrenched trees
during uprooting. However, no noticeable differences
were observed between them and intact (i.e. unexcavated)
trees. Attaching strain gauges proved a useful addition
because they can reveal how the roots are behaving during
overturning without first needing to damage to the root
system.
Anchorage mechanics of buttressed trees
10
r-
1713
2a)
H
1
1
h
BODO
over 30000 >tStrain
4000
•
1b)
2b)
2000
/
^
^
^
^
0
..•• ••
a.
•
*"•.
.-•••••
» • • •
* *•
.».•••*
• " • - • • • •
-2000
•
»
•..
. • • • • • " • '
-4000
A'.
-6000
c
"o
-B0O0
V)
-10000
*
'. '.
'. 4
•'
-12000
-14000
-18000
K
1 m up the trunk
Base of tree
"Join"
0.5 m along root
-18000
-20000
! • • • » .
' -A. ,
-22000
20
40
60
20
40
60
Trunk displacement from the vertical (°)
Fig. 7. (1, 2a) the anchorage moment generated by individual trees, both buttressed and non-buttressed, as they are pulled over and (1, 2b) the
associated strain generated along the trunk and the lateral roots. All the strains are negative on the leeward side (in compression, dotted lines) and
positive on the windward side (in tension, solid lines), ( l a ) shows the anchorage moment generated by two buttressed Aglaia affinis trees. The
dashed line shows the anchorage strength of a tree that had strain gauges positioned along its leeward buttress only and the solid line a buttressed
tree that had gauges positioned along a windward buttress only and (2a) the anchorage moment generated by a non-buttressed M. wrayi tree, with
gauges positioned along both windward and leeward laterals simultaneously.
1714
Crook et al.
1.8
D
3
1.6
1
"
4
Z. 1-2
3
E
1.0
c
o
O.B
"K 0.6
o
.*
0.4
t? 0.2
0.0
Fig. 8. The strain at the base of the trunk relative to the strain 1 m up
the trunk (on the windward side with an applied overturning moment
of 2.2 kNm) for buttressed and non-buttressed trees. Different letters
indicate significant differences between the groups (analysis of variance
with Tukey multiple comparison test).
The study revealed two unexpected findings about root
morphology. The first was that the majority of the buttress
roots did not have sinkers to anchor them firmly into the
ground and second was that the buttressed trees did have
tap roots. These findings are in disagreement with
Mattheck's model of tree anchorage (Mattheck, 1991,
1993). Mattheck also suggested that the buttresses
strengthen the anchorage by preventing delamination at
the join of the windward roots and trunk. In these studies
windward buttresses that did possess sinker roots often
remained securely in the ground and did delaminate rather
than uprooting. Presumably, without the buttresses this
would have occurred at much lower forces.
Despite these surprises the anchorage mechanics of
buttressed trees does appear to be similar to the model
proposed by Mattheck; the leeward buttresses resisting
being pushed into the ground and the well anchored
windward roots with sinkers also resist uprooting.
Although the Aglaia and Nephelium trees without sinker
roots along their windward buttresses uprooted, the trees
that did possess sinkers on their windward buttresses
were more securely anchored, fitting Mattheck's model
better. In contrast to the windward buttresses which
require sinker roots to be effective, the leeward buttresses
resist being pushed into the soil whether they have sinker
roots or not. This finding, therefore, is inconsistent with
Henwood (1973), who believed that buttresses were primarily tension members; buttresses without sinker roots
will be most effective as compression members. It must
be remembered, however, that in order to be most efficient
at anchoring the tree, the buttresses must withstand both
uprooting and being pushed into the soil because of the
dynamic loading of the wind.
One thing that was clear was the effectiveness of
buttresses at increasing anchorage strength and efficiency.
The buttressed Aglaia and Nephelium species were almost
twice as well anchored as similar sized non-buttressed M.
wrayi. This was clearly due to the contribution the buttressed laterals make to the anchorage strength of the
tree; and component tests showed that the buttresses were
responsible for generating around 60% of the total resistance to uprooting compared with only 40% for the tap
roots (Fig. 6). In contrast, in non-buttressed trees, though
the tap root contributed a similar anchorage moment as
tap roots of buttressed trees, its thin laterals contributed
much less; only around one-sixth that of buttresses.
Unlike the non-buttressed trees that reached their maximum anchorage strength around 25° trunk lean from the
vertical, buttressed trees reached their maximum anchorage strength earlier, at around 18° trunk displacement
(Figs 4, 6). This meant that the greatest resistance of
buttressed trees to overturning was produced before
anchorage failure began. In contrast, anchorage failure
had already begun in the non-buttressed M. wrayi before
the maximum anchorage strength was reached, reducing
its efficiency.
There was a trend for larger trees to possess a greater
proportion of sinker roots along their buttresses.
Although this was not statistically significant, late development of sinkers is not inconceivable because as a tree
grows taller and the anchorage requirements increase, the
tree may no longer be adequately anchored by the tap
root and leeward buttresses alone. Developing sinkers
along the laterals would increase the anchorage strength
of the tree by increasing the anchorage efficiency of the
windward buttresses. In taller tree species that develop
extensive buttresses, like many of the Dipterocarpaceae
for example (Wood and Meijer, 1964), one might expect
all the buttresses to possess sinkers. Extensive excavation
of many tree species of varying sizes is needed to test this
hypothesis.
So how does buttressing confer increased anchorage
strength? One main factor is that whereas both leeward
and windward laterals of buttressed trees contribute to
anchorage strength it is only the windward lateral roots
of non-buttressed trees that contributed to anchorage.
This is because whereas even thin roots are strong in
tension and resist uprooting, laterals need to be thick to
resist bending forces (Gordon, 1978). The laterals of
M. wrayi are thin and flexible and will generate only a
small restoring moment when they are bent. Even the
windward roots contributed little and this is probably
because of their thinness which makes them flexible in
bending and poorly orientated to resist upward movement
of the root plate (Coutts, 1983a, 1986). In contrast, the
Anchorage mechanics of buttressed trees
buttressed laterals of the Aglaia and Nephelium species
are highly resistant to bending not only because the roots
are thick but much more because their height is almost
eight times the width of the root (Table 1); this geometry
makes the roots more rigid in bending, because their
second moment of area (/), is high, just like as in the 7 '
beams used in civil engineering (Rigg and Harrar, 1931;
Gordon, 1978; Crook and Ennos, 1996). In fact, using
values of height and width of the roots from Table 1, it
can be calculated that the second moment of area of the
laterals of buttressed trees is over 20 times greater than
that of the laterals of the non-buttressed trees. Assuming
that the material stiffness of the two woods is the same
this means that the bending rigidity of buttressed roots is
also over 20 times greater than the non-buttressed roots
of M. wrayi. Even this, however, is likely to be an
underestimate because bending stiffness increases with
wood density and the wood of the buttressed species is
denser than that of M. wrayi (Burgess, 1966).
In conclusion, therefore, this study has shown that
buttresses do have a clear anchorage function. Both
leeward and windward buttresses are effective and act in
compression and tension, respectively (provided that the
windward buttresses are mature enough to have sinker
roots), and together produce more anchorage than the
tap root. As a result the buttressed trees examined had
almost twice the anchorage of similar sized trees without
buttresses.
So why do some trees have buttresses while others do
not? Clearly, size is important, since larger trees tend
to be more frequently buttressed than smaller ones. This
might be because the anchorage efficiency of tap roots
falls as trees get bigger and so larger trees require additional support (Ennos, 1993). A further study performed
on the scaling of anchorage of Mallotus will help to test
this idea (in preparation).
Wood density may also be an important factor. The
buttressed trees used in this study had wood of higher
density than the M. wrayi (Burgess, 1966). Trees of denser
wood may have thinner trunks and tap roots which will
be less effective at anchoring the tree and may require
additional support. Clearly, a survey correlating tree
morphology and wood density of a wide range of species
would be instructive to test this theory.
The factors that mediate the growth of buttresses is
also unknown. It is possible that differences in trees may
result purely from their initial pattern of rooting which
is related to factors involving water and nutrient uptake
(Scott-Russell, 1977). The Mallotus wrayi trees studied
had a larger number of thinner roots compared with the
buttressed trees (Table 1). According to the Constant
Stress Hypothesis (Metzger, 1893; Mattheck, 1991, 1993)
trees grow adaptively by laying down wood faster in
highly stressed regions. It is possible that the roots of
M. wrayi will be less mechanically stressed during growth
1715
than those of the buttressed trees and so will remain thin.
In contrast, the thicker roots of the buttressed trees will
be more highly stressed particularly at their junction with
the trunk and buttresses will automatically be produced
(Mattheck, 1991, 1993). This possibility is also being
tested by instrumenting young trees, of contrasting morphology, with strain gauges around their stem base and
laterals and investigating the differences in strains along
the root as the trees develop in natural breezes.
Acknowledgements
We are indebted to the Malaysian Economic Planning Unit and
the Danum Valley Management Committee of Yayasan Sabah
Forestry Upstream Division (RBJ) for permission to work at
the excellent Danum Valley Field Centre. We thank Ismail
Mohammed Bin Samant for invaluable and superb field
assistance, Leopold Madani (of the Forest Research Centre) for
identification of the trees, our internal collaborators Robert
Ong and Anuar Mohammed (also of the FRC, Sepilok,
Sandakan), the Royal Society and the NERC for funding
(Grant Number GR3/9446).
This paper is based on material collected (in part) whilst the
authors were participants in the Royal Society's South East
Asian Rain Forest Research Programme (program publication
No.A/141).
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